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Do you like the VIEW better through a Porro or a Roof prism binocular? (1 Viewer)

tenex

reality-based
You know, we all love using binoculars, and I actually enjoy Dennis's enthusiasm and appreciation for them, even as an eternally serial phenomenon. The trouble arises when each bin of the moment has to be justified as the best ever, and any scrap of text on the Internet is summoned to do that. I accept that this must be what goes on all the time in his own head, but like many of us I don't enjoy being subjected to it and think it really doesn't need to be shared. Perhaps that's useful to say.
 

elkcub

Silicon Valley, California
United States
RE: Posts 62 and 63.

One of the biggest advantages of Porro prisms over roof prisms is field illumination, and it is not talked about much, but it makes a big difference.

"One of the biggest differences between porros and roofs is that roofs lose full illumination very quickly once you move away from dead center. Average porros have a fully illuminated circle 20%-30% of fov. Best porros have a fully illuminated circle 50% of fov. Average roofs have a fully illuminated circle 5%-15%. The Best roofs have a fully illuminated circle 20% of fov or less. So while transmission dead center may appear to vary by only a few %, move 30-40% off axis and transmission might appear to vary easily by 10% or more. You can test this on a deep star field. Set up two equal sized binoculars, one roof and one porro, and observe for deepest stars seen at center and then deepest stars seen at 30%-50%-70% off axis. By 70% out aberrations will come into play, but in the 30%-50% area few aberrations exist in most binoculars and therefore the differences you see will better represent only a loss of illumination.
Deep star maps (M45, Cr399) are available in the Best of Threads on Limiting Magnitude."

"Edz, you are spot on. When first comparing my Leica roofs to my Fujinon 10x50's under very clear dark Austrian skies, I was shocked to see the difference in off-axis illumination between them. It was the most dramatic difference between the two. Second was size. On axis sharpness was comparable, while the Fuji's of course had superior edge of field correction, but that is not an attribute of the prisms."

I've attached a section from Freeman and Hull's book "Optics" that graphically explains how vignetting occurs in telescopes.* Starting at the bottom of pg. 153 the authors make clear that it is the locations of field and aperture stops which determine vignetting over the field of view, and that these are design variables. The particular type of inverting prism used in the instrument simply isn't mentioned or optically relevant (which is Henry's point).

It should also be pointed out that EdZ's conclusions were based on astro (scotopic) observations when his eye was dark adapted and retinal light sensitivity was nine orders of magnitude greater. That's arguably not applicable to daylight (photopic) observation.

Ed
* Using Acrobat Reader pls. rotate the view 90 deg. clockwise.
 

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Patudo

Well-known member
In the grand scheme of things arguing over optics is not really worth getting wound up over.

That's what rational folks like yourself think (and quite rightly so). Yet few things better prove "Sayre's Law" ("In any dispute the intensity of feeling is inversely proportional to the value of the issues at stake") than supposedly rational men (and yes, it is us men for the most part) discussing golf/fishing/cars/optics on the internet...
 

[email protected]

Well-known member
Supporter
RE: Posts 62 and 63.



I've attached a section from Freeman and Hull's book "Optics" that graphically explains how vignetting occurs in telescopes.* Starting at the bottom of pg. 153 the authors make clear that it is the locations of field and aperture stops which determine vignetting over the field of view, and that these are design variables. The particular type of inverting prism used in the instrument simply isn't mentioned or optically relevant (which is Henry's point).

It should also be pointed out that EdZ's observations were based on astro (scotopic) observations when his eye was dark adapted and retinal light sensitivity was nine orders of magnitude greater. That's arguably not applicable to daylight (photopic) observation.

Ed
* Using Acrobat Reader pls. rotate the view 90 deg. clockwise.
EdZ came to the conclusion that Porros have better field illumination than roofs by observing deep star fields with a Porro and a roof prism and watching for fall off in the outer FOV. Explain to me how it wouldn't be applicable to daylight observation. It may not be as apparent, but it would still be there to some degree. Anyway it would make the Porro a much better astro instrument even if you couldn't see the difference in daylight because in the outer FOV you are going to go much deeper into the sky. In Freeman and Hull's book the prisms weren't mentioned because they didn't specifically look at them or made no mention of them. The night sky is an excellent way to test for illumination and transmission performance of binoculars because many times a higher performing binocular of the same aperture will go a .5 magnitude to 1 magnitude deeper on star fields. If you don't believe it try EdZ's test on a deep star field as outlined in the next paragraph with a Porro prism binocular and a roof prism binocular.

