why not flaurite prisms?

[matt green]

It seems odd that no one has yet exploited the possible advantages of fluoride prisms?

my logic is thus...surely the manufacture of lenses is a much more costly and laboured procedure than a triangular chunk of prism that only needs to be finely polished flat on three sides.

the remaining optical lenses could be made from cheaper/regular optical glass
and still have all the usuall multi coatings etc.

not sure wether the majority of light has to pass through the combined mass of optical lenses or prisms,but surely those prisms count for a lot...

so why not make em in fluoride?

matt

ps..this question just popped strait off the top of my head with no research what so ever,so apologies to any optics designers who already make such things or if indeed zeiss also include fluoride glass into the manufacture of their prisms!

[AlanFrench]

BaK4 prisms work better than BK7 prisms because BaK4 has a higher index of refraction. In green light the index for BaK4 is 1.5712451, while BK7 is 1.51872218. The index for CaF2 (fluorite) is 1.43494007. Prisms with such a low index of refraction would not work well.

Also, the improved color correction in a lens using fluorite comes about because there are crown glasses that nicely match the partial indices of fluorite. It takes two to tango, and fluorite by itself in a lens or prism would not provide good color correction.

In low power binoculars fluorite is really just a marketing tool anyway. It is not needed.

Clear skies, Alan

[Keith Dickinson]

Optically there would be no great benefit from making the prisms from fluorite glass, as there is no refraction at the air/glass interface. The prism is deviating the light path by total internal reflection rather than by refraction. The lenses do gain optically by being made from fluorite materials and having top notch multi-layer anti reflection coatings on them. Problems like chromatic abberation are lower with the higher refractive index fluorite materials.
So as the cost of using fluorite for the prisms would bump up the price and the weight without giving any optical benefit, it's not used.
The thing I want to know is why some of the newer optical pastics materials aren't being investigated for scopes and bins...for the internal elements at least.

[AlanFrench]

[SNIP]
Problems like chromatic abberation are lower with the higher refractive index fluorite materials.
[SNIP]


Keith,

Fluorite actually has unusually low refractive indices - 1.43 and change in the visual wavelengths. But it is an ED (Extra-low Dispersion) material, so the variation with wavelength is rather low.

Clear skies, Alan

[matt green]

alan and keith...

thanks for clearing up that little mystery!

matt

[Mickymouse]

I'd like to know why they use prisms instead of mirrors.

Mick

[AlanFrench]

I'd like to know why they use prisms instead of mirrors.

Mick

Mick,

I think there are several factors that argue against using mirrors. The prism system, whether it is a porro or a roof, is there to erect the image formed by the objective. Collimation involves lining up the two halves of the binoculars so that both are looking at the same field.

The erecting system itself has to be properly "lined" up, so prisms are made to rather high tolerances. Once the prism system is cemented together it stays aligned (baring significant abuse). A prism system is also compact because the reflective surfaces are on the outside and are "supported" by the prisms themselves.

If you use mirrors, they would need an external support, which would add bulk. There would also have to be some way to line up the mirrors, and I think this would also add to the external bulk. I think it would be hard to come up with something that would hold alignment of the erecting system as well as prisms do.

The metals used in coatings on first surface mirrors, usually aluminum, are subject to attack by pollutants in the air and deteriorate with time. Some of the modern dielectric coatings are more robust, but they tend to distort the edges of the optical surfaces and dielectric diagonals are made oversize to combat this.

There are certainly reflective coatings in some prisms, but they use the surface against the glass, which is very well protected (and smoother than a first surface mirror). The external surface can be sealed and protected.

In a nutshell, I think it is harder to do, and would add bulk to the binocular. Now with very large binoculars, such as those used for astronomy, it might be doable.

Clear skies, Alan

[elkcub]

...

In low power binoculars fluorite is really just a marketing tool anyway. It is not needed.

Clear skies, Alan

Alan,

Please provide a rationale for this statement.

Thanks,
Ed

[iporali]

BaK4 prisms work better than BK7 prisms because BaK4 has a higher index of refraction.
A practical example of the above:
The difference between refractive indices of BaK4 and BK7 is quite small, but still you can easily see this difference in the "exit pupils" of binoculars. The small, round image circle formed by a BK7-bino has a bright square center surrounded by grey "cutouts". These darker areas form when the light rays that enter the "reflective" surface in such an angle that instead of reflecting (TIR), they "leak" through the surface - and cause light loss (vignetting). This would probably be much worse with fluoride glasses.

Ilkka

[iporali]

Alan,

Please provide a rationale for this statement.


