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Optical arrangement of Bushnell Legend 8x42 roof-prism binocular? (And a rant.)

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Old Sunday 9th December 2007, 18:46   #1
Dorian Gray
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Optical arrangement of Bushnell Legend 8x42 roof-prism binocular? (And a rant.)

'Evening ladies and gents,

I recently ordered the Bushnell Legend 8x42 roofs as a Christmas present for my father, whom I suppose would be best described as a casual birder. He has long wanted a decent binocular, and as he particularly enjoys walking in the rain, a waterproof roof-prism design seemed the best way to go. I understand there is severe competition in this price-range, so you might wonder why I chose this particular binocular. Well, the UK-US price disparity was even greater for this model than others from Minox, Nikon, etc., so it became the obvious choice once I'd made the decision to buy from a US dealer.

Anyway, I've become curious as to what's inside the thing. Bushnell seem to consider this a jealously-guarded secret, for they reveal nothing whatsoever about the matter in their brochures or websites. This is frustrating, but I've quickly learned that the entire binocular industry is bizarrely unwilling to impart information to prospective customers.

With the exception of Nikon who provide public access to the press-photo section of their website (though why aren't these photos made obviously available to the consumer?), it's nearly impossible to even find a decent product photo of any of the major brands (Leica, Zeiss, Swarovski, etc.). Leica make good PDF brochures available for download, though the product photos are woefully small and therefore inadequate for getting an idea of the "look and feel" and "fit and finish" of the binoculars. Swarovski's website looks like something straight from the nineties, is littered with mistakes, and generally smacks of poor quality: a peculiar state of affairs for a brand that places so much emphasis on quality. Zeiss's website is adequate, though the product photos are woeful as usual, and technical info on binoculars is drastically limited in comparison to Zeiss' other products (such as camera lenses).

Nobody seems to think it worthwhile to let consumers know the optical arrangement of any binocular. Optical performance parameters are described in subjective terms such as "unparalleled brightness", "outstanding sharpness" and other meaningless bumf. Not a single binocular manufacturer in Europe, Asia or America provides MTF charts, which are common-place for camera lenses. This leads to our present reliance on internet gurus (known to most of you, no doubt) for basic information on optical performance. Most brands don't even tell us how many lens elements and groups are in their binoculars, much less show us a diagram of the arrangement.

So: short of taking my dad's gift to bits, how can I find out the number of elements and groups, and the focal length of the objectives (and therefore the f-number). And why are binocular manufacturers so unwilling to reveal these basic data, considered essential by every camera lens manufacturer (including Zeiss and Leica)?
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Old Sunday 9th December 2007, 19:53   #2
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Great rant - Oscar Wilde himself would be proud of it...and welcome to BF.

I suspect that one of the reasons that the lack of technical literature does not impede sales is that the vast majority of binocular purchases are made following a field test of potential rivals.
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Old Sunday 9th December 2007, 21:55   #3
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Dorian

I suspect if you take the binoculars to bits, your warranty will be instantly invalidated. The optical design of things like expensive or even less expensive binoculars and telescopes are usually kept private by the manufacturers. Mainly because they don't want their rivals to know, and also because to explain the design to consumers opens up the possibility of further questions of a technical nature, which 99% of Bushnell's staff will not be able to answer. Also, the more difficult something is to understand, the harder it is to sell. They are usually happy to, as Jane says, leave testing to the media, (often a bad idea, but that is another matter).
A manufacturer will simply say great things about their own product, and then leave it at that. The vast majority of consumers don't want to know about the technicalities of the optics, particularly in lower priced and mid-priced instruments, so the manufacturers say nothing.
Even with the highest priced equipment, they are extremely nervous about saying anything that may lead to misunderstanding, and hence cost them sales.

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Old Monday 10th December 2007, 00:29   #4
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Quote:
Originally Posted by Dorian Gray View Post
'Evening ladies and gents,



So: short of taking my dad's gift to bits, how can I find out the number of elements and groups, and the focal length of the objectives (and therefore the f-number). And why are binocular manufacturers so unwilling to reveal these basic data, considered essential by every camera lens manufacturer (including Zeiss and Leica)?
How would that be important?
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Old Monday 10th December 2007, 04:52   #5
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In general most binoculars are about f4. The field of view is usually determined by the design of the oculars. Here is a binocular manufacturer that gives some information.http://www.eagleoptics.com/index.asp?pid=4899
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Old Monday 10th December 2007, 16:11   #6
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Old Tuesday 11th December 2007, 01:18   #7
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Thanks for the nice welcome!

