A comparison of prism types, and why the Schmidt Pechan seems to have won the premium binoculars (3 Viewers)

[kimmik]

Ludewig prism used in the Leitz Amplivid

Ah yes thats an interesting name. I see it shows up on google patents. The roof section is called Amici prism, so I guess the combo could be called Ludewig-Amici.

Can you provide some more info, images

Will PM you shortly.

How do you figure that so many surfaces of SP or U prisms (the ones that actually do have TIR) have AR coatings

Using the prism diagram - observe which surfaces perform both reflection and transmission.

The S-P image is a well known one from Swarovski.

The funny thing is, that SP prism is drawn wrong . I only just figured out why - they put the roof section on the Pechan half, when it should be on the Schmidt half.

John has corrected me - there are different variants of the SP prism, and roof can be placed on either the Schmidt or the Pechan half of the assembly. It seems Schmidt roof is the more common configuration.

Would you be able to show the difference between a Porro 1 and a Porro 2 binocular?

Hmm no longer able to edit original post.

I suppose the difference would be 2 additional pure transmission surfaces, unless it gets all cemented together. Slightly more compact than the regular porro.

"Casing in fibre glass-reinforced polymer for exceptional ruggedness and minimum weight"

Maybe FL will turn out longer lasting than the the ones that came before it.

 

[Vespobuteo]

The "Zeiss AK-prisms in 40-49mm bins"-list, so far.

1. 7x42 Dialyt BGAT 1981-2004
2. 7x45 Design Col. 1994-2000
3 8/10x40 Victory I, II 2000-2004
4. 7/8/10x42 FL 2004-2013
5. Conquest 10x40 T* 2005-2012 (made in Hungary)
6. 8/10x42 HT 2012-2018?

HT42 was the end of an almost 40 year old era it seems.

Who wants an SF(L) 8.5x48mm with AK-prisms, 96% transmission at 750 grams?

I think there might be a market...

Note that the latest 8/10x42 Victory RF (2018 - ) do use AK-prisms.
But they come with a premium price and the RF-feature usually reduce transmission a bit.

Range finders seem to be the area to try different prism-types.

In the current models from the "big 3" we have Perger, AK, SP and now Uppendahl in the latest Leica R.

 

[kimmik]

Zeiss uses the conventional Pechan first configuration, but has two internal prism baffles, and places them at non-conventional locations - 1 at the mirror surface of the Pechan, and 2 at the first reflection of the Schmidt.

Can you provide some more info

My best attempt at illustrating the Zeiss SF32 baffle and conventional SP baffle.

 

[William Lewis]

Maybe a bit of baffling and some other coatings is what's helping with the seil effect Holger mentioned here -

Nikon told me that with SP prisms they won't get the outstanding resolution they intended to achieve when designing the WX. I believe that this could be due to the 'Seil-effect' (see the attached pages, taken from Konrad Seil, Progress in binocular design, Proc. of SPIE Vol. 1533 (1991)). Swarovski designer Konrad Seil took several SP prisms, one of them uncoated, a single-layer coated, a double-layer coated and a multi-coated (8 layers) prism, and measured their MTF-curves. MTF values improved from uncoated to single-coated, but then they dropped with increasing number of layers. A multi-coated SP-prism showed rather poor resolution values!

The reason is that surfaces at which total internal reflection (TIF) takes place should best remain uncoated. Unfortunately, the SP-prism contains two surfaces which serve as TIF surfaces and at the same time are entrance or exit surfaces. This is the dilemma with SP-prisms: Multi-coating reduces their resolution, single-coating reduces their transmission. With the AK prism, this dilemma does not occur.

Cheers,
Holger

 

[John A Roberts]

Hi kimmik (post #34),

Thanks for the Zeiss SF32 illustration. Until now I’d been confused by your reference to internal baffles.