"One of the biggest differences between porros and roofs is that roofs lose full illumination very quickly once you move away from dead center. Average porros have a fully illuminated circle 20%-30% of fov. Best porros have a fully illuminated circle 50% of fov. Average roofs have a fully illuminated circle 5%-15%. The Best roofs have a fully illuminated circle 20% of fov or less. So while transmission dead center may appear to vary by only a few %, move 30-40% off axis and transmission might appear to vary easily by 10% or more. You can test this on a deep star field. Set up two equal sized binoculars, one roof and one porro, and observe for deepest stars seen at center and then deepest stars seen at 30%-50%-70% off axis. By 70% out aberrations will come into play, but in the 30%-50% area few aberrations exist in most binoculars and therefore the differences you see will better represent only a loss of illumination."

"When first comparing my Leica roofs to my Fujinon 10x50's under very clear dark Austrian skies, I was shocked to see the difference in off-axis illumination between them. It was the most dramatic difference between the two. Second was size. On axis sharpness was comparable, while the Fuji's of course had superior edge of field correction, but that is not an attribute of the prisms."

"I'm sure it's been posted here before, but what is the reason for this rapid drop off in illumination in even high-end roofs?"

"I think you'd have to have a layout of the design of the binocular, and ray trace it, to answer Rich's question with certainty. But, something in the path is more restrictive in the case of roofs, and I suspect that is not a physical necessity, but a design compromise made to achieve the slimmest and lightest weight final product, with a field that darkens so gradually away from center that most users don't notice it. Looking at the exit pupil of my Leica held out at arm's length, as I point it around I would say it starts to shrink about 20% of the way to the edge, but by the very edge, is a skinny football (US gridiron style) with only a quarter of the area of the circle seen at midfield. The funny thing is, I never notice this effect which might seem to be so drastic. That is probably because when hand holding, I don't really pay any attention to the view outside the central third. Good thing, since the Leica goes quite soft out there anyway!(It could also be that when viewing the center with direct vision, the outfield is viewed with averted vision, whose high sensitivity may smooth out the apparent field illumination). With a mounted astronomy binocular, you really are looking critically all over the field of view. Good field illumination becomes very important, and the prisms, etc., should be made as big as necessary to achieve that. Edge sharpness is also very important in that setting."
 
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henry link

Well-known member
Dennis, once again in post #85 you are regurgitating other people's work with no proper attribution or link to the original source.

EdZ (the first paragraph) makes the error of jumping to the conclusion that a performance difference between two binoculars can only be explained by the one design difference he happens to be aware of.

The second unidentified author is much more on the right track when he speculates that undersized prisms, not the prism type, are the cause (or more correctly undersized internal stops associated with undersized prisms).

In the real world the off-axis vignetting experienced by the eye can be quite different in daylight compared to what is experienced when observing a dark sky. That's because the cat's eye shape of a vignetted off-axis exit pupil still may have a minor axis that is wider than the pupil diameter of eye in daylight. In that case the eye experiences the full effect of the vignetting in the dark, but no vignetting at all in daylight.

Personally, I've had enough of this. I'll be happy to let others deal with Dennis on this subject from now on.
 
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WJC

Well-known member
Dennis, once again in post #85 you are regurgitating other people's work with no proper attribution or link to the original source.

EdZ (the first two paragraphs) makes the elementary error of jumping to the conclusion that a performance difference between two binoculars can only be caused by the one design difference he happens to be aware of.

The second unidentified author is much more on the right track when he speculates that undersized prisms, not the prism type, are the cause (or more correctly undersized internal stops associated with undersized prisms).

In the real world the off-axis vignetting experienced by the eye can be quite different in daylight compared to what is experienced when observing a dark sky. That's because the cat's eye shape of a vignetted off-axis exit pupil still may have a minor axis that is wider than the pupil diameter of eye in daylight. In that case the eye experiences the full effect of the vignetting in the dark, but no vignetting at all in daylight.

