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ED glass in binoculars

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#1 EdZ

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Posted 01 July 2009 - 01:51 PM

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

#2 Wes James

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Posted 01 July 2009 - 02:32 PM

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.

#3 KennyJ

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Posted 01 July 2009 - 03:11 PM

Thanks EdZ ,

That was the most interesting and informative piece I've read on this forum for an unusually considerable time .

Kenny

#4 EdZ

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Posted 01 July 2009 - 03:46 PM

Thank you Kenny.

yes Wes, the sum of the parts sums it up fairly well.

edz

#5 Man in a Tub

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Posted 01 July 2009 - 04:32 PM

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

#6 GlenM

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Posted 01 July 2009 - 04:50 PM

EdZ,

Another excellent post in plain English we can all understand.

#7 gmazza

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Posted 01 July 2009 - 05:21 PM

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.

#8 daniel_h

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Posted 01 July 2009 - 05:58 PM

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

#9 Mr. Bill

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Posted 01 July 2009 - 06:40 PM

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.

#10 brocknroller

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Posted 01 July 2009 - 07:13 PM

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.

#11 BobinKy

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Posted 01 July 2009 - 07:31 PM

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.


#12 milt

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Posted 01 July 2009 - 08:03 PM

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.../roland/ed.html

#13 ronharper

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Posted 01 July 2009 - 11:59 PM

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

#14 Mr. Bill

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Posted 02 July 2009 - 12:52 AM

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.

:question:

#15 EdZ

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Posted 02 July 2009 - 04:45 AM

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

#16 BobinKy

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Posted 02 July 2009 - 06:38 AM

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.

#17 mooreorless

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Posted 02 July 2009 - 06:48 AM

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.

#18 gmazza

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Posted 02 July 2009 - 07:18 AM

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.

#19 EdZ

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Posted 02 July 2009 - 07:22 AM

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

#20 BobinKy

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Posted 02 July 2009 - 07:25 AM

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?



#21 EdZ

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Posted 02 July 2009 - 07:38 AM

take notice in that link to the differences in opinions of performance.

edz

#22 BobinKy

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Posted 02 July 2009 - 08:11 AM

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.

#23 EdZ

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Posted 02 July 2009 - 08:26 AM

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:

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

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

#24 BobinKy

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Posted 02 July 2009 - 09:10 AM

Post deleted by BobinKy. If interested, please see my next post below.

#25 gmazza

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Posted 02 July 2009 - 09:21 AM

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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