comerade,
I think you would be surprised by some of the lower-priced binoculars. I've glass for hours through many of them.
Eyestrain can be caused by many things, including poor collimation of the two barrels, or not being able to "bend" the binocular barrels close enough or wide enough for your interpupillary distance. But often it's also due to the amount of "fuzz" around the edges of the field of view. This occurs in ALL binoculars, but in very good binoculars is extremely small. If the fuzz extends very far into the FOV, however, long-term glassing will result in eyestrain, due to the eyes trying to focus constantly, even though the FOV is sharp in the middle.
I've tested some lower-priced binoculars where the outer 1/3 of the FOV is fuzzy. I call this "donut vision," where only the center of the FOV is sharp. But there are a bunch of lower-than-alpha priced binoculars these days that have minimal edge-fuzz, so little you have to go looking for it to notice.
