EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Quote:
After some more digging, I was finally able to confirm that the Zhumell 25x100mm Tachyon binocular is in fact made in China. What's more, this bino uses BK-7 prisms. One dealer claimed the BK-7 prisms will perform equally well as the BK-4! I found that claim to be dubious, based on what I have learned here. .....
Finally, I saw a strange pair on binos on eBay. They strongly resembled the Oberwerk 22x100 with the white finish. Only these were 25x100 and "multi-coated" with MgFl only. The seller claimed that the more coatings of MgFl the more light transmits through - in other words, the seller was trying to insinuate that these single-coated binoculars were multi-coated because they had several identical coatings of MgFl. .....
MikeG
Anyone wishing to understand the applications of BaK7 and BK4 prisms should read everything that Roland Christen has published on the subject.
PRISMS
BK7 glass is probably one of the best glass in the world for prisms, however is not particularly well suited for the light cone from a fast lens. For this BaK4 performs equally well or better.
COATINGS
If a lens has several layers of MgF, then it is multicoated. But if so, it may likely not have the characteristic blue color of an MgF coated lens. A properly multicoated lens surface may reflect only 0.2% to 0.5% of the entering light. A properly single-coated lens surface may reflect only 1.5% of the light. An uncoated lens surface may reflect 4% of the light. Considering a binocular might have 12 to 16 surfaces, the importance of multicoating soon becomes evident.
Probably just as important is the precise control of the thickness of the coating process. An improperly controlled multi-coating process can produce a lens coating that reflects more light than a single coating. Although you can't truely determine anything about the precision of the coating application by the color of the coating, in general, poorly coated lens tend to look bright green. The bright green color could result from coatings being applied too thick. Improper thickness will reflect more light than it should. However, a properly multi-coated lens can give off a faint green color or a faint purple/pink color.
You'd be better off with a properly applied single coating than an improperly applied multi-coating.
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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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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deleted
edz
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Teach a kid something today. The feeling you'll get is one of life's greatest rewards.
member#21
Edited by EdZ (09/18/05 08:57 AM)
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HfxObserver
professor emeritus
Reged: 11/12/04
Posts: 624
Loc: Waterloo ON, Canada
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Bk4 prisms are made out of barium crown glass. Bk7 prisms are constructed from borosilicate glass. The Bk4 prisms tend to yield more circular Exit Pupils and less image brightness fall off towards the edges, so long as they are properly sized to the binoculars aperture. The Bk7 prisms tend to be in less expensive binoculars and the glass often produces gray areas cutting off the edges.
There are two things which make Bk7 perform poorly in a binocular. It's not that Bk7 glass is inherintly bad, it's actually pretty good for several optical applications such as an objective lens since it allows the light to pass through it, but in a binocular which uses this as a prism the light falls through the glass and doesn't bounce out the other end so we end up loosing something like 8-12% of the light and in nearly identical binoculars with the prisms being the only difference I find that even a novice observer will note a brighness difference. Secondly they often need to make the Bk7 prisms smaller as it is larger functioning prism and that tends to cut off the edges.
-Chris
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Chris
7X50 Vixen,22X100 Antares
80mm William Optics Megrez II ED
Santel MK6
Borg 125SD f6 (Pentax/Oasis version)
Tak-Lapides
Pentax XW's 40,20,14,10,3.5 3.8XP, Speers 5-8, 30mm Widescan III
Edited by EdZ (09/18/05 08:24 AM)
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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These from my notes on Roland Christen's discussion on BaK4 and BK7 glass used for prisms:
BaK4 (Barium crown) has no advantage over BK7 (Borosilicate) when used in in a situation where the incoming light beam angle is very narrow. However in wide field applications such as low power binoculars there are advantages - increasing the brightness of the edge of field while having no effect on the on-axis image. There would be no advantage to BaK4 over BK7 to any part of the image for instance in a Binoviewer because of the narrow beam angle where generally they are slow systems without wide fields.
The disadvantages of BaK4 is that it introduces more spherical aberration into the optical path than BK7. BaK4 also introduces more chromatic aberration than BK7. You may not notice this if your telescope optics are poor to begin with.