I realise some people might not be interested in the technical specifications of the binoculars they buy, just as most people buying camera lenses are blissfully unaware of what goes on inside them, and are no less talented for that. But camera lens makers nevertheless publish their product specs, and I find it hard to make meaningful distinctions between camera lens manufacturers and binocular manufacturers in terms of why they release the info they respectively do. Jane's explanation is plausible, I suppose.

Andytyle: having bought the binoculars from a US dealer, it's debatable whether I actually possess a warranty to instantly invalidate! But I'll not take them apart anyway.

Lilybeans: here's an example to answer your question. The binoculars of the Zeiss Victory FL range have acquired a reputation of having less than optimal flare-resistance. At the same time, they are generally thought to have excellent sharpness and minimal chromatic aberration. A high degree of aberration correction is more easily accomplished with many lens elements in many optical groups, which incidentally has the general side-effect of increasing flare (due to unwanted reflections at glass-air boundaries). If one knew the number of elements and groups in a particular binocular (and the price, which correlates loosely with the effort that went into the optical design), one would be able to estimate with useful if not absolute certainty the general performance characteristics of the instrument.

It would be interesting to know whether the Victory FL binoculars do in fact have more optical groups than usual. Alas, I'll probably never know.

Here are some photos of the Bushnell Legend 8x42 that arrived today, in good time for going under the Christmas tree. I didn't have time to balance the light, but the pics should give you an idea of what they're like. Namely, really quite nice for the price, snobbery aside:

http://img509.imageshack.us/img509/8...8x42topwn7.jpg

http://img509.imageshack.us/img509/1...jectiveus1.jpg

http://img509.imageshack.us/img509/7...2ocularsr6.jpg

Initial inspection suggests near-perfect barrel alignment, excellent brightness (the fact that the objective lenses are nearly invisible in the photo above is indicative of good coatings, essential for a bright image), very good sharpness in the centre of the field though dropping fairly quickly off-axis, and a not-terribly-wide-but-adequate field of view. There is plainly visible chromatic aberration off-axis if one looks for it, though I wouldn't say it's awfully objectionable. Mechanically the binocular feels very robust indeed, but less refined than the really expensive binoculars. The focus knob, for example, has a heavily-greased feel and a fair bit of backlash.

I'm sure my dad will love it.
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Old Tuesday 11th December 2007, 07:53   #8
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I've been "back processing" (thinking about it without consciously doing it) this. Binoculars are like cars. You may do some initial screening based on literature (but its pretty basic stuff), fuel consumption, time 0-60 etc, you almost certainly incorporate a whole lot of "prejudice" based on previous experiences, but in the end you'd be daft to part with your money without a test drive. You tend to get relatively superficial technical specs in car literature too.
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Old Tuesday 11th December 2007, 08:58   #9
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Hello Dorian. On behalf of all the staff and moderators, welcome to BirdForum.Enjoy yourself here. Please feel free to join in whenever you feel like it.
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Old Tuesday 11th December 2007, 14:57   #10
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Quote: 'Thanks for the nice welcome!

I realise some people might not be interested in the technical specifications of the binoculars they buy, just as most people buying camera lenses are blissfully unaware of what goes on inside them, and are no less talented for that. But camera lens makers nevertheless publish their product specs, and I find it hard to make meaningful distinctions between camera lens manufacturers and binocular manufacturers in terms of why they release the info they respectively do. Jane's explanation is plausible, I suppose.

Andytyle: having bought the binoculars from a US dealer, it's debatable whether I actually possess a warranty to instantly invalidate! But I'll not take them apart anyway.'



