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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14611
Loc: Cumberland, R I , USA42N71.4W
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This discussion continued from a recent post of mine to the refractors forum regarding ED glass
ED means that the glass has Extra-low Dispersion. That by itself has no meaning as far as color correction and no bearing on the final color correction of a doublet, triplet or multi-element lens. Dispersion does not govern the color correction of a lens. The term "ED" does not imply any particular level of color correction. There are extra-low dispersion glasses that produce bad color correction, and there are high dispersion glasses that produce excellent color corrections.
The term "reduced secondary spectrum" is used to describe a lens that has better than achromat correction, but it will still have residual color. If a lens color error is not specified as apochromatic, you can pretty much bet on the lens still being a simple achromat. In many cases the term ED is nothing more than a marketing ploy.
From the notes of a world renowned lens designer
“The term ED means Extra-low Dispersion. A lot of glass has extra-low dispersion (generally any glass having a Vd value > 70). Not all glasses having this property will produce measurably better color correction than a standard crown-flint combination achromat. Measurably better would be better than 1 part in 2000 color error over the C-F spectrum (red to blue-green). Therefore, you can easily claim to have a (Semi-ED) lens if you use the common design FK5 crown and SF1 flint, yet have exactly the same color error as any normal achromat.
It is not ED that produces better color correction. It is the fact that some glasses, notably the ones using fluorite elements in their construction, have a property known as abnormal dispersion. This property allows two dissimilar materials (such as crown and flint) to have opposite and equal color errors which cancel when they are combined. Most (not all) of these glasses with abnormal dispersion also are Extra-low Dispersion glasses, but the two properties should not be confused”.
Roland Christen
Color correction of two different achromat scopes can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.
Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens.
From this, you can see how easy it may be to market a small, not so fast, ED scope to the viewing public and have it declared at least by potentially half the viewers that is is nearly color free, and yet it is still an achromat.
ED lenses can be doublets or triplets. Even in a well made ED doublet with well-matched ED glass, a longer F ratio will show better color correction. For instance an ED doublet 80mm f/7 will have less color than an ED 80mm f/5. Generally, no ED doublet with lower Abbe# ED glass will match the performance of higher Abbe# glass.
edz
OK, so how does this relate to ED glass in Binoculars?
Setting aside what ED really means to the end user, take a moment to think about where ED glass is employed in the design and what may be the overall result. In an optical system, color error is far more dependant on the correction of the objective lens than the eyepiece. The ratio of contribution to (longitudinal) color error can be found by comparing the focal length of the objective versus the focal length of the eyepiece. Some may recognize this as magnification. So, at 10x the objective contributes 10x the color error as the eyepiece. There are many eyepieces labeled ED, however, their total contribution to color correction is quite small. In a 20x binocular ED color correction contribution from an eyepiece is limited to correcting 5% of the longitudinal CA error.
So, having two binoculars of apparently the same quality, why is it one binocular can look so much better than another? Refer to the portion above about bias. Color correction of two different achromat scopes (two different binoculars) can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.
Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens. Relate that to a 50mm or 40mm binocular and the color error is already very small due to aperture.
Furthermore, it is not uncommon at all for a binocular to have internal vignette that reduces aperture to something even smaller than the nominal stated based on objective size. Well, at the same time aperture is reduced (benefical towards reducing false color), this increases the focal ratio (also benefical towards reducing false color), reducing some aberrations, including color error. For two equal sized (nominal) binoculars, but one having greater vignette than the other, a portion of the false color is slightly suppressed by the greater internal vignette.
Many designs especially binocular designs, are faster than desirable for optimum ED color correction and their color performance suffers as a result. In the almost uncountable scope discussions that we have, it has been said data consistently shows better designs incorporate higher Abbe# ED glass in an objective of focal ratio significantly more than twice the diameter in inches. For instance, as relates to a binocular, the WO22x70 Apo (apprx f/5.8 to f/6) is specified as APO and as using FPL-51 ED glass (a lower Abbe# ED glass) in a doublet configuration. This binocular still shows some false color. It remains questionable that it produces what would be considered an apochromatic image.
There are many testimonies about scopes using FPL-51 ED glass that still show moderate to significant false color. Triplets using ED glass of various types have consistently proven to have great visual color correction. The Tak Astronomer 22x60 4 element Flourite objective shows no noticable visual color error.
So here we have not only many reasons why it may be quite easy to market ED in a scope and show good results, even though the results might not be entirely a result of a better combination of ED glass, but also we have a number of issues that are specifically binocular related that would tend to lessen even moreso the degree of performance improvement resulting from ED use in a binocular.
Don’t be so overwhelmed by what you read on some websites about the outstanding performance of ED binoculars. There are just as many, if not more, fine non-ED binoculars. How often have you seen reviews of ED binoculars in which the author isolated various tests and reported contribution to overall performance from such aspects such as focal ratio, aperture size, vignette, etc. Could it be that the ED marketed models performance is not entirely due to the inclusion of some ED in the design? I leave that to you to decide.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3391
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Good paper, EdZ.. brings to mind the term "sum of the parts"... All to easy to sometimes- even if only in the back of our minds- to feel the ED label automatically guarantees a better binocular. Not necessairly so.
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Wes
Atlantic Beach, FL
Some bino’s from Miyauchi 5x32 Binon's up through Garrett 20x110 Signature's,
Some telescopes from a Stellarvue 80mm NHNG up through a couple of 8” reflectors…
And a wonderful 4.25" Delmarva Shiefspiegler!
Some good friends, made here on C/N.
Oh- several cats and a wonderful wife!
Anyone want a cat???? :-O
"When your work speaks for itself- Don't Interrupt" -Gamble Rogers
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KennyJ
Reged: 04/27/03
Posts: 12747
Loc: Lancashire UK
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Thanks EdZ ,
That was the most interesting and informative piece I've read on this forum for an unusually considerable time .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
Captain's Helmsman 7 x 50
Nikon 10 x 42 Superior E
Swift Audubon Kestrel 10 x 50
Helios 15 x 70 Observation
Strathspey 20 x 90
Televue 76 APO
Zeiss 85 Diascope
Helios 102 f5 refractor
Various eyepieces barlows tripods mounts etc.