Personally, I've had enough of this. I'll be happy to let others deal with Dennis on this subject from now on.
So, you're not into lost causes? Coward! The way I put it: Trying to teach some people is like trying to push Gibraltar uphill ... with one hand ... at night ... without a flashlight ... in a gale ... while walking backwards and wearing skates.
 

elkcub

Silicon Valley, California
United States
EdZ came to the conclusion that Porros have better field illumination than roofs by observing deep star fields with a Porro and a roof prism and watching for fall off in the outer FOV. Explain to me how it wouldn't be applicable to daylight observation. It may not be as apparent, but it would still be there to some degree.

Dennis,

The human eye adapts to darkness by increasing the sensitivity of rod cells located throughout the retina. This can amount to an increase of 10^9 = 1,000,000,000 = one billion! Without such an enormous increase in sensitivity the eye would be completely blind, i.e., lacking the ability to perceive, the gradient under discussion. So, in daylight, the question is analogous to whether a tree that falls in the forest makes a sound if no one is there to hear it.

The eye/brain has very different perceptual capabilities depending on its state of light/dark adaptation.

Ed
 
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Patudo

Well-known member
The human eye adapts to darkness by increasing the sensitivity of rod cells located throughout the retina. This can amount to an increase of 10^9 = 1,000,000,000 = one billion! Without such an enormous increase in sensitivity the eye would be completely blind, i.e., lacking the ability to perceive, the gradient under discussion. So, in daylight, the question is analogous to whether a tree that falls in the forest makes a sound if no one is there to hear it.

The eye/brain has very different perceptual capabilities depending on its state of light/dark adaptation.

Typo, I think, has argued based on the above that differences in brightness between binoculars are effectively inconsequential, as the eye simply adapts (pardon me if I have got the gist of his argument wrong). His logic seems faultless, but I can't deny that when using some of my older single-coated binoculars in poor light I do notice the lack of light transmission, and wish they could be brighter.
 

WJC

Well-known member
Dennis,

The human eye adapts to darkness by increasing the sensitivity of rod cells located throughout the retina. This can amount to an increase of 10^9 = 1,000,000,000 = one billion! Without such an enormous increase in sensitivity the eye would be completely blind, i.e., lacking the ability to perceive, the gradient under discussion. So, in daylight, the question is analogous to whether a tree that falls in the forest makes a sound if no one is there to hear it.

The eye/brain has very different perceptual capabilities depending on its state of light/dark adaptation.

Ed
Dennis,

You keep quoting EdZ, and while there is no doubt he is a caring and knowledgeable in useful optical realities, his target audience was amateur astronomers, not birders. In addition, you speak of performance at the edge of the field. My understanding, from years as an amateur astronomer and telescope maker, is that a 40% drop-off in brightness at the edge of the field—at least in telescopes—will go undetected by MOST observers, owing to our scotopic (low-light) vision being most sensitive peripherally.

[I have not put this to a test. Thus, nullius in verba, as Ed says. But I am eager to learn. So, if what I have said is not accurate, please Ed, straighten my thinking.]

Finally, on the subject of learning: “Reach before you teach.”

Dennis, of late you keep being corrected by those who know considerably more than you about so many points of optical understanding. Thus, if your motivation on Birdforum is not to just to stay in the limelight by stirring the pot of erroneous and time-wasting controversy, you should probably spend 10% as much time in studying optical realities from classical sources as you do pontificating from the book, Hearsay Perfectica.

You certainly have the right to say whatever you want to and believe whatever you want to. When I was 12, I loved comic books. Yet, at no time did I find them relating to reality. For a time, you’ve caught so many in your web. But I think most members of Birdforum are getting overloaded by all you don’t know about optics and how you persist in trying to elevate your opinions to the level of engineers, opticians, and professional technicians. Maybe continual ego strokes are all you’re after. But if you really want to be seen as some kind of authority, you’ll need to change your direction. And the more you dispute competent practitioners, the deeper the hole you dig that’s keeping others—except the newbies—from conversing with you at all.

The first attachment illustrates the difference between photopic and scotopic vision. The second attachment is a simple spot diagram of the Cook-Houghton telescope. It was a file packaged with Zemax-EE, today the world’s most widely used optical design software.



Bill
 

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