Bear in mind that as magnification power increases, so does the binoculars' weight. A pair of 7x glasses can be hand- held comfortably for an extended period, but 10x binoculars will need some kind of support during viewing. The second binocular specification refers to its aperture, i.e. the diameter of the binocular lens. This can range from 30mm to 80mm or more. The larger the aperture, the more light the instrument can collect.
Incoming light is focused by the main lens and the image then magnified by the eyepiece.
The final image is presented to your eye as a round disk of light known as the exit pupil. The human eye has an opening, or pupil size, of about 2.5mm in bright daylight and about 7mm in total darkness. When you use a pair of binoculars for astronomy, the size of the exit pupil must match your own pupil size. If the exit pupil is larger, then all the light that is collected is not being passed into your eye and goes wasted. If the exit pupil is smaller than your eye's pupil, then you are wasting some of the eye's light-gathering capability.