Panasonic Lumix DMC - TZ5 digital camera
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14611
Loc: Cumberland, R I , USA42N71.4W
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Thank you Kenny.
yes Wes, the sum of the parts sums it up fairly well.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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Man in a Tub
Not Retired!, But a little cranky!!!
Reged: 10/28/08
Posts: 1883
Loc: San Francisco, CA
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Thanks Ed.
Yet another excellent exposition that disabuses us of the "hype" that comes into play in the marketplace.
Clear skies and clear heads!
Todd
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Todd
Brunton Eterna 15x51 ° Garrett Optical Signature Series 15x70
Nikon Action EX 12x50 ° Oberwerk 15x60 and 20x80 Standard
Orion Paragon Plus Mount and Paragon XHD Tripod
Garrett Optical Series 2000 Grip-Action Monopod
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GlenM
Vendor
Reged: 05/20/07
Posts: 1703
Loc: Lancashire UK
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EdZ,
Another excellent post in plain English we can all understand.
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Glen
www.lyraoptic.co.uk
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gmazza
member
Reged: 03/10/09
Posts: 99
Loc: RS, Brazil, 29S 51W
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Very useful info, more important data to future purchases.
The good statement that every optical system have compromises, based on what we want to observe we need to choose the most acceptable one.
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Oberwerk Ultra 10X50
Garrett Signature 15X85
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daniel_h
sage
Reged: 03/08/08
Posts: 469
Loc: VIC, Australia
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great read, my regals show no false color when observing, the bigger binos show very slight color on the moon-when you look for it
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regal 10x42, 10x50ultras, 15x70 ultras, 20x80
Oly e-500, vixen 100/1000 with 0.965"/1.25",
2 old sturdy tripods for the bins (slik & velbon)
zeiss f5.6 refractor/lens (under construction)
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3125
Loc: Just passing through.....
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IMO, we need to be more insistant on improved field edge correction (ie better eyepieces) rather than dwell on the small amount of color seen in non ED binoculars today.
False color issues become important only with the much higher magnifications commonly used with refractors up to diffraction limits.
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10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT94 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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brocknroller
Carpal Tunnel
Reged: 10/16/03
Posts: 1936
Loc: Vallée des nuages
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All that technical information above may well be true for telescopes, which have larger aperture objectives and can be pushed to high powers; however, I've owned several small 8x and 10 ED porros (44-50mm) and tried one of new Chinese ED roofs, and every one without exception had less chromatic aberration than my non-ED bins, including my Nikon SEs.
I could look at a crow against a bleak, winter's sky with the 10x ED bin and not see any color fringing on axis and even a bit off axis.
I had about a dozen bins at the time, and had them all outside in my backyard to compare, and they all produced some level of CA in that high contrast situation.
The worst of the bunch produced a colored halo around the crow while the best showed a thin green line on one side of the crow and a thin purple line on the other side.
I suspect that with the ED porros, all the ED elements were made by (or for) Vixen despite them being used in bins branded "Celestron," "Eagle Optics," and "Swift". So that doesn't provide a wide sample of ED glasses. And none listed Abbe numbers.
I don't know who makes the ED glass for the Chinese open bridged clones. I've tried only the Promaster so far, but hope to try a Zen Ray soon. Those ED glasses might also be made by the same Chinese manufacturer.
So I am admittedly dealing with a small sample of ED glass, but then again, how many companies are there that have made ED glass for bins?
For daytime use, there is another advantage of ED glass that is often overlooked (or at least not written about much on CN, though it's literally hard to overlook for daytime use), which is that colors are more intense, and you can see more shades and gradiations of the same color.
For example, in a non-ED bins, you can barely make out the subtle differences in shading between dark and lighter reds in a male Cardinal. In an ED bin, you can detect those subtle differences.
Colors really "snap" in the ED bins I've used compared to most of the non-ED bins I've tried, though I must mention that the dozens of layers of advanced coatings on premium roofs such as the Nikon LX produce about the same color depth as the ED bins.
There are other qualities such as a more robust build, better fit and finish, and a smoother, faster focuser that make the LX better than the cheaper Chinese EDs, but image-wise, the difference is small.
I could be that the Chinese ED bins use lead glass like the LX. I could be mistaken about that but lead seems to be a common contaminant in Chinese products and the Chinese don't abide by the stricter environmental standards that the US and Europe does in this regard.
Or could it be the use of ED glass corrects the skewed image tint of the lead-free glass?
For whatever reason, the color palette in the Chinese ED bin was not skewed warmer like I've typically seen in lead-free glass bins.
The color tint seen through both the Chinese ED bin and LX look like what I see naked eye, only much more vivid.
That is, fire engine red naked eye is vivid fire engine red through the bins, not slightly orangey red.
You have to be careful to compare apples with apples (literally) when testing color, because nearby objects tend to influence what tint we see, as show in this chart:
tints of red
Note how the red squares near the dark squares look darker and the red squares near the light squares look lighter.
I can also use this chart to illustrate the differences I see in the SE vs. EII and LX vs. LX L (that is, lead vs. lead-free glass).
Dark reds through the SE and LX remain dark red and very close to what I see naked eye, whereas through the EII but especially the LX L, the reds have a lighter tint with a bit of yellow added.
The upside is that the warmer colors in the lead-free glass bins give the impression of the image being brighter.
However, in an actual color extinction test at twilight, the SE outperformed the EII by about 8-10 minutes, perhaps because of the slightly larger aperture or more efficient light transmission.
In daylight, the EII looks "brighter" to my eyes.
I didn't have the 10x42 LX and the 10x42 LX L at the same time, so I couldn't compare them at twilight.
The bins with lead-free glass I've tried also show more CA than lead glass bins of comparable quality. I suspect this has to do with the lack of lead in the glass.
