It's not the quality of the target that I'm asking about, it's the distance. Is there a difference if the target is ten feet or 100 feet away? If so, what causes that difference....c'mon Brock....
Your eyes.
A long time ago when I was born a little bitty baby in the same hospital as John Travolta, just a few months apart, my entrance pupil opened to 8mm (particularly when big busted nurses passed by
, and if somebody had thought to hand me a pair of IF EP bins with a very narrow IPD range, I could "set and forget".
Now that I'm older than a giant Galapagos tortoise, my exit pupils open to 5mm at best, and if I use an IF EP bin, even a 6x30, I can no longer "set and forget," I got to keep refocusing the diopters at different distances.
Hence, the same principle applies to diopter focus setting. For me, at 10 ft., it's one setting, at 200 ft. another setting, and at 1,000 ft., a third setting. Not a great variance, mind you, +2 at best on most roofs, +1 at best on most porros, with just a nudge in between.
So at what distance should I set the diopter? At the distance in whatever environment I'm in that I'm most likely to find birds. Close in my backyard, medium in the field, and far if looking at circling birds of prey.
The ZR 7x36 ED2 was the only roof I've tried that I didn't have to keep fiddling with the diopter settings. I did with the 8x30 SLC and 8.5x EL, Nikon 10x42 LX (not too bad with the 8x42, but I never gave it as good a workout at different distances), Regal LS, Noble, A LOT with the 8x32 LX. Don't have to reset the diopter on my 9x63 roofs, but that's because the close focus is 1 light year.
Didn't give the 8x32 EL a really good workout either, but I looked on this side of a lake (100 ft. at the shore) and at the mountains on the other side of the lake and an eagle riding the thermals on the ridge, and I never had to readjust the diopter. It was a bright, sunny day.
My focus accommodation falls with the light levels, so on a sunny day, I'm not fiddling with readjusting the diopter as much as much as on an overcast day or in the winter. YMMV and will probably be less, according to the EPA.
Brock