Not sure I agree with the need to publish the optical configuration of binoculars unless there is an unusual system that requires explanation. Yes there are a few of us who would like to know what the system is if for no other reason than interest, the majority though would just find it essentially meaningless and thus confusing. There are some binocular/telescope manufacturers that do supply exploded illustrated diagrams of their products, just to show the light path and to illustrate some points that enhance the desire to own one. In terms of technical details for the sake of technical details, then I think this would be just time and expense from the manufacturer but without a financial reward.
With camera lenses it is different. The photographer often needs to know the focal length and aperture setting range of a lens, in order to compose a photograph. There are obvious benefits to this knowledge in terms of being aware of depth of field, exposure times etc. so it is important that this type of info is made available for consumers.
With regards to camera lens manufacturers giving info on lens groups etc. well that is not really giving anything important away to their rival manufacturers. Also, how does knowing whether an instrument has groups and configurations of lenses or not contribute to an understanding of image quality? You need to know how these lenses work in a theoretical mathematical sense, i.e. the prescription. The important bit is the lens prescription. The radius of curvature of each lens surface, glass type, refractive index, what the Seidel aberration correction equations are. These are known by the lens designers and guarded by the manufacturers.
With binoculars, the situation is a little different. Compared to designing a wide-angle camera lens, or a zoom telephoto for example, binoculars systems are relatively simple. Either a basic achromatic doublet objective + porro prisms + eyepiece, as in the basic porro prism design, or an achromatic triplet/semi APO objective + or - internal focusing lens + roof prism + eyepiece, or a variation on either. There is much more design work needed in a camera lens because of complicated varying field stops, varying aberrations with change of focal length, complicated scatter reduction and internal reflection properties.
Making binoculars more complicated than they need to be will often result in a lower optical performance than the basic designs. The reason some may appear somewhat sophisticated is usually just marketing. The best binoculars (for optical performance) I have ever seen have all been porro prism design.
So, a manufacturer can make a big point of explaining the optical train of their binoculars in their brochures. But unless it is understood by the consumer, then it is just as easy for a rival manufacturer to cast doubt on a rival's product by using their own baffling technical jargon as evidence. It works that way!
If it doesn't increase sales, or if it reduces sales, then don't use it. Use what works, usually b***s**t.

'Lilybeans: here's an example to answer your question. The binoculars of the Zeiss Victory FL range have acquired a reputation of having less than optimal flare-resistance. At the same time, they are generally thought to have excellent sharpness and minimal chromatic aberration. A high degree of aberration correction is more easily accomplished with many lens elements in many optical groups, which incidentally has the general side-effect of increasing flare (due to unwanted reflections at glass-air boundaries). If one knew the number of elements and groups in a particular binocular (and the price, which correlates loosely with the effort that went into the optical design), one would be able to estimate with useful if not absolute certainty the general performance characteristics of the instrument.'

Sometimes objectionable aberrations can be controlled by internal correcting lenses, sometimes not without creating a new set of problems. There is no general rule here. Again, with camera lenses there is a difference because the needed outcome is different. With simple refracting instruments such as spotting scopes and binoculars, simple is often the best.
Internal 'flare' is something that can be caused by many different reasons - lens/gas surfaces, lens edges, spurious reflections in poorly designed prisms, eyepiece design, reflective metallic or plastic surfaces, poor baffling, diffuse light at the focal plane, and can be present in simple or complicated designs. There are many high quality, high performance very expensive telephoto lenses that have no flare problems at all. There are also simple doublet refractors that have very annoying flares and reflections.
The second part of the paragraph is again too general to be true. For example, if you had two achromatic binoculars to choose from, and one came with literature that shouted in a loud voice about having several different elements or groups that minimised aberrations leading to a sharper image, and the other just said that this is a simple achromatic binocular with a basic Fraunhofer doublet objective, which would you expect to be the best?
The point is that generalistions can not be sensibly made on these subjects because the subject itself is complicated. When the subject is complicated, unless there is a real need (e.g. the use of an instrument is not realised properly unless certain info is given), providing technical info just serves to confuse.

If the consumer can look though an instrument and then distinguish performance level differencies between instrruments, then that is the sensible method of choosing.

Your Bushnell binoculars look attractive enough, if the multi-coatings are understated in their "presence" as you say, then you should enjoy nice bright images. I am somewhat envious as no-one is going to buy anything optical for me this Christmas. I am constantly nagged into selling some of the optics I already have. My excuses for not doing so are having to get more and more sophisticated as time goes by.

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Old Tuesday 11th December 2007, 20:23   #11
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Other than interest most of us now leave lens construction parameters to the manufacturer's computers and their accountants.

IMHO, The interest in lens design from a photographic perspective really only applied when deciding to invest in a prime (non-zoom) lenses for film cameras where you had to have a fair idea of how a lens was likely to behave under a range of conditions based on the type of design and the manufacturer. This was really important as lens reviews gave you little more than a feel for resolution at various apertures and occasionally field flatness.

This interest continued into the zoom era from the point of view of physical construction when a quick field strip down was required to restore functionality following screws etc. shaking loose during heavy work.

Autofocussing zoom lenses for dslrs mean that you can now only rely on reviewers (and the ruggedness of construction).

With binoculars it is a lot easier to assess performance from a quick test and you don't generally have a screaming customer if you can't stretch the optics to get the results they expected!