"Even if lead-free glass material with an equivalent refractive index and Abbe’s number is developed, it is impossible to achieve the desired Apochromat performance especially for highest-grade objective lenses with a high aperture number when the anomalous dispersion degrades."
Here's the rest of the article:
Lead in optical glass
So the "ED craze" we are now seeing in roofs might be a result of manufacturers trying to reduce CA in their lead-free glass bins.
If you look back at reviews of the pre-HD/FL bins by Zeiss and Leica, you will see criticisms about excessive CA in the Zeiss Victory and Victory II and the Leica Ultravids (even in the Trinovid BN vs. BA).
I also detected more CA in the LX L than in the LX.
So while the color fidelity difference between high grade ED glass bins and high grade non-ED bins may not be that noticeable except in high contrast situations, the color saturation difference between the ED bins I've tried and non-ED bins, with the exception the Nikon LX, is quite noticeable.
For binocular stargazing, vivid color is limited to bright stars (most nebulous DSOs appear bluish-white, as do faint stars).
For daytime uses, ED binoculars can show you the entire spectrum of visible color.
Turn those ED bins on a Yellow Finch or a Cedar Waxwing or on colorful bed of flowers and be prepared for a "Wow" moment.
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B'rock, son of Grilka
Member of the House of Kozak
Klingon Poet-Warrior
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"The character of a Klingon poet-warrior is measured not only by the metal of his blade--but also by the mettle of his words."
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BobinKy
Carpal Tunnel
Reged: 04/27/07
Posts: 1672
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Brock...
My experience agrees with your report. I have had the opportunity to compare ED/nonED versions of two different models and, in both comparisons, the ED versions showed better views than their nonED brothers. The two pairs are the Swift Audubon 820 8.5x44 Porro CF ED and nonED versions, and the Pentax DCF 10x50 Roof CF ED and SP (nonED) versions. Now, in both comparisons, there may be more optical features at work than ED glass, as both you and EdZ discuss. But, for me, the end result in both comparisons was the ED versions produced more contrast, better color saturation, no CA when viewing a bird against a bright sky, no CA when viewing the Moon, and . . . well, just a better observing experience.
In my opinion, if an observer can afford model versions with ED glass, I recommend they go ED. There is a difference, particularly during daytime viewing.
However, as with all binocular purchases, only the individual observer can make the decision as to whether the ED experience is worth the ED price.
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Bob
38°N
Edited by BobinKy (07/03/09 07:44 AM)
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milt
professor emeritus
Reged: 09/13/04
Posts: 550
Loc: Arizona
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Quote:
From the notes of a world renowned lens designer....Roland Christen
Ed, nice summary. For those who want to read more from Roland I found this very helpful:
http://geogdata.csun.edu/~voltaire/roland/ed.html
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Clear skies, Milt
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ronharper
Pooh-Bah
Reged: 02/14/06
Posts: 1267
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Ed,
That is a great summary of the astro scope issue, longitudinal CA. I believe this is the effect that puts a haze of poorly focused blue and violet around a bright object. I have seen this in my achromat scope, and also in binoculars, mostly around Venus in my 16x70. It rarely seems to be a problem in binoculars.
But in the daytime, in common high contrast situations, like a bird sitting high in a tree against a bright cloud, I often see color fringing that gets worse the farther from the center of the field you go, and is yellow-green on one side of the object, and purple on the other. That is annoying. That's a different kind of CA, the lateral kind.
The origin of lateral CA is not clear to me. Some people blame the eyepieces, and that can clearly be a culprit, because Nagler type eyepieces show lateral color even with a mirror objective. Also, if the exit pupil is partly occluded by an off center eye, an achromatic objective will contribute to the effect. Whatever the origin, that is the kind of CA that bugs me, and I think most people, in binoculars. And whatever the explantion, most people report that lateral CA is significantly reduced in ED glass binoculars. Some critical observers have gone so far as to say that, for example, the Zeiss FL series completely eliminates this effect over the central half of the field of view!
I am apparently rather sensitive to color fringing, and blessed be those who aren't. In high contrast situations always I am careful to center my eye, and the object. Good technique helps, but doesn't always work. This morning I looked at a bird in a tree in such a bad situation with my 8x42 Trinovid BA. I just couldn't get rid of the color fringing. The view was confused, yet I thought I could just see a trace of reddish color on his head. Then he flew to a better visible location, and turned out to be a brightly colored Western Tanager. The bird's yellow was very similar to the yellow green color of the fringes. I can't help but wondering if a Zeiss FL, or similar, would have solved that problem.
Ron
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Mr. Bill
Post Laureate
Reged: 02/09/05
Posts: 3125
Loc: Just passing through.....
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I thought this was a bino forum for astronomy use, not birding. Certainly, two different uses with different criteria as far as tolerance of false color.
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10x50 Fujinon binos + 16x70 Fujinon binos + UA UniMount
Oberwerk BT94 45 degree +24mm Pans + Hercules fork mount
120mm f/5 Orion achromat + Moonlite focuser
130mm TMB 130SS APO refractor
140mm f/5.7 Vixen NeoAchro Petzvel refractor
150mm f/6.5 Antares achromat
150mm f/8 homemade achromat....EE Barnard MW Sweeper
8 inch newt with f/5 Swayze mirror
10 inch f/4.7 Orion newt + Paracorr
15 inch f/5 Discovery "Galactic Cannon"
Member IDA
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14611
Loc: Cumberland, R I , USA42N71.4W
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Quote:
the Zeiss FL series completely eliminates this effect over the central half of the field of view
I wouldn't expect that Zeiss FL is one of those we need to be trying to determine if the ED formula is really responsible for performance or not. I'd more than likely give them some credit that they've got it. BTW, how much are those worth? Would they perhaps be at the pinnacle of the highest end of design/cost? Would those be a good example to argue against the points I made above? I think not.
Since there are both very expensive and not so expensive glasses that can be used to satisfy the combinations of ED lenses that can be designed, have you thought about why it cost so much for a really effective ED apochromat lens, or for that matter even an effective ED achromat? Although not the only reason, adding an ED element increases spherical abberation and coma. So the design configuration works towards reducing CA, but now there are other problems that must be overcome. This requires additional lens design and working and therefore higher cost. Or perhaps it is simply considered by some the compromise that must be overlooked in order to deliver a cost effective product to the public. I'm sure there are examples of both on the market.