I would also be interested to hear ;)
Could it be that in astronomical use or ray tracing there is usually nothing in front of the subject or behind it. I believe that at focal plane the improvement by ED may well be beyond the eye's resolution - or even wave physics, if you say so. But I guess I agree with Ed that a "terrestrial" 3D image which mostly consists of more or less out-of-focus planes, may actually benefit from ED. It is a little hard to explain, but some optics just seem to be more comfortable than others. Ed has described it with his ED-Swifts and I feel the same with one Leica APO Televid spotting scope (even compared to my ED-doublet scope). And I can easily see the longitudinal coloration of tree branches even with low power binoculars. Is it "bokeh" or something else - I don't know, but until I hear a better explanation I believe it has something to do with longitudinal CA.

Best regards,

Ilkka

[mak]

The thing I want to know is why some of the newer optical pastics materials aren't being investigated for scopes and bins...for the internal elements at least.

Because at this moment in time, it is more likely that there are no plastic lenses containing suitable refractive indices to be used in high quality binoculars.

[kabsetz]

Like Ed and Ilkka, I prefer to have as little as possible CA in the image my binoculars provide. Providing other aberrations are equally well corrected, I see a difference in the view between non-ED and ED/fluoride lens binoculars. To me, this difference is significant enough. However, in night-time use the differences, while there, are perhaps not that relevant. It is pretty easy to enjoy a view of the moon or star clusters even if there is a bit more CA, and at the magnifications typical binoculars provide, you don't see Airy disks and in-focus diffraction rings anyway.

Kimmo

[matt green]

incidentally,how many prisms are there in a roofprism binocular?

I've always been under the (false?) impression that roofprisms had three sets of prisms and porros had two,because they don't need to bend the light so many times,thus explaining why porros give a superior image?

matt

[henry link]

The original Leitz Trinovids used Uppendahl roof prisms which are made from three prisms cemented together. All the current roof prism binoculars I know about use either Schmidt-Pechan or Abbe-Koenig prism clusters which are made from two air spaced prisms. The Schmidt-Pechan prism has 6 internal reflections, one of which has to be mirror coated. Abbe-Koenig prisms have 4 internal reflections and no need for mirror coating, the same as a Porro. I think the main problems with roof prisms compared to Porro are loss of contrast and sharpness from destructive interference when the light cone is split and recombined at the roof faces (which phase coating is supposed to fix), lower light transmission from extra reflections and mirror coating in the S-P type which is almost but not completely cured by dielectric mirror coating and finally the requirement for a very high level of accuracy in the roof angles and a very fine roof edge which is expensive to do.

[Tero]

Roof prism
http://en.wikipedia.org/wiki/Schmidt-Pechan_prism

It really does have a little house roof at the top, the right prism. The left prism is mirrored on the bottom side, the side that only reflects.

(I took a cheap pair apart)

[matt green]

The original Leitz Trinovids used Uppendahl roof prisms which are made from three prisms cemented together. All the current roof prism binoculars I know about use either Schmidt-Pechan or Abbe-Koenig prism clusters which are made from two air spaced prisms. The Schmidt-Pechan prism has 6 internal reflections, one of which has to be mirror coated. Abbe-Koenig prisms have 4 internal reflections and no need for mirror coating, the same as a Porro. I think the main problems with roof prisms compared to Porro are loss of contrast and sharpness from destructive interference when the light cone is split and recombined at the roof faces (which phase coating is supposed to fix), lower light transmission from extra reflections and mirror coating in the S-P type which is almost but not completely cured by dielectric mirror coating and finally the requirement for a very high level of accuracy in the roof angles and a very fine roof edge which is expensive to do.

goes a long way to explain why lower end and even some mid priced roofs rarely equal the optical quality of the best budget porros!

we definately need some more high end porro designs!

many rave about the nikons,but only two porroprism models are currently made with internal focusing and are completely water proof/nitrogen purged.

I'm refering to opticrons magnificant HR WP 8/10X42 and the minox BP 8X44.

matt

[AlanFrench]

Alan,

Please provide a rationale for this statement.

Thanks,
Ed

Ed,

The color fringing people complain about is lateral color, and the main source of lateral color is the eyepiece (it has to cover 50 to 67 degrees, while the objective covers 5 to 8 or so).

The use of ED material (either ED glass or fluorite) reduces secondary color, which is a variation of focal length with wavelength. When a system is carefully focused on-axis, the colors around green are in-focus and the colors at the blue and red ends of the spectrum are out-of-focus. At low powers, however, the out-of-focus blur is too small to be detected by the eye. During the day it is the diffraction effects of the eye that determine the size of the diffraction image, not the size of the objective.

As a reality check I asked two people who make a living designing and making optics. The both said there was not need for ED material in a low power binocular, although one suggested the perception among buyers made it desireable.