Looking at the diagram, the light path runs from left to right. And the first baffle (sic) would be externally located, outside the mirror coated surface.
If you look through the objective, you’ll be able to see the front face of the prism, and perhaps part of the front edge of the external plate *

However, if you look through the front face of the prism at the mirrored surface, by definition you can’t see what’s behind the reflective coating.
So if you can see a circle of light with a black surround, then it’s a reflection of something further up the optical train.
In this instance it’s going to be the external baffle, shown outside the rear face of the second prism.


John


* A clarification (after a night's 🛌 and subconscious cogitation):
If there is a plate present, you may be able to see part of the front edge of it.

 

[tenex]

Maybe a bit of baffling and some other coatings is what's helping with the seil effect Holger mentioned here -

Unfortunately, the SP-prism contains two surfaces which serve as TIR surfaces and at the same time are entrance or exit surfaces.

Thanks, this is what I missed before, answering my question above. Now kimmik's numbers for "Imperfect TIR" add up for me.

 

[henry link]

Looking at the cutaway view of the Zeiss SF (right) and Swarovski NL below I see three baffles in both binoculars associated with the SP prisms: one at the entrance, one at the exit and one at the gap between the Schmidt and Bauerfeind prisms. I don't see a baffle on the mirror coated surface of the Schmidt prism.

 

[kimmik]

Hi Henry, correct. The SF42 and SF32 use different prism configurations.

For one, the SF42 puts the Schmidt first.

SF32, EL and NL puts Schmidt second.

SF42 prism seems to be same configuration as Noctivid. I don’t have SF42 to examine in detail, however. Seems there is very little in common between SF32 and SF42.

 

[Robert Moore]

The Swarovski in the image is EL.

 

[kimmik]

Regarding AK, one thing I don't understand, is why Nikon chose to use full glass, when the size means significant weight, and also unnecessary blue absorption. This diagram isn't nikon, but shows how significant the front half of AK is for weight.



.https://www.allbinos.com/174-binoculars_review-Carl_Zeiss_Dialyt_8x56_B_GA_T*_ClassiC.html

.https://imaging.nikon.com/lineup/sportoptics/wx/interview/p3/

Surely it is better to replace the front half, with a dielectric mirror? I can understand dielectric mirror being costly pre-2000, but in astronomy 90degree mirror has been dielectric for some time now, and they are big and affordable.

If the question is of collimation, the two AK halves are not always cemented anyway, nor is SP prism. So one mirror and one prism should be as easily aligned and maintained, while saving weight and improving blue transmission.

The one downside of using mirror hybrid, is you lose the beautiful publicity shot above! a wimpy roof and a cheap looking mirror haha.

And at 1kg of prisms for the two barrels, changing over to mirror could save at least 0.5kg from the overall bino weight, ie sub 2kg WX.

 

[Patudo]

Right, so the folks at Nikon went for an all glass prism system to get a "beautiful publicity shot". Haha indeed.

 

[John A Roberts]

Hi kimmik (post #50),

Zeiss introduced the use of dielectric coatings on the prism surfaces in binoculars in 1999, and others quickly followed.
So as you indicate, that long predates the introduction of the Nikon WX in 2017.

And among others, Zeiss and Swarovski have also introduced new A-K prism binoculars since 1999.
Since none have gone to a hybrid external mirror/ prism combination, the implication is that there's at least one Alchilles' heel to such an approach.

Mounting a mirror so that it's clamped sufficiently secure (to be able to resist the shock forces that a handheld binocular may be subject to)
but not overly stressed, may be a factor.


John

 

[William Lewis]

This could be said for all prism surfaces, I think it's the quality of the mirrors that's not up to the job or would be too expensive to make well enough to minimise refraction.

They also tend to degrade over time more than a lump of glass and would be tricky to keep in alignment as you and John have mentioned.

Collimation with mirrored optics isn't quite as much of a problem as is made out but I'm talking telescopes here, my dob doesn't go out of whack too quickly but then I'm not yomping up a hill with it dangling off my neck.