BK7 is the clearest, most defect-free optical glass available for prisms. BaK4 is close, but not quite as good. That's ok for low power applications of a typical binocular, but not for high power low contrast situations seen in telescopes. For a high powered high f# telescope, with a very narrow light beam, BK7 would be a better choice for a prism. Similar applications are found in binoviewers.
So why is BaK4 glass normally found in binoculars. It's cheaper.
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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KennyJ
Reged: 04/27/03
Posts: 10163
Loc: Lancashire UK
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Ed ,
Your latest extract from Roland Christen's discussion may well come as a surprise to some readers , but not to me .
And Bill Cook tried to tell us basically the same thing a while back .
It is perhaps a little unfortunate that " Bak4 " has become something of a ( often mis-representative ) buzz word for QUALITY , in the same way as " multi -coated " and that even more vague expression " aspheric " have .
Kenny
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If everyone is thinking the same thing , no-one is thinking - General George S.Patton
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Glassthrower
Vendor - Galactic Stone & Ironworks
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EdZ, Chris, and Kenny -
Thanks for the heads-up regarding the Bk-7 vs. Bk-4 prism issue. Like many others, I fell prey to the misconception that Bk-4 is "better" because so many manufacturers tout it as so.
MikeG
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Michael Gilmer - Member of the Meteoritical Society & Collector of Falling Stars.
Galactic Stone & Ironworks - Buy/Sell/Trade Meteorites, Moon Rocks, Mars Rocks, & 35 different falls and types!
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Well,
agian I would caution readers not to read something into this that is not there.
BaK4 is generally used in binoculars because it can handle the light cone of an f/4 instrument. BK7 cannot.
Of course as noted by several here, that doesn't make BaK4 better. It's not. But it does serve this particular application better.
BaK4 adds to two aberrations that BK7 would not. But BK7 prisms, if made to the same size, would drastically cut off light at the edges. Same size BK7 prisms would be a better glass to use to eliminate adding anything to abberrations, but at the expense of light loss at the edges. Roland explains that what BaK4 adds to the aberrations, for several reasons might not be so noticable, especially in low power binoculars. And in this case low power would include all commercial fixed power binoculars. Even a 22x60 is less than 10x per inch.
So I suppose BK7 prisms could be made larger to eliminate the light cutoff at the edges. Then you could say having BK7 prisms in your binocular would be better. But that would cost more. And it would result in an oversized prism housing. How much larger? I don't know. And don't forget that would add not only the weight of the oversized prism glass itself, but also the extra metal of the larger prism shelf and the complete prism housing.
If you have in your hands a binocular with BK7 prisms and you do not see any light cutoff at the edges of the exit pupil, then you probably hold an instrument that includes a good application of BK7 prism glass. If you can see the light cutoff in the exit pupil, you would maybe have been better off with BaK4.
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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refractory
Pooh-Bah
Reged: 02/05/05
Posts: 1016
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I have a question about mirror-folding of light paths- given enhanced and dielectric coatings, why haven't more high-end binoculars utilized these instead of glass- then you wouldn't run into the issues of chromatic and spherical aberration, light cut-off at edges etc. And big binos could be made even shorter than they are now, if one of the mirrors is placed close to the objective. Essentially just a paired folded refractor.
Jess Tauber
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HfxObserver
professor emeritus
Reged: 11/12/04
Posts: 624
Loc: Waterloo ON, Canada
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Edz, Thanks for elaborating and expanding on what I said.
One thing which I would be interested to know is, Why the light comming out of a prism made of Bk7 is less brilliant compared to Bk4? We are talking about the beam inside a properly sized prism, not a prism which is cut off. From what Roland says, and all else being equal, if the Bk7 prism is larger then the Bk4 then the Bk7 could outperform the Bk4. Can anyone tell me if that is the case?