I admit there is a chance I am still not completely understanding the situation - but I've sure tried to make sure I am not way off base, and I'd love to hear from someone who is an optical engineer and designer in the binocular industry. It is easy to find out about telescope design. Binocular design seems out of our view.

Unfortunately, marketing is much more about perceptions than reality. If buyers perceive something as being beneficial, the marketing folks are going to take advantage of that perception.

Clear skies, Alan

[John Russell]

[QUOTE=Keith Dickinson]Optically there would be no great benefit from making the prisms from fluorite glass, as there is no refraction at the air/glass interface.

As much of the light falling on and exiting the prisms of a binocular with a short focal ratio around f/4 is not perpendicular to the prism surfaces, there is likely to be some colour dispersion.
I have often wondered if this plays a significant role in longitudinal CA.
Does anyone have any views on this?

John

[elkcub]

Ed,

The color fringing people complain about is lateral color, and the main source of lateral color is the eyepiece (it has to cover 50 to 67 degrees, while the objective covers 5 to 8 or so).

The use of ED material (either ED glass or fluorite) reduces secondary color, which is a variation of focal length with wavelength. When a system is carefully focused on-axis, the colors around green are in-focus and the colors at the blue and red ends of the spectrum are out-of-focus. At low powers, however, the out-of-focus blur is too small to be detected by the eye. During the day it is the diffraction effects of the eye that determine the size of the diffraction image, not the size of the objective.

As a reality check I asked two people who make a living designing and making optics. The both said there was not need for ED material in a low power binocular, although one suggested the perception among buyers made it desireable.

I admit there is a chance I am still not completely understanding the situation - but I've sure tried to make sure I am not way off base, and I'd love to hear from someone who is an optical engineer and designer in the binocular industry. It is easy to find out about telescope design. Binocular design seems out of our view.

Unfortunately, marketing is much more about perceptions than reality. If buyers perceive something as being beneficial, the marketing folks are going to take advantage of that perception.

Clear skies, Alan

Alan,

Thanks for the reply. As I understand it, the extent of lateral CA of the objective is magnified by the eyepiece so that color fringing becomes more obvious at higher power. Conversely, according to opticians, there is a lower power (who knows where exactly?) below which, as you said, "... the out-of-focus blur is too small to be detected by the eye." In short, opticians have settled upon a practical rule-of-thumb that sounds quite logical. It is, however, only a heuristic, not a truth.

I have been unable to find a single scientific article concerning the detection or perception of lateral/longitudinal CA from optically magnified images. They may be out there, but until located there is litttle basis for the heuristic, other than perhaps reported consciousness of color fringing. But consciousness, in this instance, is neither a basis for what can or can't be detected in more subtle ways, nor does it ultimately determine color perception. Indeed, we are not consciously aware of most factors that contribute to the inner world of color perception — which as we know is created entirely by the brain.

The science of color perception is extremely complex, and I make no claim to be an expert. However, anatomical evidence is compelling that the distribution of different types of color receptors within the retina, as well as their sensitivities and physical orientations, is a result of a remarkable evolutionary adaptation to one overriding problem: chromatic aberration. Were it not for this issue the nature and distribution of color receptors would probably have been quite different for the species.

So, in the interest of providing magnified images, which is the only purpose of telescopes, one also introduces chromatic aberration via these external sources that stresses the eye's evolutionary mechanisms to deal with it. As we know, some people are more "sensitive" or annoyed by consciously perceived color fringing than others, but I would maintain that we are all equally affected by it visually. The effect shows up in image dullness and lack of color gradation, which there is evidence to show can be reduced by the proper use of ED glass. The one and only example I have of this is my Swift 804ED, but Henry Link and possibly others have made similar observations about Zeiss' FL series. I would hasten to add, however, that the underlying issue is minimizing CA, and ED glass is only one way to accomplish that. Perhaps other manufacturers do it a different way. Proprietary designs make it difficult to know.

Ed

[AlanFrench]

The performance of a binocular is certainly complicated. We have the binocular itself, our eye - which enters the picture in a big way with low power optical instruments - and the way our eye and brain work together to form the image in our mind. (There is no image sent from our retina to the brain - only information about the image formed on the retina, which is used to construct our essentially virtual view of the world around us.) This makes it difficult to make objective measurements.

In the end, it isn't the opinions of other people, the writings of optics experts, or what your birding friend carries around, that should determine what binocular you pick. You should make the choice based on your own experiences with the binocular in hand. If it has the attributes you want, provides the view you like, and makes you happy, then you have picked wisely.

