TIR, prism choice and its effect on image quality (1 Viewer)

[hopster]

I have become interested in the effects of prisms on image quality for the very best binoculars, and developed a liking for prisms that feature Total Internal Reflection (TIR) like Porro and Abbe-Koenig over the increasingly ubiquitous Schmidt-Pechan types used in most roof optics. I seem to see a better transparency, more 'immediacy' in the image and also possibly a better ability to differentiate subtle contrast differences. I know that glass and coating quality and other aspects of the design are also important but this does appear to be a pattern in the binoculars that I have looked through.

From a physics or signal processing perspective one can wonder what happens to the energy that is lost or dissipated in a prism with lower transmission than one with TIR (perhaps ~5% of all the energy entering the prism). In relation to this point, usually only brightness is considered but I believe there may be other more subtle effects on image quality. Interested in the thoughts of others.

I am also curious why alternative TIR prisms are not used often (or even at all) by binocular manufacturers. With fully sealed roof prisms binoculars that use internal focusing I would imagine that both of these would be more compact than Abbe-Koenig which will always tend to increase the length and weight of the system:

Abbe-Porro

Perger

I'm sure there are others which the experts here will know about.

​

 

[Canip]

one can wonder what happens to the energy that is lost

Recalling my physics lessons, I think energy can never be „lost“, only transformed into other forms of energy🧐

 

[Canip]

one can wonder what happens to the energy that is lost

Recalling my physics lessons, I think energy can never be „lost“, only transformed into other forms of energy

I seem to see a better transparency, more 'immediacy' in the image and also possibly a better ability to differentiate subtle contrast differences.

If you can see this kind of differences, your eyesight must be superb, or at least miles better than mine.

 

[John A Roberts]

Some context to help others follow what’s being discussed . . .

For a quick overview of prisms, see posts #28, 32 and 33 at: New Horizons II

Prisms with Total Internal Reflection, include:
• the various non-roofed Porro and Perger prisms, and;
• the roofed Abbe-Koening, along with the more obscure Hensoldt Penta and Leman-Sprenger prisms.


As Canip has previously pointed out, Leica currently has exclusive rights to Andreas Perger’s prism designs,
see post #10 at: Perger Prisms


The Abbe-Porro prism is another name for the Porro Type II prism, see the Abbe patent in post #30 in the first link above.
And Canon currently uses Porro Type II prisms in its image stabilised binoculars, see two examples in post #4 at: High end porro's


John

 

[hopster]

Recalling my physics lessons, I think energy can never be „lost“, only transformed into other forms of energy

Well indeed, so then transformed into what?

One could suggest heat but after all this is just another EM wavelength beyond red and who knows how this - or whatever else is generated - interferes with the optical spectrum.

If you can see this kind of differences, your eyesight must be superb, or at least miles better than mine.

I'm not claiming superb eyesight but I think I do see this consistently.

 

[Lerxst]

For those interested in a derivation of roof prism image degradation and its solution, I recently wrote an article on this, located here:

The Physics of Roof Prisms and Phase Coatings - Michael Hurben, PhD

I've written a technical article on the topic of why the roof prisms used in binoculars require specialized coatings in order to improve their performance. I was prompted to pull this together because I couldn't find a single source that traced through this interesting problem in a non-trivial...

legallyblindbirding.net

 

[hopster]

For those interested in a derivation of roof prism image degradation and its solution, I recently wrote an article on this, located here:

The Physics of Roof Prisms and Phase Coatings - Michael Hurben, PhD

I've written a technical article on the topic of why the roof prisms used in binoculars require specialized coatings in order to improve their performance. I was prompted to pull this together because I couldn't find a single source that traced through this interesting problem in a non-trivial...

legallyblindbirding.net

Thanks for this, I have downloaded the PDF and will have a look. I am also mathematically trained so it should make sense and hopefully put into objective terms some of the things that I believe I perceive, though that is not always possible as I have experienced at length in the world of audio recording and reproduction when I worked in that field.

 

[John A Roberts]

Just to be clear, the lack of TIR of some roof prism surfaces (the issue raised by hopster),
and the de-phasing resulting from the two roof surfaces (the main topic of Lerxst’s paper), are separate matters *

There is also the complication that some roof prism surfaces perform both transmission and reflection functions,
and so require a compromise coating e.g. with Schmidt-Pechan and Uppendahl prisms.
See post #39 at: A comparison of prism types, and why the Schmidt Pechan seems to have won the premium binoculars


John


* Perhaps of interest to some, the 1988 paper by Weyrauch and Dörband of Zeiss that heralded the introduction of phase coating
on consumer optics, is linked to in posts #228 and 229 at: Let's talk PORROS!

