Roof Prisms Used in Binoculars (1 Viewer)

[WJC]

I disagre. It is a good idea! Schmidt-Pechan vs. Abbe-Koenig prisms. I have already some questions.

After seeing how much trouble I got into just trying to explain dioptric accommodation and how much verbiage was wasted, I would just say what I wanted to mention was how the two prisms got around spiking on bright objects, and that I’ve have never considered myself a prism expert. I’m just a noclar fixer with SOME engineering experience.

In looking through earlier productions of roof prism binoculars, one was prone to notice the “roof” in the field of view. My first roof prism binocular was a Swift 7x35 Tri-lite. For daylight observations, such as birdwatching, it performed fine optically and was structurally well-made.

And, concentrating on a target, and not the roofs, there was no problem for me as the roofs disappeared.

Having occasionally taken them under the stars, however, a disconcerting problem developed. In viewing a bright star, the roof would cause the star to “spike”* when the image of the star passed over the roof (see attached).

As time went on and roof prism instruments started taking over the binocular market, roof prism technology improved.

As it stands today, the Schmidt-Pechan is the most popular. It allows the binocular to be shorter and doesn’t place a roof edge in the middle of the field of view. HOWEVER, while the Abbe-König needs an anti-phase shifting coating in order to provide its best resolution, the Schmidt-Pechan needs two to best do its magic. But ... “The times, they are a changing.”

* This image is over-stated in that I couldn’t make the spikes small enough to represent a realistic image.

 

[John A Roberts]

Hi Ted (post #58) *

For a chronology and images of the different roof prism designs used in handheld binoculars, see posts #32 and 33 at: New Horizons II

An alternate set of images of the reflection paths by Stefan Emsel, can be found in posts #116 and 117 at: Zeiss: Collection of cross-section and cutaway images

And see post #122 and on for calculations about comparative differences between various types of prisms.


- - - -
Comparing Schmidt-Pechan to other roof prisms, the main attractions are:
• Physical compactness, allowing for a more compact binocular especially front-to-rear, and;
• Less volume, allowing for a slightly lighter binocular.


However, optically S-P prisms are the most compromised choice:
• As with many other roof prisms (notably excluding Abbe-Koening prisms), there’s one surface that does not provide Total Internal Reflection, and so it needs to be reflectively coated.
Traditionally aluminium or silver was used, but now typically many layers of narrow wave length dielectric coatings are used, to create a cumulative effect of total reflection e.g. the 30 layers of Swarobright indicated in the image below.

• Prisms can be made in several parts and then cemented together, often to create one unit (as with A-K’s). However, an S-P pair must be air-spaced to enable the simultaneous reflection and transmission functions of the 2 facing surfaces (see the second image below).
This dual function necessarily requires a compromise in coatings to enable the contradictory requirements.

• And a third S-P surface also performs a simultaneous reflection/ transmission function (see the last/ exit face shown in the second diagram).
For some more detail, including some history of the evolution of coatings for the three surfaces (from 1 to 3 layers as used by Swarovski), see post #62 at: Wish List...


- - - -
The two S-P diagrams demonstrate some other points of interest:
• The order of the prisms in the optical path can be reversed (in the first diagram, the light enters through the Schmidt prism) **

• The location of the two roofed surfaces can be changed (in the first diagram, the Pechan prism is roofed).

• The location of the non-TIR surface can also consequently be changed (in the first diagram, the Schmidt prism has the non-TIR surface).


In addition, while it’s generally considered that S-P prisms have no offset of the optical path (the light column remains on the same optical axis, before and after passing through the prisms), that’s not necessarily so. See post #5 at: [Tech] Are most/all roof binocular oculars and objectives collinear?

And S-P prisms can also be designed to include a significant amount of offset, see examples from Nikon and Pentax in posts #69 and 71 at: What is the SMALLEST non-Galilean bino you know of? - Page 3 - Binoculars - Cloudy Nights


John


* now at: Dioptre Setting: Fallacy and Fact

** and also see an image from Gijs comparing the order of the prisms in an 8x42 Swarovski EL SV with those in an 8x42 Zeiss SF; the units are from Jan's collection.

 

[Troubador]

I have moved these two posts from another thread to this one to allow discussion of this topic and make it easier to locate in future.

