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Abbe-Koenig vs Schmidt-Pechan? (1 Viewer)
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Huang_Lingyun
Member
Abbe-Koenig prisms system will increase the size and weight of binoculars. Now I think vacuum coating techniques can resolve the weakness of Schmidt-Pechan prisms system.
Such as FBMC, phase corrected, super high reflection dielectirc coating, etc.
Such as FBMC, phase corrected, super high reflection dielectirc coating, etc.
Tero
Retired
They are just pirsms, of two sorts. All prisms cause light distortions. I think the Conquests had CA. In any case, it had something in the view that was not pleasing. They did their best, but there was not a big WOW in looking through them. About the same as 300 dollar pair from Nikon, Pentax.
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Not long ago there was a good deal of discussion on the roof prism issue, and some interesting papers were discussed on this forum. Here are some things I learned, hey I'm not smart, this is just a great place to hang out:
Schmidt-Pechans make a very compact binocular, because the light bounces all around in them and the optical path gets strongly "folded", in just a little space of length. Abbe-Koenigs are long and skinny, and the light goes more nearly straight through them. That is why some of the older pre-fluorite Zeiss Classics are so long.
Longer focal length is good, optically, because it decreases aberrations like color and spherical aberration. If an AK bino is as compact as a SP bino, it has a shorter focal length. To get back the image quality, some other trick must be pulled, such as the use of Fluorite in the objective of the Zeiss FL.
All the reflections in the AK are total internal, not requiring coatings for perfect reflection. One reflection in the SP requires a coating, and this being of aluminum or silver is the main reason that SPs of the past were dimmer. Now, multilayer dielectrics even that up, but you still have to wonder if complexity can ever equal simplicity, in perfection.
Also, one of the surfaces of the SP does double duty, both admitting light from the outside, and also providing internal reflection on the inside. The use of the best antireflection coating on this surface, to optimize the admission of light into the prism, unfortunately compromises the internal reflection. So the coating on that surface is often compromised, to give the best overall image.
One would hope that with today's miraculous technologies, the practical differences between the designs are extremely subtle to nonexistent, but they've been saying that since Galileo.
I am trying to talk myself into an FL here, can you tell?
Ron
Schmidt-Pechans make a very compact binocular, because the light bounces all around in them and the optical path gets strongly "folded", in just a little space of length. Abbe-Koenigs are long and skinny, and the light goes more nearly straight through them. That is why some of the older pre-fluorite Zeiss Classics are so long.
Longer focal length is good, optically, because it decreases aberrations like color and spherical aberration. If an AK bino is as compact as a SP bino, it has a shorter focal length. To get back the image quality, some other trick must be pulled, such as the use of Fluorite in the objective of the Zeiss FL.
All the reflections in the AK are total internal, not requiring coatings for perfect reflection. One reflection in the SP requires a coating, and this being of aluminum or silver is the main reason that SPs of the past were dimmer. Now, multilayer dielectrics even that up, but you still have to wonder if complexity can ever equal simplicity, in perfection.
Also, one of the surfaces of the SP does double duty, both admitting light from the outside, and also providing internal reflection on the inside. The use of the best antireflection coating on this surface, to optimize the admission of light into the prism, unfortunately compromises the internal reflection. So the coating on that surface is often compromised, to give the best overall image.
One would hope that with today's miraculous technologies, the practical differences between the designs are extremely subtle to nonexistent, but they've been saying that since Galileo.
I am trying to talk myself into an FL here, can you tell?
Ron
FrankD
Well-known member
Ron,
Thank you for the wonderful insights. I did not read through all of the threads you mentioned and find it very convenient to see a summary of the information.
I have to agree with Arthur though...the image through the FL/AK combination is quite addicting. I am going to be very sad to see my Zeiss 7x42 FL go. :-(
Thank you for the wonderful insights. I did not read through all of the threads you mentioned and find it very convenient to see a summary of the information.
I have to agree with Arthur though...the image through the FL/AK combination is quite addicting. I am going to be very sad to see my Zeiss 7x42 FL go. :-(
John M Robinson
Well-known member
Frank, where is your 7x42 FL going, are you selling it?
John
John
Kevin Purcell
Well-known member
Nice to see you again, Huang Lingyun!
The AK prisms need phase correction too on the roof surface.
The difference was bigger in the past. An SP prism with Al coating had perhaps 10% lower transmission than AK. With silver coated SP prism perhaps a few percent lower. The mirror coatings on the SP prisms especially dielectric coatings with 99.9% reflectivity are now very good they're still not quite total internal reflection.
That said the SP prisms are manufactured in bulk they're a commodity which makes them cheaper.
As ronh points out both the entrance and exit faces on an SP prisms are both transmission and reflection faces so you have to compromise the AR coating on those faces. This is were the real difference lies. There is also an "extra" air gap in the SP design that causes some loss. So the AK prisms have 4 TIR reflections plus 2 air-glass transitions (none of which are compromised) whereas SP prisms have 5 "TIR" reflections (two of which are compromised so they're not "total" TIRs), 1 mirror reflection and 4 air-glass transitions (3 of which are compromised). I would guess a compromised glass-air transition (and a compromised TIR) are probably worth about 1% loss in each case so the SP I would guess is something like 5% down on the AK in the best case. I think these differences and compromises are the main reasons for the transmission difference between the two prism types today.
