Originally Posted by Kevin Purcell
This is an interesting optical illusion that I've seen a few times. It's very vivid.
I think the issue here is where the final aperature that just matches the vignetting aperture.
I've even seen this effect on "narrow" field bin with pop up eyecups. If you set the eyecups just right (just about vignetting the FOV) it appeared a lot bigger.
This points to the perceived AFOV being a perceptual issue which rather puts it beyond objective measure. A bit like the "roof magnification illusion" where the magnification of bins appears to change depending on objective lens separation.
That's why it's better to use my (birding) standard of 3m/10feet to infinity. Pretty much all bins can do that so you get the same measure for all bins.
There is a thread with such measurement I've made in it ... you could add to it.
If you search here you can also find the thread on why depth of field is only dependent on magnification. Some believe it and some don't.
I didn't know that you already had this idea of standard focus speed measurement! Now we're two!
I guess the construction of the optical system will have some impact on the DOF. The magnification has the most effect, though. A lesser magnification will also not magnify the blurred parts of the image, and this is why they appear sharper.
Probably the size of the exit pupil matters too, and possibly the range of usable eye-relief.
"The roof magnification illusion" partially comes from the offset of the bin's barrels (each tube will expand the lateral field of view as compared to a roof prism bin) , but also from the fact that a eyeball convergence is needed when you look at medium or close distance through a porro bin.