Lee
MODERATOR

 

[Tringa45]

As it stands today, the Schmidt-Pechan is the most popular. It allows the binocular to be shorter and doesn’t place a roof edge in the middle of the field of view. HOWEVER, while the Abbe-König needs an anti-phase shifting coating in order to provide its best resolution, the Schmidt-Pechan needs two to best do its magic. But ... “The times, they are a changing.”

The roof edge bisects the field of view in both Schmidt-Pechan and Abbe-König prisms.
In both prism types both roof surfaces require phase coating to reduce/eliminate iterference effects due to partial polarization and to optimize contrast.

John

 

[GrampaTom]

But ... “The times, they are a changing.”

Bill, tell us more, please....

 

[henry link]

The Uppendahl roof prism is is not quite dead yet. It's just barely hanging on in the Leica Geovid R models.

 

[Gijs van Ginkel]

Perhaps also the following prims are worth mentioning, source dr. Hans Seeger:
-1 Uppendahl roof prism used by Leitz
-2- Möller roof prism used by Möller, Wedel in its Tourox and Tourixem binoculars, a very efficient prism and as a result a higher light transmission. These prisms were also used by Zeiss in its Telita and Turita binoculars around 1927-1928.
-3- Leman roof prism also indicated as Spinger-Leman prism used in the Zeiss Teleplast from 1907 , the Zeiss Stenor from around 1914 and the ZEISS Stenotar from around 1928..
Gijs van Ginkel

 

[WJC]

Bill, tell us more, please....

hi, Tom,

That comment was a stand alone. Think of all the changes in the last 100 years! Have all been "UPGRADES"?! NO!!! But then, some people see every change as an upgrade, even when they MAY or MAY NOT be able to recognize a difference in optical PERFORMANCE.

 

[John A Roberts]

Hi Henry (post #6),

For completeness . . .
Leica also uses Uppendahl prisms in their line of 7x24 Rangemaster monoculars that have been in production since 2006.
See three images, with the first two showing the simple eyepiece.

In contrast, the earlier 7x21 RF monocular with a horizontally orientated body, seems to have used a modified version of the Leman prism
i.e. 4 reflections, with no crossing of the light path and significant offset to the optical axis *


- - - -
Hi Gijs (post #7),

Does Seeger indicate why he considers the Moller (and the two Zeiss Telita variants) very efficient and resulting in higher transmission?
All three have 6 reflections verses the 4 of the earlier Hensoldt Penta of which they are variations *

What the Moller/ Telita series does have is less axial offset than the Penta, and in turn less than the Leman
(the only other roof prism in common use at the time was the 4 reflection Abbe-Koenig, with either no or very little offset).


John


* see the images in the initial link in my previous post.

 

[Tringa45]

One of the often cited advantages of Abbe-König prisms is that they rely entirely on total internal reflection, whereas Schmidt-Pechan (and Uppendahl) prisms require one mirrored surface, which will provide 99% reflection over most of the visible spectrum at best.

However, I suspect that Abbe-König might offer a small additional advantage due to the obtuse angles of incidence for TIR that would allow the use of lower index glass in the prisms. The incident angles on Schmidt -Pechan prisms are quite acute and would require high index glass for TIR such as BAK4.

BK7 in the right application offers some advantage over BAK4. It not only has lower dispersion (an advantage for the designer) but also much better transmission at the blue end of the spectrum.

Here are some transmission figures I extracted from Schott's data sheets for 25 mm glass path at 400 nm. Transmission would fall accordingly with increased glass path.
N-BK7: 0,992, i.e. 0,8% absorption, N-BK7HT: 0,996, 0,4% absorption
N-BAK4: 0,980, 2% absorption, N-BAK4HT: 0,983, 1,7% absorption
It seems, btw, that all the hype about HT glass was rather exaggerated.

John

PS:- I have never understood why Baader Planetarium offer erecting prisms in BaK4 with a silver coating. As most astronomical scopes are "slower" than f/5 this would appear to be two belts plus braces (suspenders)!

 

[Gijs van Ginkel]

John, post 9,
Seeger describes the Möller prism as follows: Ein modifiziertes Pentaprisma mit Dachkante, das aus zwei mit einander verkitteten Teile besteht. 6 Reflexionen, keine verspiegelte Fläche. Nur der zentrale Lichtstrahl eingezeichnet, der direkt an die Dachkante fällt". So no mirror coating necessary. It was my conclusion that this can be the cause of a better light transmission, although the modern mirror coatings on the Schmidt-Pecan prisms are very efficient and will most likely take away the slight advantage of the Möller prism.
Gijs van Ginkel

 

[tenex]

In viewing a bright star, the roof would cause the star to “spike”* when the image of the star passed over the roof

So why was that, and how was the problem fixed? (I know about phase coatings but it's not obvious to me how this symptom would result from phase interference.)