Every wavelength of light has a different focal length. Since a lens presents rays across not a flat plane at the focal point, but a curve, if it were corrected to have CA minimized at the center, it will still present the focal point of the same wavelength rays at a different position to the eyepiece as you move further off-axis. An eyepiece cannot focus across all the points of a curve at the same time, so while focused on any given point, some other point must be out of focus. The presence of lateral color is showing the amount of off-axis curve that cannot be accomodated by the eyepiece, focal plane junction.
I'll assume it is for this reason, often the CA correction in a doublet is corrected at the position 70% off axis. This at least helps minimize this effect. Perhaps a lens design that moves the CA correction away from the 70% off-axis position inward is one that shows considerably greater lateral color. But this is now considerably off-topic to the discussion of ED glass. This is lens design.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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BobinKy
Carpal Tunnel
Reged: 04/27/07
Posts: 1672
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Quote:
I thought this was a bino forum for astronomy use, not birding. Certainly, two different uses with different criteria as far as tolerance of false color.
Mr. Bill
I have never considered this forum to be a binocular forum exclusively for astronomical use. Maybe I have missed something over the years. Many of the regular participants who hang out here use binoculars to observe diverse objects during the day, as well as objects in the night sky. Some of the regulars who participate in this Cloudy Nights Binocular Forum, also participate in Bird Forum, 24hourcampfire, Astronomy.com, Yahoo, and other forums. I have always considered this binocular forum to offer more because of the knowledge and experiece of our forum moderator and the regulars. I hope future discussion will not be restricted to astronomical use.
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Bob
38°N
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mooreorless
Just worried
Reged: 07/05/05
Posts: 1808
Loc: Cornpropst Mills,Huntingdon,Pa
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Hi Bob I had posted something before and then deleted it about this. You can not find much on daylight use of optics on this forum. Cloudy Days does not come up on google search. google search
I agree with you Bob.
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Regards,Steve M
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gmazza
member
Reged: 03/10/09
Posts: 99
Loc: RS, Brazil, 29S 51W
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Tolerance with daytime use is a feature of any binocular astronomy forum, as far as the discussions do not turn the forum an birding and landscaping forum (ie. the discussions always return to astronomy) we could forget about this.
There are binoculars considered "speciality" for astronomy which can be considered "all around" for daytime and vice versa. As are the users, some nature lovers eventually will point glasses to night sky. I think there's no need to discuss this, all users of this forum in particular always managed well this duality of binocular use.
Fortunately I am not so sensitive to color nuances as some who previously posted, hope some day have a more trained color vision.
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Oberwerk Ultra 10X50
Garrett Signature 15X85
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14611
Loc: Cumberland, R I , USA42N71.4W
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Quote:
I have never considered this forum to be a binocular forum exclusively for astronomical use.
Although we are primarily a binocular forum, we have always for years been open to all types of discussion. Some topics cannot possibly ignore dual use. This is not an issue here. Please don't make it the topic of discussion in this thread. If you wish to discuss this, an appropriate forum might be Feedback.
So, back on topic please.
Thanks, edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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BobinKy
Carpal Tunnel
Reged: 04/27/07
Posts: 1672
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I think this is on topic.
For those who care to look, here is a thread from another forum about performance opinions of ED/nonED versions of the Swift Audubon 820 8.5x44 model: Audubons. Are ED's worth the extra money?
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Bob
38°N
Edited by BobinKy (07/03/09 07:30 AM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14611
Loc: Cumberland, R I , USA42N71.4W
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take notice in that link to the differences in opinions of performance.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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BobinKy
Carpal Tunnel
Reged: 04/27/07
Posts: 1672
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EdZ...
How right you are. I posted the previous link because of its diversity of opinions about the ED/nonED versions of a specific model. I think discussion and different opinions make online forums worth the time and effort to participate and keep up with the topics.
. . .
Now, regarding the ED/nonED question, I think we cannot really do the topic justice without entering the discussion of color sensitivity among various observers. Simply put, how much do differences of color sensitivity by individual observers affect the decision to purchase an ED or nonED model? If the observer's eyes are sensitive to one or more colors, maybe the ED binocular may be worth the premium price. On the other hand, if the observer has limited sensitivity to colors, the nonED binocular may see just as much as the ED model.
EdZ, I know you discussed color sensitivity differences among observers. However, here is a link from another forum with links to medical research that also discuss the many nuances of color sensitivity. I direct any interested reader to the links in my first post of the thread in the link below--(1) How we see colors
(2) Intertwining of color sensitivity with the other senses
(3) Possible gender differences in color sensitivity. Here is the link: What determines our color sensitivity as looking through bins? .
The link also discusses brand manufacturer bias toward specific colors, as well as atmospheric influences upon brand manufacturer's color rendering. Some of the participants in the link have actually done tests (with photos) of brand manufacturing bias.
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Bob
38°N
Edited by BobinKy (07/03/09 08:00 AM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14611
Loc: Cumberland, R I , USA42N71.4W
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Quote:
Now, regarding the ED/nonED question, I think we cannot really do the topic justice without entering the discussion of color sensitivity among various observers. Simply put, how much do differences of individual color perception affect the decision to purchase an ED or nonED model?
I addressed that issue in my report. It's not simply human perception (which by the way we cannot quantify, so I won't bother discussing), but it is color bias in the correction of the instrument. How many users hate the Fujinon FMT-SX becuase of the broad blue false color fringe, and yet how many does it not bother at all? That individual color perception is not relegated to ED/nonED choices. But as I reported above:
Quote:
So, having two binoculars of apparently the same quality, why is it one binocular can look so much better than another? Refer to the portion above about bias. Color correction of two different achromat scopes (two different binoculars) can be biased towards the red end or the blue end of the spectrum, but still be equal. Some people are more turned off by red CA and others more by blue CA. Some people (IIRC, older people with smaller pupils) are less sensitive to blue and may not even see the full extent of blue CA, and therefore find a blue CA biased instrument seems to be without color. Yet it may have exactly the same color error as another biased towards the red, that, to the same eyes, seems to have lots of false color.
