A comparison of prism types, and why the Schmidt Pechan seems to have won the premium binoculars (2 Viewers)

[kimmik]

The prism is the heart of a prismatic binocular, the most critical piece of the technology that converts an inverted and inaccessible image, to a natural extension of the eye of the expected orientation and light path. This is achieved through reflections, many reflections, especially if one wishes to look straight through rather than at 45° or 90° to the source of the light. In the search for the big eyes experience, the ultimate augmented reality, an immediate extension of the mind and body, is Schmidt Pechan the future?

Zeiss seems to think so, having nearly fully converted the premium line up to SP. Leica tried for a modern Uppendahl but eventually went with SP. Swarovski has been nearly all in on SP for a long time now. Nikon also, with the EDG being SP, added a giant Abbe Koenig WX as a halo product which is impractical for handheld use (for most).

Porro being the grand father of prismatic binoculars, was thought to be fully matured until Perger patented a version which is water proofable - great, but here is the problem, it is the same problem as why Abbe Koenig is phased out. It is too elegant, too simple, too perfect, with too few reflections, and that is bad for two big reasons:

1. less reflections means the binocular will be too long, lacking compactness. This wasn't noticeable before ED glass, as the focal length was quite long anyway.
2. it is already optically perfect, with little room to improve. I even wonder, whether Zeiss at least, knew of this design all along, as it seems to be a roofless and narrower FOV version of abbe-koenig.

With Abbe Koenig, one of the final iterations being the Zeiss FL, was considered by Zeiss to be too good to be made with metal. It was downgraded to plastic housing which will slowly leave circulation and make room for the next era of Zeiss binoculars, the Schmidt Pechan.

	Schmidt Pechan	Uppendahl	Abbe Koenig	edit: Mirror-AK?	Perger Porro	Porro
Reflections (total number)	6	6	4	4	4	4
Perfect total internal reflection TIR	0	1	2	0	4	4
Imperfect TIR	5 (2 phase, 3 AR)	4 (2 phase, 2 AR)	2 (phase coated)	2 (phase coated)
Mirror reflection	1	1		2
Pure transmission surfaces	1	4 (0 if cemented)	4 (2 if cemented)	2	4 (2 if cemented)	4 (2 if cemented)
Partial transmission surfaces	3	2		0
Compactness in order	A	B	C	B/C	D	E

Schmidt pechan uses 6 reflections, lengthening the light path with sideways bounces, while shortening the physical length. The latter is desirable, while the former is where magic separates "alphas" from the "good". This separation is what the current premium brands live on. If they were making traditional porros, they would struggle to differentiate themselves with the newer makers who have caught up with broadband multicoat antireflection. I think we will see AK and UP return in 20 years, when mass market SP has caught up sufficiently that Zeiss and Leica need a new way to differentiate themselves, then these inherent strengths will be of value.








By Fred the Oyster, CC BY-SA 4.0, File:Schmidt-Pechan prism.svg - Wikimedia Commons
By Fred the Oyster, CC BY-SA 4.0, File:Abbe-König prism.svg - Wikimedia Commons
By Holger Merlitz - Own work, Copyrighted free use, File:Uppendahl prisma randstrahlen.png - Wikimedia Commons
By Merlitz (Diskussion) - Own work (Original text: Selbst erstellt, nach Vorlage der Patentschrift EP2463692A1 künstlerisch erweitert), Public Domain, File:Perger prisma.png - Wikimedia Commons
By Fred the Oyster, CC BY-SA 4.0, File:Double Porro prism.svg - Wikimedia Commons

 

[Richard D]

Interesting, but Swaorvski still sell three AK prism models, that I don't think they have plans to discontinue - is it just that the bulk of a big objective makes the length less important? Of course they still see Porro's too...

 

[Binocollector]

I have a number of AK-prism models (7x50, 8x56, 9x63, 10x45 and 15x56). Some old some new. My DDoptics "Pirschler" 10x45 sits on the window sill in the kitchen and is frequently used. It is not that much longer than a normal 10x42 would be and fits me perfectly ergonomically. I have no idea who makes these, I suspect some Chinese company (there are similarities to my Kite 8x56 which seems to be the same as the GPO 8x56 with AK-prisms).
They are still very nice. DDoptics has a whole line of these "Pirschler" models. From 8x45 up to 15x56.
I think the modern coatings greatly reduced the advantages of AK-prisms and modern S/P-binos can be made just as good with the same kind of light transmission and more compact.
I like the size of the old 9x63 models especially for star-gazing. Not as shaky as a 10x50, great balance, bright and does nicely show the colors of the stars.
Another favourite -- old 7x50 Japanes AK-prism model. Super eye relief -- more or less the only vintage bino that I can use with glasses. And it has a flat field which makes it great for astronomy. Of course my Fuji FMTR is much brighter. So on vintage models the old coatings destroy the advantage of the AK-prisms.
That's why I am still on the look-out of one of the newer Optolyth 9x63 models that were made shortly before Optolyth went belly-up.