-Chris
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Chris
7X50 Vixen,22X100 Antares
80mm William Optics Megrez II ED
Santel MK6
Borg 125SD f6 (Pentax/Oasis version)
Tak-Lapides
Pentax XW's 40,20,14,10,3.5 3.8XP, Speers 5-8, 30mm Widescan III
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Quote:
I have a question about mirror-folding of light paths- given enhanced and dielectric coatings, why haven't more high-end binoculars utilized these instead of glass- then you wouldn't run into the issues of chromatic and spherical aberration, light cut-off at edges etc. And big binos could be made even shorter than they are now, if one of the mirrors is placed close to the objective. Essentially just a paired folded refractor.
Jess Tauber
Wouldn't you have to deal with
inverted images?
extremely expensive dielectric coatings? Assuming for instance it could be done with two mirrors in each barrel, wouldn't that cost about $100-150 per mirror or $400 to $600 just for the mirrors?
If you used standard mirrors, the light loss due to reflectivity would be an insurrrmountable hurdle. Standard mirrors have about 85% to 88% refectivity. You would loos 25-30% of the light in each barrel. Properly multi-coated prisms pass nearly all the light that enters them.
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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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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Quote:
One thing which I would be interested to know is, Why the light comming out of a prism made of Bk7 is less brilliant compared to Bk4? We are talking about the beam inside a properly sized prism, not a prism which is cut off. From what Roland says, and all else being equal, if the Bk7 prism is larger then the Bk4 then the Bk7 could outperform the Bk4. Can anyone tell me if that is the case? -Chris
Do you have an example of light coming out of BK7 being less brilliant?
The BK7 if oversized to permit all edge light to pass or when applied in a LONG focal length application would not add any aberrations and would therefore outperform BaK4. However, it's use in standard fixed power binoculars just may not be practical, from a manufacturing standpoint.
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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Joad
Wordsmith
Reged: 03/22/05
Posts: 11923
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"Probably just as important is the precise control of the thickness of the coating process. An improperly controlled multi-coating process can produce a lens coating that reflects more light than a single coating. Although you can't truely determine anything about the precision of the coating application by the color of the coating, in general, poorly coated lens tend to look bright green. The bright green color could result from coatings being applied too thick. Improper thickness will reflect more light than it should. However, a properly multi-coated lens can give off a faint green color or a faint purple/pink color.
You'd be better off with a properly applied single coating than an improperly applied multi-coating."
This is very interesting information. I had read somewhere that improperly applied multi-coating could decrease performance, but I didn't know that it *might* be detected by a bright green look to the lens surfaces. I've been envying those bright green lenses in the newer Oberwerks; now, perhaps, I'll be more satisfied with my rather ordinary bluish-purple Obies.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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maybe just a little more about this statement also found in the above quote will help others to understand how we see color reflected from the coatings
Quote:
Although you can't truely determine anything about the precision of the coating application by the color of the coating
The most common refractive substances are aluminum oxide, zirconium oxide and magnesium fluoride. They are applied using electron beam coating machines. Thicknesses of the layer must be precisely controlled to get optimum results, or rather than the intended result, you will get higher reflection.
Some properly applied refractive mixtures provide near zero reflectivity on either side of the central wavelength (green) with a slight rise in reflectivity at the middle wavelength. It is important to consider that the thickness of the coating is 1/4 wavelength of the light you are trying to maximize. The wavelength (color) of light reflected will be different than the wavelength you are trying to optimize. That is why often we see the best coatings may appear with a purple/blue tint. Generally we are trying to optimize light in the green wavelength.
Some properly applied refractive mixtures produce a broader near zero or minumum reflectivity all the way from blue to yellow with a slight rise at the red and violet ends of the spectrum. For these what you end up seeing is the typical purple colored coatings or slight pink colored. It is probably worthy of note here that these coatings reflect very little if any green/yellow light, which is the wavelength of light to which we have the greatest sensitivity. This is the typical color reflected fron Nikon SE, Fujinon FMT-SX, Oberwerk BT100 and some Pentax.
In any case, the reflections from coatings that show you the color of properly applied multi-coatings would be on the order of 10 times less than reflections from an uncoated surface and 3-4 times less than a single coated surface.