This brings up a good point. This issue on bias and the following I answered above just this moring on lateral color
Quote:
Every wavelength of light has a different focal length. Since a lens presents rays across not a flat plane at the focal point, but a curve, if it were corrected to have CA minimized at the center, it will still present the focal point of the same wavelength rays at a different position to the eyepiece as you move further off-axis. An eyepiece cannot focus across all the points of a curve at the same time, so while focused on any given point, some other point must be out of focus. The presence of lateral color is showing the amount of off-axis curve that cannot be accomodated by the eyepiece, focal plane junction.
I'll assume it is for this reason, often the CA correction in a doublet is corrected at the position 70% off axis. This at least helps minimize this effect. Perhaps a lens design that moves the CA correction away from the 70% off-axis position inward is one that shows considerably greater lateral color. But this is now considerably off-topic to the discussion of ED glass. This is lens design.
Take a moment and think about both these issues. They can both be accommodated very well in either an ED binocular or a non-ED binocular. BUT more importantly, neither one really is dependantt on ED glass. So here are two examples of how someone can perceive that the binocular marketed as ED glass so much improves the view, yet the reason for the improved view might have nothing to do with ED.
There are some not so expensive glasses that can be employed in design to market as what may be called ED binoculars. Not all of them really give all the improvements that may be perceived by the user. Few users really expend the effort to actually quantify what causes results. If you read clearly what I summarized in my very first post, you may notice that one of my points is that ED glass may not be responsible for all that is perceived. Isolating cause and effect is not so easy. Perception can often be mis-perception.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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BobinKy
Carpal Tunnel
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Post deleted by BobinKy. If interested, please see my next post below.
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Bob
38°N
Edited by BobinKy (07/03/09 08:01 AM)
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gmazza
member
Reged: 03/10/09
Posts: 99
Loc: RS, Brazil, 29S 51W
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Quote:
Few users really expend the effort to actually quantify what causes results.
In a post months ago we all were greatly benefited from your measures of exit pupil ilumination profile of 3 premium binoculars.
Variable subjective opinions require objective testing.
If few people bother to quantify by perception the color differences of the different solutions exposed above, even less to measure it in quantitative and qualitative data. Just by testing is possible to determine if a precise measure of the spectrum delivered by different areas of exit pupil could prove if a ED design is better than stopped one (or other solutions already mentioned). As already stated there are different glasses for use in ED merchandized lenses, so if a quality non ED is better than ordinary ED is just subject to testing.
I don't know about the instruments and method to do this.
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Oberwerk Ultra 10X50
Garrett Signature 15X85
Edited by gmazza (07/02/09 10:29 AM)
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ronharper
Pooh-Bah
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Sorry for sending things off topic, Cloudy Days would be the place for the lateral CA issue. CA is not much of a problem for astronomical use of typical small through medium large binoculars, and doesn't warrant expensive "solutions".
However, the finest and most expensive Binoculars made shouldn't be barred from discussion. There are apparently some pretty rich dudes here! Not me, dang it.
Ron
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gmazza
member
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Posts: 99
Loc: RS, Brazil, 29S 51W
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Quote:
CA is not much of a problem for astronomical use of typical small through medium large binoculars, and doesn't warrant expensive "solutions".
I think this is one reason of the previous shifting of opinions to daytime use. The kind of CA discussed is more noticeable at:
1. high magnifications
2. Astrphotography
3. lateral CA.
Astronomy binoculars are low magnification, not used for astrophotography, this left only the lateral CA for discussion.
Lateral CA is more a issue in Miky Way scan, it is almost negligible in steady, framed objects at such low magnifications.
We could even speculate about the use of doublers, thus increasing magnification and making the aberrations more noticeable, if the ED models are more resistant to boosted magnification or not, but in daily practice, and even more in astronomy, I have seen very few reports of doubler use.
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Oberwerk Ultra 10X50
Garrett Signature 15X85
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BillC
on a new path
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It is too bad that we are SOOOOO trusting that with take popular buzzwords to be of GREAT value.
WE take "ED Glass, BaK4 prisms, semi-apochromatic, Multi-coated . . ." to mean better viewing. Not one word is said about the dozens of OTHER features that go into making a good instrument.
Because "aspheric lenses" are used in a particular instrument, does it mean the aspheres were calculated correctly, make correctly, or made with the composition of the OTHER elements having been taken into consideration.
Let's get REAL here. Good binoculars are made from the sum of their design concepts and parts--not the coolness of their buzzwords!
Cheers,
Bill
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William J. Cook, Chief Opticalman, USNR-Ret.
Founding Editor, Amateur Telescope Making Journal
20-year vet. of Captain's Nautical Supplies, Optics Dept. Mgr.
Optics Machanic, WG11-3306, Ft. Lewis, Tacoma,WA
Yata, Yata, Yata . . .
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KennyJ
Reged: 04/27/03
Posts: 12747
Loc: Lancashire UK
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Thanks for that reminder , Bill !
Another thing I've noticed with regard to colour perception is that I notice a considerable difference between what I see through my right eye and what I see through my left .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
Zeiss 7 x 42 BGAT
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Pinewood
scholastic sledgehammer
Reged: 12/07/04
Posts: 810
Loc: 40.77638º N 73.982652 W
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Kenny,
It is the same with me: one eye shows more saturated colour. I have sent you a PM about a possible cause.
Happy observing,
Arthur
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Bread is not enough. We demand circuses!
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Wes James
Post Laureate
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Posts: 3391
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Quote:
It is too bad that we are SOOOOO trusting that with take popular buzzwords to be of GREAT value.
I suppose that could be blamed upon the constant heap of advertising that is heaped upon us over an entire lifetime-
Quote:
WE take "ED Glass, BaK4 prisms, semi-apochromatic, Multi-coated . . ." to mean better viewing. Not one word is said about the dozens of OTHER features that go into making a good instrument.
