I've read various preferences for exit pupil size. For instance, one person posted:

"you're better off with a 12x60 than with a 15x60 if you want to see dim objects, as a 12x60's exit pupil is 5, while a 15x60's exit pupil is only 4. The higher the exit pupil, the more light will reach your eyes. For astronomy, you'll want a binocular with an exit pupil of 5 or better."

Other posts note that 4 to 5mm is preferred, while yet others note that anything larger than 5 or 6mm (for an old person, like myself) is wasted (since old eyes don't dilate like young eyes). What is a "good" range for astro use ?

Robert

It is just my opinion, but I think you need to take into account the sort of objects you preferr to view.  For bright objects like the moon or planets I'd think that the smaller exit pupil would focus more of the image on the fovea of the eye where there is a greater concentration of cones which excell at picking out detail and recognizing color.  For dimmer objects like nebulae the larger exit pupil might be more advantageous as it would spread the image out over more of the rod dominated surface of the retina where light sensitivity is at its best.

Smaller exit pupils typically work better for light polluted skies because they show more contrast, at least that is my understanding. The binoculars forum over at cloudy nights is fairly active, and there are some topics that have been posted as sticky discussing exit pupil sizes and their importance.

http://www.cloudynights.com/documents/binoexit.pdf

Dj

Thanks for the comments and references. I'm still comparing three binocs, and haven't entirely made up my mind, but I'm leaning toward one pair, for general night and daytime use.

Robert

Consider the fact that during bright, sunny, daytime use your eye pupils will be rather small, around 2mm's.

Daytime exit pupil is mainly taken into consideration by those who are wanting to see more in low light situations, such as dawn & dusk.

One of my favorite night time stargazing binoculars is my 30x80mm with an exit pupil of only 2.66mm.

Speaking for myself, those of us who have astigmatisms, the smaller exit pupils show less distortion, and sharper images.

http://www.birding.com/exitpupil.asp

http://www.cloudynights.com/item.php?item_id=348