Clear skies, Alan

[Mickymouse]

In a nutshell, I think it is harder to do, and would add bulk to the binocular. Now with very large binoculars, such as those used for astronomy, it might be doable.

Clear skies, AlanThanks for your explanation, it more or less supports what I would have imagined as the reason.

Mick

[kabsetz]

Ed,

Nice post!

If you do find scientific articles on the perception of CA, I trust you let us know about them.

When it comest to resolving power, there also seems to be a heuristic in place about what levels of resolution are above/below the ability of the human eye to detect. Granted, this area has been studied very extensively, but I still feel that there is a tendency for the optics manufacturers to make excessively straightforward assumptions of what the resolving power of a piece of optics with magnification X needs to be in order to give viewer Y an optimally "sharp" image.

My field experience with low-CA optics is rather extensive by now. Most of it comes from ED/FL/Apo spotting scopes. As for binoculars, my long-term reference has been the Nikon SE, which of course is not an ED-glass, but does have considerably less CA than virtually all the roof-prism binoculars which do not employ special glasses. I have had all the 32-42mm models of the Zeiss FL range for field testing at various times, for periods ranging from a couple of weeks to a couple of months, and have compared them to the SE as well as various Leica and Swarovski models. All of this has shown me quite clearly that at least for my eye-brain system there is a decided advantage to be had from a low CA design.

Lately I have been using the Canon 10x42 L, which utilizes two UD-elements per side and has a porro II prism system, and which both subjectively and in an out-of-focus star test shows even less CA than the Zeiss 10x42 FL. With the Canon as well, much of my enjoyment of the view comes from the purity and subtle gradation of the colors as well as the enhancement of perceived contrast which accompanies good CA correction.

Kimmo

[elkcub]

Ed,

Nice post!

If you do find scientific articles on the perception of CA, I trust you let us know about them.

When it comest to resolving power, there also seems to be a heuristic in place about what levels of resolution are above/below the ability of the human eye to detect. Granted, this area has been studied very extensively, but I still feel that there is a tendency for the optics manufacturers to make excessively straightforward assumptions of what the resolving power of a piece of optics with magnification X needs to be in order to give viewer Y an optimally "sharp" image.

My field experience with low-CA optics is rather extensive by now. Most of it comes from ED/FL/Apo spotting scopes. As for binoculars, my long-term reference has been the Nikon SE, which of course is not an ED-glass, but does have considerably less CA than virtually all the roof-prism binoculars which do not employ special glasses. I have had all the 32-42mm models of the Zeiss FL range for field testing at various times, for periods ranging from a couple of weeks to a couple of months, and have compared them to the SE as well as various Leica and Swarovski models. All of this has shown me quite clearly that at least for my eye-brain system there is a decided advantage to be had from a low CA design.

Lately I have been using the Canon 10x42 L, which utilizes two UD-elements per side and has a porro II prism system, and which both subjectively and in an out-of-focus star test shows even less CA than the Zeiss 10x42 FL. With the Canon as well, much of my enjoyment of the view comes from the purity and subtle gradation of the colors as well as the enhancement of perceived contrast which accompanies good CA correction.

Kimmo

Kimmo,

Many thanks for your comments. I certainly share your suspicion that even in the domain of "resolution" optical designers and manufacturers use heuristics that don't fully address the biological capabilities of the eye. An expression that gives me a shiver every time I read it is "... all the eye can see (or use)," which, of course, is based completely on the anatomical size and spacing of the retinal matrix in the fovea. It is an oversimplification to think that the eye (i.e., the retina and the brain of which it is an anatomical extension) can necessarily see no more. The eye is known to be able to abstract much more information from the scene, provided it is presented to the retina for higher brain processing and not removed, let's say, by overzealous application of an engineering heuristic. Fortunately, high quality instruments have substantial spare capacity, so the rationale appears to work quite well in practice, regardless of it's veracity. But, as in the case of ED glass, there is a tendency to make heuristics into immutable truths. A thought provoking and concise article written in 1975 by Prof. Gerald Westheimer is worth contemplating along these lines: http://www.iovs.org/cgi/reprint/14/8/570.pdf#search=%22visual%20hyperacuities%22. All these years later he is still running a graduate laboratory at UC Berkeley to address this robust area of vision research.

Addendum 9/26. You've probably seen this post on CN some time back, but it's worth bringing up here: http://www.cloudynights.com/ubbthreads/showthreaded.php/Cat/0/Number/695753/page/0/view/collapsed/sb/5/o/all/vc/1. Note the statement near the end "... Given that COLOR variation is a key field mark identifier for birdwatchers, particularly when trying to discern different species in the same genus, I'm surprised there are so few ED binoculars made." Yes, indeed, I'm surprised too!

Ed