Also it would at least double the amount of separate elements than a prism needs, Porro's - 4 prisms in 2 lumps or 8 separate mirrors. Then there's keeping the surfaces spotlessly clean and dead flat and the gaps between them dust free etc etc. Still wouldn't mind seeing a modern amplivid though...

And as you say prisms look better!

 

[tenex]

Seems there is very little in common between SF32 and SF42.

Indeed. SF32 even focuses with a moving objective. No wonder there was such a delay.

 

[Zedster]

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.....

having worked at an optics engineering and manufacturing firm as a quality engineer I can tell you that magnesium is VERY difficult to work with. Expensive and finicky. It also has issues with fasteners and surface coating. Aluminum is the ideal though heavy metal. Plastics are the real star. Lighter and much more adaptable. I’m surprised that Zeiss gave up on the composite route. There is definitely a learning curve to designs based on composites but when you nail it down it’s worth it

 

[kimmik]

Right, so the folks at Nikon went for an all glass prism system to get a "beautiful publicity shot". Haha indeed.

Considering the decorative spoilers and aero vents in sports cars, I wouldn’t rule out this possibility lol.

Halo products are made to excite people not to be practical.

WX has one heck of a prism

 

[Zedster]

Hi kimmik (post #50),

Zeiss introduced the use of dielectric coatings on the prism surfaces in binoculars in 1999, and others quickly followed.
So as you indicate, that long predates the introduction of the Nikon WX in 2017.

And among others, Zeiss and Swarovski have also introduced new A-K prism binoculars since 1999.
Since none have gone to a hybrid external mirror/ prism combination, the implication is that there's at least one Alchilles' heel to such an approach.

Mounting a mirror so that it's clamped sufficiently secure (to be able to resist the shock forces that a handheld binocular may be subject to)
but not overly stressed, may be a factor.


John

Now I've never taken one apart but I would be very surprised if they were not all glued into place. Certainly that's the way I've seen optics assembled in my work. The one I worked with that was clamped had a nasty tendency to go out of alignment.

 

[Binastro]

Mirrors have to be an optimum thickness otherwise they sag and deform.

So an optical mirror may not be very thin.

My 14.5 inch Newtonian as bought secondhand had rather poor optics.

Hysom gave me a very reasonable quote on refiguring the primary and making a high quality secondary flat mirror, as the original mirror was too thin.
The resulting optics were exceptional.

Not sure if the diagonal was glued but probably not as both primary and diagonal could be collimated.

I gave the telescope to a friend who had bought a noted astronomer's house complete with dome in a very dark Wiltshire village.

When I delivered the Zeiss 120cm f/7 triplet on a late night 130 mile trip and after tea and conversation, I sat outside the noted astronomer's house for an hour looking at the Milky Way and naked eye clusters.
He was surprised to see me still there.
It was very dark.

The Zeiss was used for photographing comets on 5x4 inch film.

Then at 1 a.m. I drove the 130 miles home. I loved night driving on empty roads.

The Yukon 6-25-100x100 spotting scope and 30x50 binocular probably have glued mirrors.
The mirrors work well, although I suspect they are only selected float glass with not very bright mirror surfaces.

Regards,
B.

 

[Dennis Mau]

Discussing prism types is an interesting subject, but the SP prism has won in the marketplace and all the top birding binoculars all use them, so really the point is mute. Ultimately, the manufacturers know what is best, and I don't see any of them changing back to an AK or some other prism type except for some specialty binocular like the Zeiss HT. The SP prism is the future of modern birding binoculars. Learn to love them, or get a porro prism.

 

[kimmik]

SP seems to come in many flavours, which impact the image but usually go unadvertised.

The two main types seem to be Schmidt first vs Pechan first.

Pechan first seems more common, more compact, and wider FOV, with lower contrast.

Schmidt first has higher contrast, but narrower FOV, heavier due to the need for physically larger prism. Possibly sharper.