 

[hopster]

Just to be clear, the lack of TIR of some roof prism surfaces (the issue raised by hopster),
and the de-phasing resulting from the two roof surfaces (the main topic of Lerxst’s paper), are separate matters *

There is also the complication that some roof prism surfaces perform both transmission and reflection functions,
and so require a compromise coating e.g. with Schmidt-Pechan and Uppendahl prisms.
See post #39 at: A comparison of prism types, and why the Schmidt Pechan seems to have won the premium binoculars


John


* Perhaps of interest to some, the 1988 paper by Weyrauch and Dörband of Zeiss that heralded the introduction of phase coating
on consumer optics, is linked to in posts #228 and 229 at: Let's talk PORROS!

Lots more interesting reading here. I think John's second point is the one that sticks in my mind.

Need to note also that in point 2 of post #39 that he refers to on the other thread, Porros require no phase coating whereas even with TIR Abbe-Koenigs do. This would appear to leave Porro and Perger as the gold standard or reference to which others should be compared.

 

[Binastro]

There are transmission losses in glass depending on glass thickness, but also the quality of the glass.

The Barr and Stroud binoculars were notorious for rather dirty glass.
I think that the best Scottish sand was not as good as Wetzlar sourced sand.

Ross also used British glass, but it was better quality.

I have two 15x60 Bresser Porro binoculars,, I think, Chinese binoculars where the glass is grey. Just dirty glass with unwanted impurities.
Besides very poor coatings.
The Minolta 12x50 Activa shows stars about as faint as the 15x60 Bressers.

As to energy visible light loss going in to heat or infra red, I don't know if the infra red affects visible light.

Also the blue and red light comes to different focus points. I am not sure how this interacts.

As to TIR, is there really 100% reflection?
How flat are the surfaces?
How smooth are the surfaces?
Some USSR prisms in simple Porro binoculars were measured by my professional friends as 1/20 wave.
How good are old Japanese surfaces?
Is there a loss at 1/20 wave?
The best measured surfaces I have seen mentioned are around 1/40 wave, when it becomes difficult to measure.

Horace Dall was able to measure curves to 1/40 wave.

In addition some 10x50s are 10x45 and some 60mm binoculars are actually 52mm.
The Optolyth 12x50 is 12x42.

Regards,
B.

 

[Hermann]

Porros require no phase coating whereas even with TIR Abbe-Koenigs do. This would appear to leave Porro and Perger as the gold standard or reference to which others should be compared.

Absolutely. And it's a shame there are only a very few high-quality porros made anymore.

Hermann

 

[hopster]

Absolutely. And it's a shame there are only a very few high-quality porros made anymore.

Hermann

I tried the SW Habicht 10x40 today and liked the image a lot (extra sparkle, 3D and transparency compared to two other very good 10x42 roofs) despite a bit of softness and CA near the edges. It's also smaller and lighter than I imagined and I would seriously consider it as my main 10x optic if it wasn't for the small and very stiff focus wheel which I think would be a pain in everyday use.

 

[Hermann]

I tried the SW Habicht 10x40 today and liked the image a lot (extra sparkle, 3D and transparency compared to two other very good 10x42 roofs) despite a bit of softness and CA near the edges. It's also smaller and lighter than I imagined and I would seriously consider it as my main 10x optic if it wasn't for the small and very stiff focus wheel which I think would be a pain in everyday use.

Try the Habicht 7x42.

Hermann

 

[hopster]

Try the Habicht 7x42.

Hermann

I would like to but not easy to find in the shops. Is the focuser easier to turn?

Also the AFOV sounds very narrow - too much of a compromise maybe.

M

 

[[email protected]]

Absolutely. And it's a shame there are only a very few high-quality porros made anymore.

Hermann

I agree with you Herman and that seldom happens! A big prism porro is the best prism you can find. They are better than an AK, Perger, Uppendahl or an SP. Many roof prism binoculars in an effort to make them small and compact like the Kowa Genesis 8x33 have prisms that are too small, making the view truncated.

 

[[email protected]]

I tried the SW Habicht 10x40 today and liked the image a lot (extra sparkle, 3D and transparency compared to two other very good 10x42 roofs) despite a bit of softness and CA near the edges. It's also smaller and lighter than I imagined and I would seriously consider it as my main 10x optic if it wasn't for the small and very stiff focus wheel which I think would be a pain in everyday use.

Exactly. You need a vise grip to turn it, but not too many people that own them will admit it. The focuser is precise, though.

 

[[email protected]]

I would like to but not easy to find in the shops. Is the focuser easier to turn?

Also the AFOV sounds very narrow - too much of a compromise maybe.

M

No, the focuser is just as tight, and the AFOV is probably the narrowest you will find in any binocular in existence. A 45 degree AFOV, REALLY? They are like looking down a soda straw or like entering a mountain tunnel that is 3 miles long, but they are very bright. One of the brightest binoculars around.

 

[hopster]

Exactly. You need a vise grip to turn it, but not too many people that own them will admit it. The focuser is precise, though.

I have heard that they loosen up with use. By how much?

 

[Thotmosis]

Having fun with my Porro’s in Sweden

 

[[email protected]]

I have heard that they loosen up with use. By how much?

None, that I noticed, and I sat and watched TV and turned the focuser lock to lock about 1000 times. I still needed a vise grip to turn it.