BK7 in the right application offers some advantage over BAK4. It not only has lower dispersion (an advantage for the designer) but also much better transmission at the blue end of the spectrum.

Do we know whether BK7 is in fact used in Abbe-König bins like FL/HT or SLC 56? I've never seen that specified.

 

[WJC]

So why was that, and how was the problem fixed? (I know about phase coatings but it's not obvious to me how this symptom would result from phase interference.)


Do we know whether BK7 is in fact used in Abbe-König bins like FL/HT or SLC 56? I've never seen that specified.

It was MOSTLY fixed with improvements in coating technology. However, the P* coatings are to synchronize the light paths split by the roof. However, the spiking in earlier roof-prisms was caused by the apex of the roof on Abbe-Konig prisms being not as SHARP as it later became and the light from a bright star traveled along its edge. It was NOT a problem for dimmer stars or in binos using Schmidt-Pechan prisms. But it was a killer in the otherwise great Swift Trilyte.

 

[John A Roberts]

The images that I attached to post #9, seem to have rearranged themselves (sic) into a different order,
one that doesn't correspond with the text!

The correct order is:



As can be seen the order has been reversed. The same thing recently happened with a post in a Leica thread, though the effect was less significant.


John

 

[Tringa45]

Do we know whether BK7 is in fact used in Abbe-König bins like FL/HT or SLC 56? I've never seen that specified.

No, just pure speculation.
In a Porro binocular rays parallel to the optical axis would meet the reflecting prism surfaces at 45°. Holger Merlitz calculated that there would still be TIR in BK7 at 5,7° less than this. This puts the limit for Porro bins at about f/5 to avoid vignetting, inadequate for most. BaK4 would cover focal ratios down to f/3,3.
However, angles of incidence in Abbe-König prisms would appear to be significantly greater than 45° with the possible exception of the roof surfaces, so I guess BK7 would be feasible.

John

 

[Tringa45]

It was MOSTLY fixed with improvements in coating technology. However, the P* coatings are to synchronize the light paths split by the roof. However, the spiking in earlier roof-prisms was caused by the apex of the roof on Abbe-Konig prisms being not as SHARP as it later became and the light from a bright star traveled along its edge. It was NOT a problem for dimmer stars or in binos using Schmidt-Pechan prisms. But it was a killer in the otherwise great Swift Trilyte.

Bill,
Spiking can be seen in many S-P prism binoculars just as in examples with A-K prisms. Sometimes differences can be seen in left and right barrels, but I have the impression that Swarovski, at least, have improved their manufacturing precision in recent years and it's barely detectable on a couple of my bins.
That link, btw, is incorrect. Phase coating is applied to both roof surfaces. I have a phase coated prism, which was given to me a few years ago by a Leica representative on their stand at Photokina. P-coating can be seen on both roof surfaces.

John

 

[WJC]

Bill,
Spiking can be seen in many S-P prism binoculars just as in examples with A-K prisms. Sometimes differences can be seen in left and right barrels, but I have the impression that Swarovski, at least, have improved their manufacturing precision in recent years and it's barely detectable on a couple of my bins.
That link, btw, is incorrect. Phase coating is applied to both roof surfaces. I have a phase coated prism, which was given to me a few years ago by a Leica representative on their stand at Photokina a few years ago. P-coating can be seen on both roof surfaces.

John

Hi, John,

What link are you speaking of?

 

[Tringa45]

Hi, John,

What link are you speaking of?

Hi Bill,
The one from "Best Binocular Reviews".

John

 

[WJC]

Hi Bill,
The one from "Best Binocular Reviews".

John

I assume you are speaking of the one in post #13. Certainly my fault; I should have grabbed one fro a little farther up the optical food chain. But for myself: I have seen the problem in older Abbe-Konig, but not Schmidt-Pechan. Admittedly, I am a Porro guy. Consider it deleted as it will be in less than 10 seconds. Oops, I guess I can't! All I can do is fall on my sword!

 

[Ted Y.]

Some binocular models are presented as having a bigger prism than the standard, therefore a better image. Is this a marketing thing only?