Now back to poorly applied coatings. They are most commonly seen as bright green. Layers applied too thick or too many layers resulting in an overall too thick layer will produce a bright green reflection. This layer will reflect more light than a properly applied single MgF coating.
This is one of the cautions we have pointed out for several years here now in this forum. And it falls under the heading "just because they look the same doesn't mean they are the same". You can pick out 3 or 4 different binoculars, same size, that look the same, and I'm sure you could find a dozen people who will say they are all the same, why should I pay $150 or $250 for that binocular when I can buy the same thing for $79. Well, ignorance is bliss. An uninformed consumer is the best friend of the mass producer of cheap products. This is just one of the differences to be found.
Our recommended test for coatings is to look for a reflection of your face in the coating. The more you can see of your face, the more light is being reflected off the coating. That's a bad thing. The best coatings I've seen, Nikon SE, Fujinon FMT-SX and Oberwerk BT100 produce so little reflection that is is very difficult to even see your face reflected in the coating.
edz
Edited by EdZ (07/06/06 03:22 PM)
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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Interesting. I test bino coatings on Luna, if there are reflections then they are not so great, if not they are fine. Now just because there are some reflections doesn't mean the bino-s are un-use-able either. Many of the cheapie 10x50 under $20 binos do a fine job of sweeping star fields and general DSO observations, yet give reflections on Luna.
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EdZ
Professor EdZ
Reged: 02/15/02
Posts: 12601
Loc: Cumberland, R I , USA42N71.4W
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When you say they give reflections on Luna, exactly what do you mean? Are you walking around in front of the binocular to look at the objective lens and see if the moon is reflected off of the objective lens?
Seems to me it would be a lot easier to do that in daylight with the bright sun on your face and using the objective lens as a mirror. That's what I mean. If you can see your face all light up and reflected in the objective lens to the point you can see details of the features in your face, the coatings are not up to par.
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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HfxObserver
professor emeritus
Reged: 11/12/04
Posts: 624
Loc: Waterloo ON, Canada
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Looking through a pair of recent expensize Celestron binos with full sized BK7's, I think the outland 10X50's, the exit pupil appeared to be fully illuminated but the images were noticably dimmer compared to any other, Pentax Nikon etc. 10X50 with Bk4's.
We are talking prisms here, if the Bk7's tend to pass more light through, does this effect their performance as a prism? It may be fine for an optical lens which Roland is talking about but the different characteristics of the prism could lead to other performance issues couldn't they?
-Chris
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Chris
7X50 Vixen,22X100 Antares
80mm William Optics Megrez II ED
Santel MK6
Borg 125SD f6 (Pentax/Oasis version)
Tak-Lapides
Pentax XW's 40,20,14,10,3.5 3.8XP, Speers 5-8, 30mm Widescan III
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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Ed, I'm pretty sure he's talking about internal reflections. Having much less internal reflections on the Moon is the first thing I noticed when going from a dirt-cheap no-name 10x50 to a better-performing model.
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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Mark9473
Carpal Tunnel
Reged: 07/21/05
Posts: 2703
Loc: 51°N 4°E
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hey it has to be true, that post made me a "sage" :-)
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Mark
Leica 8x20; Vixen 8x42; Swift 8.5x44, 10x50 and 20x80; TS 7x50; Orion 15x63
WO Megrez II 80 FD + Baader 90° T2 Amici
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refractory
Pooh-Bah
Reged: 02/05/05
Posts: 1016
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About mirrors- everything EdZ say is true. Inversion I can live with. Cost of dielectrics is problematical (maybe someday people will have low cost versions?). Weight reduction might be significant if the prisms are otherwise very large, though. Wonder if folks with deep pockets might be willing to spring for the trade-off?
Jess Tauber
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Joad
Wordsmith
Reged: 03/22/05
Posts: 11923
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Actually, I put asterisks on each side of my subjunctively intended "might" to indicate that I did not intend it in indicative certainty. I am a very careful reader and user of language and was trying to write a pleasant post. I knew perfectly well that simply looking at the color of coatings doesn't indicate anything with certainty. But in my experience as a professor, I find that pleasantness and a relaxed attitude about things is appreciated by my students.
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