That's probably because those OTHER features are not as easy to quantify as "BAK-4", "ED", "APO", etc. Advertising dictates that we have to have buzzwords that people can quantify/identify with. The "OTHER" things that are not as easy to quantify come- silently assumed with a brand name or price tag- or reviews on wonderful forums such as this.
Mechanical quality, optical quality, longetivity- or a company's reputation is not so easy to quantify with a buzzword.
Quote:
Because "aspheric lenses" are used in a particular instrument, does it mean the aspheres were calculated correctly, make correctly, or made with the composition of the OTHER elements having been taken into consideration.
Let's get REAL here. Good binoculars are made from the sum of their design concepts and parts--not the coolness of their buzzwords!
Goes without saying, I think- but good to be reminded of every now and then.
Nice to see you here again, Bill- hope you're doing well! We miss you when you're not around.
Wes
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moynihan
Carpal Tunnel
Reged: 07/22/03
Posts: 1594
Loc: Wisconsin
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Quote:
...
I could look at a crow against a bleak, winter's sky with the 10x ED bin and not see any color fringing on axis and even a bit off axis. ....Turn those ED bins on a Yellow Finch or a Cedar Waxwing or on colorful bed of flowers and be prepared for a "Wow" moment.
I would have to agree with that from my limited experience with ED binos.
I have the Swift Audubon 8.5x44 in three flavors, regular, (older non-armoured), ED, and roof.
The above quoted text is true, for me. Of course, as EDZ pointed out, other factors may be at play between ED/non-ED models, even from same manufacturer/brand. Also, there can be variation within copies of the same model...
But i should add, that regarding my three binos mentioned above, the ED difference is very clear (for me, your mileage may vary) when looking at for example the rim of the moon, also.
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"Gosh, you've... really got some nice toys here"
Dual mount/ambient temperature Hominid Widefield Photon Collectors®
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Pinewood
scholastic sledgehammer
Reged: 12/07/04
Posts: 810
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Quote:
However, in an actual color extinction test at twilight, the SE outperformed the EII by about 8-10 minutes, perhaps because of the slightly larger aperture or more efficient light transmission.
In daylight, the EII looks "brighter" to my eyes.
Hail B'rock son of Gr'lka,
If you were testing the 8x SE and EII, in Cloudy Valley, I would be surpised that 2 mm. difference in aperture would make an 8-10 minute difference in twilight colour distinction. In Copenhagen, or another spot, at a high latitude, such a conclusion would make far more sense.
Happy observing,
Arthur
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Bread is not enough. We demand circuses!
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Gordon Rayner
scholastic sledgehammer
Reged: 03/24/07
Posts: 955
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In #3194486, , we read " ..... often the CA correction in a doublet is corrected at the 70% position off axis..."
It has been a while since I read about lens design, but I am recalling that the 70% is at the ray height 70% of the lens radius, and is for rays parallel to the lens axis, called paraxial, and is not a correction chosen for field rays.
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NDfarmer
member
Reged: 04/02/09
Posts: 23
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This thread has been very interesting in its discussion of the newer ED products. Many are binoholics here, not all are into astronomy, but birding, hunting and everything else. One conclusion that can be drawn is that the newer ED bins coming out mainly China certainly impress. With color, brightness, clarity and many of the basic things by
which all binoculars are judged. I am just waiting for the smoke to clear before making my first pick into the large and growing choice comning into view. Right now the ZenRay Ed with new dielectric coatings in 7x36 has my interest, will be out soon but who knows, maybe if I wait till fall there will be something else. In the meantime I'll use my Nikon 8x32 SE, and 10x42 SE, the Swar. 8.5x42 EL, so I won't be wanting or needing anything better. (Brocknroller, keep writing, Klingon, you keep me smiling!)
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brocknroller
Carpal Tunnel
Reged: 10/16/03
Posts: 1936
Loc: Vallée des nuages
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Quote:
Quote:
However, in an actual color extinction test at twilight, the SE outperformed the EII by about 8-10 minutes, perhaps because of the slightly larger aperture or more efficient light transmission.
In daylight, the EII looks "brighter" to my eyes.
Hail B'rock son of Gr'lka,
If you were testing the 8x SE and EII, in Cloudy Valley, I would be surpised that 2 mm. difference in aperture would make an 8-10 minute difference in twilight colour distinction. In Copenhagen, or another spot, at a high latitude, such a conclusion would make far more sense.
Happy observing,
Arthur
Sir Arthur of Manhattan Island,
Next week, I hope to take some photos of my backyard rain forest (which is almost entirely covered in foliage) and post them on the Cloudy Days Forum.
I recently bought a pith helmet to protect me from falling coconuts (British Royal Marines surplus).
The light levels are quite low on a cloudy day, but it's a pastoral setting rather than a cityscape so I will throw in a scaled down version of "Den Lille Havfrue" to make it look more like Copenhagen. :-)
After seeing the pix, it will become obvious why I purchased a CZJ 8x50 Octarem (though in hindsight it would have been much cheaper to attach a flashlight to my 8x32 SE since the images in both bins are fairly similar except for brightness).
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B'rock, son of Grilka
Member of the House of Kozak
Klingon Poet-Warrior
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"The character of a Klingon poet-warrior is measured not only by the metal of his blade--but also by the mettle of his words."
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BobinKy
Carpal Tunnel
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Posts: 1672
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Another point I want to throw upon the table is the observer's awareness of their own color sensitivity. In my mind, this is similar to the observer's awareness of their own interpupillary distance, eye pupil size (dark adapted and daytime), facial bone structure, or binocular weight and size they can comfortably hold in their hands.
The point on the table--now hear this--I think observers should develop an awareness of their personal sensitivity to colors. This probably will not come with the purchase of the first optical instrument. And this awareness may not come until they own several instruments and have done a considerable amount of optical observing.
The awareness of our unique color sensitivity, along with our personal field comparisons of several ED/nonED models, may influence whether we decide to add one or more ED binocular models to our selection of optical instruments.
