A minimum of a 4mm exit pupil is needed to see reasonable detail in dim light.  (It does no good to magnify something 10 times if the image just looks like a larger mystery-blob.)  We can figure out the exit pupil’s size by dividing the objective lens diameter by the magnification.  An 8x32 binocular, for instance, has the minimum 4mm exit pupil, while a 10x32 does not — one reason I regard even the best 10x32’s in the world as a poor choice for most hunting, even though most binocular manufacturers report 10x as the top seller all across the board.  The tiny exit pupil of the 10x32 also makes long-term glassing uncomfortable, since our eyes are essentially locked into one position: staring straight ahead.  Just as it’s much more comfortable to walk rather than stand in place for 20 minutes, it’s much more comfortable to glass through a larger exit pupil, allowing our eyes to roam around a little.

Eventually we had 30-some binoculars arrayed on the living room couch, next to the big picture window that looks out across the street.  Our friends showed up late in the afternoon on a dark, cloudy day, ideal for really testing binoculars.  The view through many binoculars looks OK on sunny days, because the pupils of our eyes “stop down” to tiny dots, much like the aperture that sets the f-stop in a camera.  The big reason for this eye-reaction is to keep too much bright light out of our eyes, but a secondary effect is to sharpen the view.  

The smaller pupils in our eyes cut down stray light at the edges of our eyes, and stray light (rays that don’t all arrive at the same place) is one major reason for a fuzzy view, whether through our eyes or through binoculars.  When our eyes “stop down” in bright light, they make even cheap binoculars look pretty good.  So to truly test binoculars, we should look through them in dim light, when our pupils open up and allow all the defects of a binocular’s optics to show up.  

The binoculars ranged in magnification from 8x to 10x, with objective lenses from 32mm to 44mm.  All were roof-prism models.  Why?  Because roof-prism binoculars are tougher, more compact and easier to waterproof than Porro-prism binoculars, the reason roofs have just about taken over the serious hunting market in the past decade.