With handheld bins, the expectation is that you move them around and point them at the objects you wish to view rather than hold them steady and look at the extreme edges. Your eyes are much worse than any bins having only ~2 degrees of high accuity FOV. Prove it to yourself by staring at a word in this post and seeing if you can read adjacent words. Your eyes need to dart around to take in detail over a broader FOV. Bins need to have enough sharp FOV to accomodate this short term darting, for larger angles, move the bins. Mounted bins and scopes are a somewhat different story.