 

[Vespobuteo]

With Abbe Koenig, one of the final iterations being the Zeiss FL, was considered by Zeiss to be too good to be made with metal. It was downgraded to plastic housing which will slowly leave circulation and make room for the next era of Zeiss binoculars, the Schmidt Pechan.

Intersting summary. Many sides of the prism to consider here. I guess size and weight is a major factor.
And older SP-prisms using mirror-reflective coatings in silver added a color cast.
When that was solved the SP-prims became fully competitive.
Also better AR-coatings degrades the value of AK-prisms somewhat, especially in normal/daylight bins.
But AK:s still have their place in the HT and other big guns it seems.

BTW, The Zeiss FL housing is not made of "plastic", it is made of carbon reinforced composite. More durable and lighter than magnesium alloy.
Definitely not a downgrade from a technical point of view,
but the production was most likely more complicated and expensive and not viable for that reason.

From an engineering perspective mag. alloy is the most convenient, but from a user perspective,
nothing beats a pair of FL:s when it's cold outside...my Swaros get sooo coooold.....


A miracle like the Zeiss FL 7x42 with AK-prims will never occur again I guess.
And I will continue to cherish my 7x42 FL:s for the technical marvel and pinnacle of true engineering they are.

Nowadays, products doesn't even appear to be fully finished or even thought through before they hit the market.

 

[Swissboy]

Intersting summary. Many sides of the prism to consider here. I guess size and weight is a major factor.
And older SP-prisms using mirror-reflective coatings in silver added a color cast.
When that was solved the SP-prims became fully competitive.
Also better AR-coatings degrades the value of AK-prisms somewhat, especially in normal/daylight bins.
But AK:s still have their place in the HT and other big guns it seems.

BTW, The Zeiss FL housing is not made of "plastic", it is made of carbon reinforced composite. More durable and lighter than magnesium alloy.
Definitely not a downgrade from a technical point of view,
but the production was most likely more complicated and expensive and not viable for that reason.

From an engineering perspective mag. alloy is the most convenient, but from a user perspective,
nothing beats a pair of FL:s when it's cold outside...my Swaros get sooo coooold.....


A miracle like the Zeiss FL 7x42 with AK-prims will never occur again I guess.
And I will continue to cherish my 7x42 FL:s for the technical marvel and pinnacle of true engineering they are.

Nowadays, products doesn't even appear to be fully finished or even thought through before they hit the market.

Another very happy AK equipped FL x42 user here. And the convenience of the non-metal housing is also very much appreciated. Will be interesting how I accustom to the SFL magnesium body in the future.

 

[kimmik]

Agree FL is superb.

Regarding AK, UP and SP, here is the crux of the matter, imo.

Wide angle.

SP wide angle, of which I have two, leave something to be desired in colour gamut, even though peak transmission and brightness are good. The complexity of wide angle binocular reduces transmission, which is compensated first by losing the violet and/or deep red.

This is possibly why Leica and Nikon have capped themselves at 8degrees. Perfect colour wide angle is on my radar, but I feel possibly beyond the theoretical limits of SP.

It is also no coincidence that Nikon chose AK for the WX ultra wide.

 

[[email protected]]

With modern coatings, the AK prism has very few optical advantages over the SP and the SP is smaller and more compact, so the binocular can be made smaller and more compact. AK prism binoculars tend to be longer than SP prism binoculars because of the bigger prism. Some say also that AK prisms have more CA than SP prisms, and that is another reason the binocular is longer to correct for it with a longer focal ratio. From Cloudy Night's.

Light transmission vs. CA or AK-prisms vs. ED glass - Binoculars - Cloudy Nights

Page 1 of 2 - Light transmission vs. CA or AK-prisms vs. ED glass - posted in Binoculars: Hi guys and gals, I need a little input from the members of this fine forum. Its not that I need another bino but when was it ever a question of need? I noticed a gap in my fast growing collection and that...

www.cloudynights.com

 

[kimmik]

AK in a mirror-roof configuration is the way to go for its return I feel. Not sure what the proper name is, mirror-AK?

Basically Leitz got it right 60 years ago, with the amplivid then trinovid 6x24.

Use dielectric mirrors to save significant weight. Only a small roof pentaprism needs to be solid glass.

How much are the manufacturers hiding from us...

