?? about exit pupil size

If I understand the exit pupil formula correctly it's the objective lens dia divided by magnification. So if I have a given optic with an obj. lens dia of 56mm and mag of 24x then the exit pupil is 2.33mm If my eyes are capable of dialating to 5mm does this mean my eyes are only receiving about 50% (2.33mm divided by 5mm) of the light they are capable of? Does this mean the image ( terrestial viewing) would be dim? Is this what causes the black that surrounds the visible image that you see in an eyepiece at times and as you move your head and eyes the position of the visible view and the black that surrounds it seems to move as well? I'm trying to understand the significance of exit pupil data and why mfgrs. provide it?

The folks selling binoculars give exit pupil because it is supposed to imply something about the abilities of the binoculars. During the day, at least when it is reasonably sunny, our eye is open to 2 to 3mm. If you use an instrument with an exit pupil larger than this, you are stopping down the objective diameter. You are, however, still getting all the light and all the resolution you could possilby get at that power.

During deep twilight, your eye's pupil opens up to 5 to 7mm, depending loosely on age, and a larger binocular will deliver more light to the eye.

For astronomy, at least, the exit pupil does not seem as useful. Binocular users have found that increasing either objective diameter or magnification increases your ability to see celestial objects. In spite of its smaller exit pupil, a pair of 12x36 binoculars will show more than a pair of 7x35s will.

The blackouts are generally because your eye is not correctly posistioned at the exit pupil. With 7x42s I almost never have this happen, but with my 12x36s I have to be careful positioning my eye. Incorrect eye position caused by incorrect IPD can also produce added chromatic aberration.

Clear skies, Alan

"Does this mean the image ( terrestial viewing) would be dim? "

Well, "dim" is relative. For daylight viewing, 2.33mm exit pupil would be fine, and it's doubtful your pupil would be open much larger than that anyway even in cloudy daytime conditions. For low-light conditions, larger exits pupils start make a difference. But, yes, there's greater image brightness the larger the exit pupil (up to the actual size of your pupil).