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Old Wednesday 12th December 2007, 02:58   #12
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Thank you for interesting and informative answers.

FWIW, I have Bushnell Legend 8x32 and I think they are excellent. Granted, I'm no expert, but to me they are bright, sharp, waterproof and have rainguard. Wide FOV too. I'm sure your father will love them.

I didn't even realize the diopter was locking until the other day.
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Old Wednesday 12th December 2007, 12:17   #13
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Technical spec for Bushnell Legend

It is a pity you did not ask your questions a few weeks ago as I had a Legend in the workshop for realigning and I could have examined the eyepiece. I believe the eyepieces have 5 elements in 3 groups but when the next one comes in, I'll try to remember to make a drawing. I'm surprised you detected some chromatic abberation as the Legend range have PC-3 phase correction coating which should elliminate it. I don't recall ever seeing an f number quoted for a binocular. Brightness is usually quoted as a number obtained by dividing the diameter of the front lens by the magnification so your 8x42 works out to be 5.25, which is the diameter in mm of the disc of light leaving the eyepiece and entering your eye. The 10x42 for example is 4.2 for comparison which means your 8x42 is about 25% brighter. With regard to the guarantee, you are correct in thinking that the UK distributor who is an independent company, won't be too happy sorting out a problem but Bushnell have a European HQ who will help you if necessary. Fortunately the Legends are very well built and unlikely to go wrong unless you drop yours.
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Old Thursday 13th December 2007, 21:55   #14
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Hello again.

Quote:
Originally Posted by Jane Turner
Binoculars are like cars. [...] You tend to get relatively superficial technical specs in car literature too.
Detailed information on cars is readily available though, from the manufacturer and from sources such as the Haynes manual for the model. But regardless, binoculars surely have more in common with camera lenses than cars, and camera lens manufacturers provide a wealth of information about their products. Here are examples from Zeiss and Leica (PDF files), both of which include MTF data.

Quote:
Originally Posted by andytyle
In terms of technical details for the sake of technical details, then I think this would be just time and expense from the manufacturer but without a financial reward.
With camera lenses it is different. The photographer often needs to know the focal length and aperture setting range of a lens, in order to compose a photograph. There are obvious benefits to this knowledge in terms of being aware of depth of field, exposure times etc. so it is important that this type of info is made available for consumers.
Well, it would be a rather small expense, and one that might be recouped by extra sales to interested customers, and also to customers swayed by the apparent technical rigour of a company that publishes such information.

A photographer doesn't really need to know the MTF performance of her lenses any more than a birder needs to know the MTF of his binocular. So I'm not sure I appreciate the extent of the difference between the two markets.

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Originally Posted by andytyle
Also, how does knowing whether an instrument has groups and configurations of lenses or not contribute to an understanding of image quality? You need to know how these lenses work in a theoretical mathematical sense, i.e. the prescription. [...] The point is that generalistions can not be sensibly made on these subjects because the subject itself is complicated.
True. I strongly overstated the effect of the knowledge in an attempt to explain why I want this info! Basically, I want it because I'm interested.

Quote:
Originally Posted by Lilybeans
FWIW, I have Bushnell Legend 8x32 and I think they are excellent. Granted, I'm no expert, but to me they are bright, sharp, waterproof and have rainguard. Wide FOV too. I'm sure your father will love them.

I didn't even realize the diopter was locking until the other day.
Yes, the 8x42 seems pretty good too, though it has a narrower field of view. I noticed the diopter-locking mechanism too. Is it perhaps a tad too course in adjustment though? Seems like one might want a position between the indented stops, though I'll have to test properly to be sure.

richard866945, thanks for the vote of confidence in the build of the Legend binocular, and the remark about the warranty. Useful to know. I hope it won't need the warranty, of course. So far it looks great. I tried it last night against a Minolta Classic 7x50 (cheap Porro), and not only was the Legend 8x42 much sharper, it was also slightly brighter to my dark-adjusted eyes, despite the significantly smaller exit pupil.
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Old Friday 14th December 2007, 01:31   #15
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Quote: 'Well, it would be a rather small expense, and one that might be recouped by extra sales to interested customers, and also to customers swayed by the apparent technical rigour of a company that publishes such information.