 

[Canip]

I think the modern coatings greatly reduced the advantages of AK-prisms and modern S/P-binos can be made just as good with the same kind of light transmission and more compact.

Yes, I think mainly this, plus the form/shape factor are the main reasons why S-P is advancing in all areas formerly occupied by A-K.

The future of Perger, which on paper has lots of potential, is unclear to me, for some reasons it hasn't been used beyond Leica's Geovid rangefinder binos (there were early hopes that Leica might come out also with "normal" Perger binos), although in my experience, the existing Perger application (Geovid) exhibits superb optical performance, at least in the larger x42 models (I haven't seen the x32 models).

 

[William Lewis]

Yes, I think mainly this, plus the form/shape factor are the main reasons why S-P is advancing in all areas formerly occupied by A-K.

The future of Perger, which on paper has lots of potential, is unclear to me, for some reasons it hasn't been used beyond Leica's Geovid rangefinder binos (there were early hopes that Leica might come out also with "normal" Perger binos), although in my experience, the existing Perger application (Geovid) exhibits superb optical performance, at least in the larger x42 models (I haven't seen the x32 models).

How do those Geovids compare with non rangefinder Bono's? Are there any downsides other than weight and complexity to having the rangefinder function - i.e if you were to use a geovid for birding would you notice it was a rangefinder when viewing without using the rangefinder or ballistic functions?


Will

 

[tenex]

Good thread. As I understand TIR, reflection is either total or not depending on the critical angle, and the mirror reflection simply is not. So I don't know what you mean by "imperfect TIR: AR" or how there would be so many cases of it in SP/U prisms.

I have suspected that rather than competing on quality of similar products, postwar Zeiss/Leica sought to distinguish themselves by producing ultracompact designs that Jena or Japan couldn't, and then (with P* finally) by producing them distinctly better for a while. That is, it wasn't just catering to consumer preference but deliberate marketing, emphasizing compactness even over optical quality. (Even the Oberkochen 10x50 could be seen as another instance.) Now that era has ended, similar products are coming even from China, and they're finally left merely trying to find some tech spec (like FOV or edge sharpness) where a distinct advantage can be claimed at additional cost. Full circle, except the insidious SP prisms are still there. Actually, SF/NLs are already becoming less compact in order to achieve this, so SPs make ever less sense. Do Pergers really have a FOV challenge?

 

[Scott98]

Have the AK dropped off that much? Zeiss and Swaro still use them for 54 and 56mm in multiple sizes. 45 and 56mm available from Maven and others. Isn't it just Zeiss that switched to SP's for their mid-aperture binos? Didn't Leica and Swaro and Nikon go directly from porro to SP? (I'm not that clear on the history 10-20 years ago).

It is interesting that the 56mm's still use AK when they could use SP. Could AK be cheaper? Dielectric coatings are expensive, it looks like AK is still the preferred choice as long as they can fit it into the body and keep the weight reasonable. Maybe Zeiss used the plas....uh, "composite" frames in Victory FL because it was the only way to keep the weight down with AK's in the 42mm size?

actually that doesn't make much sense because the 54 HT's and SLC's are very light binos in that size range. I guess we'll never know why Zeiss went with SP's in the SF line.

 

[Vespobuteo]

Yes, I think mainly this, plus the form/shape factor are the main reasons why S-P is advancing in all areas formerly occupied by A-K.

The future of Perger, which on paper has lots of potential, is unclear to me, for some reasons it hasn't been used beyond Leica's Geovid rangefinder binos (there were early hopes that Leica might come out also with "normal" Perger binos), although in my experience, the existing Perger application (Geovid) exhibits superb optical performance, at least in the larger x42 models (I haven't seen the x32 models).

Perger is an interesting choice, but haven't tried that model.

Leica seem to avoid AK-prims...maybe the think they are too "Zeissy"....?

"This improved straight prism system is known as the Abbe-Koenig prism and was protected by ZEISS’ trademark from July 7, 1901"

I also noted that the new "budget" Leica Geovid R (42mm < 20k) uses "Modified" Uppendahl, maybe they are easier/cheaper to manufacture?
Made in Portugal like the retros that also uses Uppendahl.

 

[Canip]

Are there any downsides other than weight and complexity to having the rangefinder function

Not in my opinion - the one I have (Leica Geovid 8×42 HD-R (Typ 402) – Binoculars Today) is a bit larger and on the heavy side compared to many other 8x42s, but optics and handling make it an excellent birding bino with good sharpness and contrast and superb image brightness.

 

[Canip]

Made in Portugal like the retros that also uses Uppendahl.

The “retros“ use Schmidt-Pechan.