I did this with two separate models--Swift Audubon 820 ED 8.5x44 porro and Pentax DCF ED 10x50 roof--and I decided to invest additional money and go for the ED versions. Now, two years later, I am glad that I did.
Others may see it differently.
. . .
A related issue to the observer's awareness of color sensitivity is the observer's color preference.
Myself, I find that I enjoy a Minox color rendering in the fall and winter seasons, and the Pentax color rendering in the spring and early summer. Of course, what I am seeing is really a type of chromatic aberration--albeit, in my eye, a very pleasant chromatic aberration.
Others report they prefer neutral color or no color bias in their binoculars. I must confess that most of the time I prefer neutral color when I view. This is one reason I frequently observe the night sky with Nikon SE binoculars. Of the binoculars in my selection, the SE is most neutral. Someday, I would like to compare the SE color rendering with the Leica color rendering, which is advertised as the most neutral glass around.
As for my selection of telescope eyepieces, I have decided to build a complete set of the Pentax XWs, which some observers report to be more neutral than the Televue assortment. With telescope observing, I am learning the eyepiece quality is an important component to the observing experience.
. . .
How about ED glass for night sky observing?
Much has been written in this thread about daytime observing with ED binoculars. I confess I have contributed to this more than most by posting links of similar discussions from the Bird Forum.
However, I find ED binoculars have their niche in night sky observing with 8x or 10x ED binoculars: Sweeping the Milky Way
Moon
Planets
Constellations (particularly those with bright color stars)
Double stars
Large open clusters
Meteor showers Sure, my 12x50, 16x70, or 22x85 will see more stars and DSOs than either my 8.5x44 or 10x50 EDs.
But when I desire to relax and enjoy the color of the night sky, I cover my Dob and large binoculars, head for the chaise lounge, and let the ED binos fill me up with color. I particularly like the way the orange and red stars pop in the Pentax DCF ED binocular. I also enjoy the color of the night sky with a 70mm ED refractor and a Pentax XW 40mm wide-field eyepiece (11x, 6.6 exit pupil, and 6.5° FOV).
But I have found that when I pass the ED optics to family and friends, they do not rave about the color as I do.
I wonder if they see what I see?
I am finding this to be a difficult question to discuss. I have tried to leave the optical engineering to others who know much more than I ever want to learn. Particularly our forum moderator EdZ, who is fantastic. And Brock, who never ceases to amaze me with bi-lingual postings.
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Bob
38°N
Edited by BobinKy (07/03/09 08:25 AM)
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Andresin150
sage
Reged: 08/14/07
Posts: 298
Loc: Bogotá - La Calera / Colombia
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what an interesting thread this is....... learning a lot here.. 
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25x150 MT
30-50x120's
Ultra 15x70's
GOTO NEX Planetarium
1 Macaw, 1 Toucan, 1 parrot and many other little ones...
And the nicest greenhouse!
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FrankKD
member
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Very interesting. Please keep the discussion going gentleman.
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EdZ
Professor EdZ
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I was asked in another thread (Comparing 22x85, 22x70 and Tak22x60) if I intended to mask the objects of the larger binoculars to compare the slower resulting f# to the smaller and slower Tak. Let's look at that for a moment.
See this note from my original post in this thread
Quote:
Also, the amount of color error seen is dependant on focal ratio, an f/4 having greater potential color error than an f/5. Finally, aperture plays a significant role in color correction. An f/4 80mm will show one half the color error as an identically designed f/4 160mm lens.
If I were to mask down the objective of my f/4.7 22x85 to 60mm I would get an f/6.7 60mm, nearly identical to the Tak f/7 22x60. And I'm certain the color correction of the GO22x85 would improve. But I would not be able to isolate how much of the improvement is related to smaller aperture and how much is related to slower f ratio.
We know that given equal design quality and equal f ratio, a smaller objective produces less color. So the color of the 22x85 when masked to 60mm, as a result of smaller aperture should be reduced to 60/85, or to 70% of original.
We also know that slower f ratio in an equal sized equal quality scope would produce less color. The f/6.7 60mm masked objective would produce less color than the full open f/4.7 85mm objective.
The difficulty would be in attributing how much color correction is a result of which changed aspect.
These issues raised here are not specifically relative to ED binoculars. However, you can see from this example, it is quite difficult to assess how much color correction in a binocular is attributable to ED, how much to aperture and how much to f ratio. And although not needed in my example since all are 22x, there would be a difference attributable to power if it were different. If any of those are different in comparison binoculars the assessment becomes rather difficult. And as I stated earlier in this thread, few reviewers are taking the steps to isolate each cause when comparing binoculars.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
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milt
professor emeritus
Reged: 09/13/04
Posts: 550
Loc: Arizona
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Quote:
The difficulty would be in attributing how much color correction is a result of which changed aspect.
Ed, actually it's not difficult at all. The angular color blur diameter depends only on focal ratio, not on aperture. As Roland says in the link I cited earlier, it's the decrease in Airy disc diameter as aperture increases that makes the RATIO of color blur to Airy disk get worse. But the actual color blur doesn't change.
Milt
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EdZ
Professor EdZ
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Quote:
The angular color blur diameter depends only on focal ratio, not on aperture. As Roland says in the link I cited earlier, it's the decrease in Airy disc diameter as aperture increases that makes the RATIO of color blur to Airy disk get worse. But the actual color blur doesn't change.
"it's the decrease in Airy disc diameter as aperture increases that makes the RATIO of color blur to Airy disk get worse".
While total color blur may be controlled by f ratio, the visual display of that color error is dependant on the size of the Airy disk. As aperture changes that also changes. Therefore, visual color error varies as aperture varies.
If you had an 80mm f/4 aperture with a color blur twice the size of the Airy disk, the color would be prominent. Keep f the same, but now reduce the aperture to 40mm, so you now have an 40mm f/4; yes color blur is still the same size, but now the Airy disk is twice as large, the same size as the color blur. There would therefore be no false color outside the Airy disk.
