As a user of
binoculars for over forty years, especially
birding binoculars, I have become sensitive to some features more than others. Number one sore spot with me is focusing. I cannot abide a binocular that focuses too slowly or a binocular that has rough or uneven focusing or a binocular that cannot be focused with one finger. I'm a birder and have no time to stumble or waste when a rare bird pops into view. Seconds count in the birding game. Yes, it is hard for me to be objective about an otherwise good binocular if the focusing is not up to par. Indeed, I have returned more than one binocular that did not live up to my
fussy requirements for focusing.
Another sore spot for me with any
binocular is edge of field sharpness. This is not really a matter of binocular performance as it is a case of me being spoiled by using premium binoculars for so long. Once you've used a binocular that is sharp nearly to the very edge of the field, you get a little fussy, I suppose. To be sure, I cannot make a case that I would identify fewer birds with a binocular with average edge sharpness, but, like I said, I am spoiled.