A photographer doesn't really need to know the MTF performance of her lenses any more than a birder needs to know the MTF of his binocular. So I'm not sure I appreciate the extent of the difference between the two markets.
'

I agree about the idea of making technical detail available, its just that I don't think the manufacturer's/suppliers do. The problem is one of marketing. If they thought that doing so would increase sales, then it would have happened many years ago. I thlnk the over-riding fear is that making potentially complicated information available as a matter of course is more likely to deter the majority of potential customers. We live in an increasingly dumbed down consumer society. There are exceptions but generally we want it easy. As consumers we want straight black or white answers in order to make choosing easier for us. The current system of "I trust the logo/company and their product is expensive therefore the best" is the simplest route to take to arrive at consumer confidence. The very essence of bling. Back that up with regular printed or published reviews agreeing time after time that the logo/price thing is an OK marker, because "we have tested them all for you - and yes, the trusted/expensive makes are better", and you have confirmed reassurance in the simple system. To you and I, and a few others in the birding community, this isn't good enough, but we are in a tiny minority.

I think with the issue of technical info given with camera lenses, I was talking more about info like the variety of focal ratios and focal length being useful for picture composition. Focal ratio and focal length are not really going to help birders decide which binoculars they want.
With qualitative info like MTF charts for camera lenses, yes its fun to compare to see whether lens A is a little better than lens B, but this practise has been around for a few decades now and is accepted as a definitive test for a given lens to be used with a specific detector - flat film or a flat CCD chip. The other relevant matter is that the camera market is enormous compared to "Sports Optics" as they call it, the vast majority of people who purchase a camera simply don't know about MTF graphs, and gloss over them even if they are made aware of them. Those in the minority who are interested, and find one lens by Leica having a slightly better result than one by Canon may well have to change camera systems completely to take advantage. The interaction between consumers and manufacturers varies with the size of the market. The manufacturers can get away with some rules for one sector of the market, and other rules for other sectors because each sector represents a large market in itself, and each will have a distinct mindset.
In the birding optics market, the market itself is very small. Unlike the photo market, the minority sector of the market, those that want higher quality and are prepared to pay for it are essentially in the same "announcement" arena as the mass sector in terms of marketing, and this affects what is said and what is revealed within the marketing, because all sectors of the interested market are going to read the ads and see the reviews. In other words, majority rule wins. Don't rock the boat.
I agree with the idea of having more technical detail published, but the manufacturers are too concerned with affect on sales. I spoke briefly to Kowa Europe about it this year with regards to maximising the interest in the 88mm scopes, but they prefer to stick with the same methods their rivals use. I fully expect it will happen eventually, perhaps when someone with a bit of vision heads a company. More likely it will involve an increase in the market size first though.

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Old Friday 14th December 2007, 03:36   #16
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There are actually a lot of parts in a binocular. Even a 15 dollar 8x20 had a three part eye piece, parts of it shown here
http://i9.photobucket.com/albums/a57/Tero1111/bino1.jpg
the black ring is a spacer between some of the lenses

and half the prism, out of order, is here
http://i9.photobucket.com/albums/a57/Tero1111/bino3.jpg
a light frame holding the prisms and baffles were also inside

a prism has these parts
http://en.wikipedia.org/wiki/Schmidt-Pechan_prism

In the cheap binocular, the objective lens, 20mm, moved back and forth.

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Old Friday 14th December 2007, 09:12   #17
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I'm surprised you detected some chromatic abberation as the Legend range have PC-3 phase correction coating which should elliminate it.
Richard,

I have to disagree that CA could be eliminated by phase (or any other) coating. IMO it is used to correct contrast-reducing phase shifts on the roof prism surfaces. CA is a result of chosen glass material and lens design, and usually the most noticeable "lateral" CA originates in the eyepiece. Careful IPD adjustment and eye-placement can reduce perceived CA.

Best regards,

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Old Friday 14th December 2007, 21:11   #18
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The chromatic aberration that I observed with the Legend wasn't particularly bad at all (I suspect many people wouldn't notice it at all: someone I know has been wearing regular reading glasses for a few years, but had never noticed the colour-fringing they cause until I pointed out that it simply must exist when looking through the edge of the lenses!).

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Careful IPD adjustment and eye-placement can reduce perceived CA.
Yes; I think this is largely because chromatic aberration gets worse off-axis. By adjusting the IPD until the object of interest is centred in the field of view for both sides of the binocular simultaneously, one can eliminate most colour-fringing for that particular object. But of course that requires using a smaller IPD for close objects than for distant objects.
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Old Wednesday 19th December 2007, 18:20   #19
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Iporali - thank you for correcting me. Of course colour abberation is caused by the object lens and reduced by the correct use of two different types of glass elements. Phase coatings only correct the inherent error in roof prisms. I should have remembered that before posting my reply. Happy Christmas. Richard
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