For ray diagrams see here:
http://en.wikipedia.org/wiki/Abbe-Koenig_prism
http://en.wikipedia.org/wiki/Schmidt-Pechan_prism
But they're close enough for most companies and the transmission differences are outweighted by other problems.
Ron has mentioned most of the AK advantages and disadvantages.
One implied but not made explicit is the longer bins (e.g. see a Zeiss Dialyt) tends to put the objectives further way from the face and make the bin longer and so more objective end heavy. So they can result in worse balance in the bin which can be annoying. Users generally prefer more compact bins. And we've seen Zeiss make efforts in this direction with the Victory and (Victory) FL: they're not much longer than the average SP bin.
One nice extra feature of the AK prisms is you can arrange for the light path not to be coaxial in and out of the prisms but offset slightly. Zeiss have used this feature in the Victory and the (Victory) FL to place the objectives a little further apart than the IPD unlike most roofs (about 75mm for a 65mm IPD). This gives a little "porro-like 3D" effect to their bins that many find pleasing.
Zeiss is pretty much the only bin maker to spend time developing AK prisms. Prof Abbe was an original optical designer for Zeiss and prof at the local university but the current designs came from Hensoldt (IIRC?). So there is some history there that drives the continued development of the AK prism. Other manufacturers don't have this historical attachment.
But if you can make a decent SP prism there's no technical difficult in making a decent AK prism. Other manufacturers don't use AK I think partially to avoid some of the disadvantages above and because they put all of their effort behind one prism design that they can perfect.
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Kevin,
Thanks for the added information. Now that you mention it, the AK ought to be even brighter than a Porro, which, like the SP, consists of two separate pieces of glass.
AK seems just flat better in a number of ways. Zeiss, however, apparently feels obliged to make their binocular as short as everybody else's, even though the prisms make it difficult. The fluorite fixes, or even more than fixes, the aberrations of the very fast objective, but the eyepieces are still saddled with the rapidly converging light cone. This is likely why the FL is often criticized for relatively poor edge of field sharpness.
Ron
Thanks for the added information. Now that you mention it, the AK ought to be even brighter than a Porro, which, like the SP, consists of two separate pieces of glass.
AK seems just flat better in a number of ways. Zeiss, however, apparently feels obliged to make their binocular as short as everybody else's, even though the prisms make it difficult. The fluorite fixes, or even more than fixes, the aberrations of the very fast objective, but the eyepieces are still saddled with the rapidly converging light cone. This is likely why the FL is often criticized for relatively poor edge of field sharpness.
Ron
Attachments
henry link
Well-known member
Tom,
I think Ron is referring to cementing the two prisms together. The SP design requires an air space, but both AK and Porro can be cemented. Porro Type II must be cemented. Porro Type I can be, but seldom is. Two examples I recall of cemented Porro I were the Swarovski SL series and the later CJZ Nobilems.
Henry
I think Ron is referring to cementing the two prisms together. The SP design requires an air space, but both AK and Porro can be cemented. Porro Type II must be cemented. Porro Type I can be, but seldom is. Two examples I recall of cemented Porro I were the Swarovski SL series and the later CJZ Nobilems.
Henry
No, my saying the AK was a single piece was just an error, and thanks for straightening me out Thorlo. I see the interface in the Wikipedia diagram now. But, if they could, it looks like they would cement the AK pieces. In fact, if they can be cemented, why not just make them in a single piece? Different glass types seem unlikely. Besides avoiding a bit of light loss and resulting scatter, it would save having to accurately mount a separate piece, and save having to finish the two mating surfaces.
Ron
Ron
Kevin Purcell
Well-known member
My point was the AK has no air gap ... see the rest of the text (not quoted) where I enumerate the differences.
Plus you don't need two prisms to rectify an image. You can do that with one
The AK prism has two parts cemented together (as Henry points out). The SP requires an air gap because that face is used as an entrance face and a IR reflection face.
The air-glass transitions are a critical difference between the designs.
Henry posted on another thread that Zeiss claim a residual 2% difference between AK and SP in their designs (as they use both).
Tero needs to read more optical text books
The other type of cemented porro prisms are most often used in microscopes. And yes you can polish all sorts of oddly shaped pieces of glass (so long as you can get the polishing equipment to the surface!).
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chartwell99
Well-known member
Great summary, Ron. I do have two unrelated questions. Given the true benefits of the AK prism, why is Zeiss the only current maker of note (I remember certain Optolyth roof models used AK prisms in the past, but I can think of no others)? And, are images bouncing around either type of prism "reflected" or "refracted"? My understanding may well be wrong, but I thought the only reflected image was off the coated roof surface of either the AK or SP prism while other images (and all images within porro prisms) are refracted.
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Pinewood
New York correspondent
Hello Chartwell99,
As I recall my high school physics, refraction occurs when light passes from one medium to another of a different refractive index. Light entering and leaving the prisms, must be refracted, while there are internal reflection as the light bounces off prism walls.
Does that help or confuse?
Happy bird watching,
Arthur Pinewood
Tero
Retired
Light hits the prisms mostly perpendicular to the glass surface, so refraction is minimal. Inside the prism, reflection.
http://en.wikipedia.org/wiki/Schmidt-Pechan_prism
http://en.wikipedia.org/wiki/Schmidt-Pechan_prism
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