 

[Vespobuteo]

Have the AK dropped off that much? Zeiss and Swaro still use them for 54 and 56mm in multiple sizes. 45 and 56mm available from Maven and others. Isn't it just Zeiss that switched to SP's for their mid-aperture binos? Didn't Leica and Swaro and Nikon go directly from porro to SP? (I'm not that clear on the history 10-20 years ago).

It is interesting that the 56mm's still use AK when they could use SP. Could AK be cheaper? Dielectric coatings are expensive, it looks like AK is still the preferred choice as long as they can fit it into the body and keep the weight reasonable. Maybe Zeiss used the plas....uh, "composite" frames in Victory FL because it was the only way to keep the weight down with AK's in the 42mm size?

actually that doesn't make much sense because the 54 HT's and SLC's are very light binos in that size range. I guess we'll never know why Zeiss went with SP's in the SF line.

AK-prisms seems mostly to be a Zeiss thing by history.
And I suspect you are right about that Swaro went from Porros to SP, and not used AK in the smaller models.
The Swaros were famous for their color cast in the 90-ies.... (= silver on the mirror-coated SP-prisms, perhaps).

As long as I can remember back, Zeiss always used AK-prisms in 40mm+ models.

Im pretty sure the Zeiss 10x40 BGAT (Manufactured in 1975-1998) had AK-prims (EDIT: They were SP)
But back then you just said "roof-prims" They were only 730 grams.

The tall 7x42 model definitely had AK prims.

Carl Zeiss Dialyt 10x40 B/GA T* - binoculars specification - AllBinos.com

The best binocular tests on the net. The comprehensive database of binoculars with their parameters and users opinions. Interesting articles and comparisons.

www.allbinos.com

The consecutive model-series Victory I/II also had AK-prisms in the 40mm versions
and they were very compact and light. The Victory I + II were probably not a big success (manufactured 2000-2004) and soon replaced by FL.

Carl Zeiss Victory 10x40 B T* - binoculars specification - AllBinos.com

The best binocular tests on the net. The comprehensive database of binoculars with their parameters and users opinions. Interesting articles and comparisons.

www.allbinos.com

The Zeiss Victory 10x40 B*T*P* AOS and the Leica Ultra Trinovid 10x42 BN - Under 50mm

I conducted a practical comparison of these two new, highly advanced, waterproof binoculars. First handling revealed that the Zeiss 10x40 Victory is almost a featherweight glass when compared with the Leica 10...

www.cloudynights.com

 

[Vespobuteo]

The “retros“ use Schmidt-Pechan.

Ah yes, sorry, it's the original model that uses Uppendahl.

So I guess the "expectations" was "Uppendahl"...

Not so Retro after all then...

 

[Binocollector]

Could AK be cheaper?

From what I read -- yes. They used to be hard to make to precision (my vintage ones from the 70's seem not too bad but they all show spiking) but manufacturing seems to be good enough nowadays that even Chinese companies can make decent AK-prisms like in the GPO 8x56, Kite 8x56 and probably all the DDoptics "Pirschler" models (if those were made in Japan, I think they'd mention it).
So AK-prisms are cheaper because they don't need the mirror coatings -- that's what I gathered from reading about them at least. So not sure it's true.
A number of German companies used them BTW -- like Hensoldt and Optolyth (so it wasn't just Zeiss). One of my aunts still has an old AK-prism Hensoldt "Dialyt" 8x56 and it is pretty good -- fantastic edge sharpness, it demolished my Kite Cervus 8x56 -- the Kite is brighter of course because of the modern coatings.

 

[tenex]

I guess we'll never know why Zeiss went with SP's in the SF line.

It certainly puzzled me, but perhaps not surprising having hired former Swaro designers.

The consecutive model-series Victory I/II (composite frame) also had AK-prisms in the 40mm versions and they were very compact and light.

They sound very impressive but I never tried them, or followed why they weren't hugely successful. I've gathered that they were Zeiss's transition away from leaded glass and they didn't get it quite right the first time?

So AK-prisms are cheaper because they don't need the mirror coatings -- that's what I gathered from reading about them at least. So not sure it's true.

And not just mirror coatings, but others that allow the surface to transmit in the other direction as well. So it would make sense if AK were less costly.

 

[[email protected]]

I am sure Zeiss went with the SP instead of the AK prism in the SF line to keep the size smaller and more compact. The WA in the SF already required a big eyepiece. The FL 8x42 had AK prisms and the FL 8x32 had SP prisms, most likely because of the more compact size of the FL 8x32. The transmission of the FL 8x32 and FL 8x42 were almost identical, even with the different prisms. Zeiss told me that AK prisms were less expensive than SP, so you are correct. Also, another disadvantage of AK prism versus SP is they have more CA, so an AK binocular has to be longer to compensate for that.