The visual change in false color in this instance is not dependant on f ratio, since that remains the same. It is dependant on the change in size of aperture.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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gmazza
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Loc: RS, Brazil, 29S 51W
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Hi EdZ,
Thank you for a comprehensive response, I'm sorry if I started some statements which drove topics in the wrong direction (actually we have topics in glass features, masks in binoculars, desired magnifications, all at same time) some times people mix the things, I will try to keep my responses and questions more strict on topics.
In that question I also wondered about the glass features of the 3 binoculars compared (The 22X60 has state of art quadriplet lens with fluorite, the WO has FPL-51 ED, the Garret I do not find glass specifications, but the views are praised by everyone here). Last week happened in refractors forum the same, about ED glasses and about the "best 80mm" with crossing ideas. In that forum some people called FPL53 APO's (remember FPL53 Abbe number is about the same as fluorite) as overkill for visual observing, and also that a slower FPL 51 well designed could match a FPL 53. There is even difficult to define what is "ED" in terms of abe number, and if different glasses could be called "ED" it's hard to measure the real benefit.
I see binocular astronomy as just about immersion, we use most heavy binoculars (exclude military) of the bino world. We mount them to scan the sky and enjoy color and correct bright. We could even change focal lenght by masks for specific targets. Equatorial mounted binos for astrophotos are not the bulk reality at this time. Of course is human nature try to find some usefulness to new features as we always want better views. The glass specifications of binoculars are a very important topic to discuss here, but, we can conclude that for this particular niche some glasses are overkill, here the importance of testing.
Also seen some topics of 2005, about TAKs 22X60, and awaited 22X65 from other brands which never come into market, these binos anchieved a superb color performance in small optical designs, but were discontinued and never launched one reason must explain this.
In the "porro binocular" world of nature watching, the shorter focal lenghts probably benefit from all color correction possible, but, even them, there is diversity of opinions.
My final thoughts on this would reiterate the importance of well, arguments reinforced, reviews of each model before any buy.
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Oberwerk Ultra 10X50
Garrett Signature 15X85
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milt
professor emeritus
Reged: 09/13/04
Posts: 550
Loc: Arizona
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Quote:
If you had an 80mm f/4 aperture with a color blur twice the size of the Airy disk, the color would be prominent.
Ed, actually not. This would be a good example of an ED doublet, and I doubt that anyone would notice color at binocular magnifications. However if this same objective was a standard C-F corrected achromat, the color blur would be a whopping 7.5 times the diameter of the Airy disc and color would become evident on bright stars.
Binoculars are saved by low magnifications and the rapid falloff in eye sensitivity away from green-yellow. In Roger Ceragioli's words:
"so long as a broad range of the brightest wavelengths comes to a true focus or is concentrated into a spot whose size the eye cannot resolve (because of low magnification), the brain will sense this as white".
In other words, the color blur has to get really big, many times larger than the Airy disc, before it is noticed in binoculars. So I stand on my statement that the size of the color blur (set by focal ratio) is the controlling factor, not the size of the Airy disc (set by aperture).
As you have pointed out in this thread, ED glass is not a panacea for binoculars.
Milt
Edited by milt (07/08/09 07:06 PM)
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EdZ
Professor EdZ
Reged: 02/15/02
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Loc: Cumberland, R I , USA42N71.4W
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Quote:
Quote:
If you had an 80mm f/4 aperture with a color blur twice the size of the Airy disk, the color would be prominent.
Ed, actually not. This would be a good example of an ED doublet, and I doubt that anyone would notice color at binocular magnifications. However if this same objective was a standard C-F corrected achromat, the color blur would be a whopping 7.5 times the diameter of the Airy disc and color would become evident on bright stars.
Milt
Milt, sorry, I didn't mean that as any specific lens example, only as an example to show the doubling, or halving, of the lens produces such and such an effect... as I think I pointed out in that last post.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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milt
professor emeritus
Reged: 09/13/04
Posts: 550
Loc: Arizona
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Ed, just edited my previous post to make more clear what I was saying. This stuff isn't easy... 
Milt
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 14611
Loc: Cumberland, R I , USA42N71.4W
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Milt makes a very good point above.
Color Blur or CB value is a ratio of Airy disk size. So a CB=1 is same size as Airy disk. CB=2 is twice the size of the Airy disk.
The forumla for color blur states that for a color free instrument you need to have CB=1, or explained another way, no residual color can fall outside the diameter of the Airy disk. Essentially all the light is contained within the Airy disk.
What's not mentioned in any of this is low power viewing. That's what is brought up by Milt in his post above.
When viewing at low powers, the human eye cannot perceive sizes as small as the Airy disk. Human eye resolution at 30x is not controlled by scope diameter, it is controlled by acuity and hence magnification. The spot size that can be resolved at 30X might be 4 to 5 arcseconds, not the scope Airy disk size of 1 arcsec. So the latitude of how wide the color blur can get before the eye sees false color, when magnification is not high enough to truly resolve the point, is dependant on how large a point the eye makes the object appear to be.
For example, for most people, regardless of scope size and hence it's potential resoltion, (which could be a 5 inch scope with res = 1 arcsec), the eye cannot resolve a 4 arcsec pair at 30x. It will be seen as one. As Milt points out, as long as the color blur is as small as the spot that can be resolved by the eye, (at 30x, perhaps 5 arcseconds), then false color is minimized. Oh, you may see some residual color around the edges, but not nearly as bad as you might think.
It doesn't take too much effort to think thru, as magnification increases, even though it may still be too low to reach the resolution limit of the scope, the spot size resolved by the eye gets smaller. So, for instance at 50x, the eye might be able to resolve 3" and at 75x, the eye might be able to resolve 2". The extent of color blur would need to be progressively smaller in each case to match the spot size, if the observer were to desire a color free image.
But as you can also see, that presents a very wide latitude in CB factor when using powers below that needed for the resolution limit. Low power binoculars are well down the chart. Even high powered BT style instruments are generally used in the wide latitude range, but may approach the resolution limit.
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
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