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."
In the grand scheme of things arguing over optics is not really worth getting wound up over.
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.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.
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.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.
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.
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.
Dennis,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