If the lens caliber (mm) is divided by the magnification, such as "35 / 7", "50 / 8", "70/15", then you can get the diameter of the beam in the millimeter of the eye through the telescope. The bigger the value is, the more light your eyes will receive or the information you will observe. This value is called the exit pupil of the telescope. What is the use for our selection of telescopes?
Suppose you are going to buy a binoculars for watching birds, and you want to use it to see birds at dawn or evening, and then the birds often fall in the trees and hide in the shadows. If you buy a 10x25 binoculars, the pupil diameter is 25 / 10= 2.5 (mm), and the diameter of the pupil of our eyes varies from 2mm to 7mm under different light and dark conditions. The darker the light, the greater the diameter of the pupil. If you are looking for a binoculars in the dark, you should choose binoculars from the pupil of the telescope and the pupil of your eyes in the dark, in order to make the most effective use of the information received by the telescope. So what about the "7X 50" binoculars? Its pupil is 50/7=7.14mm, almost equal to the eye of the eye in the darkest pupil. The light energy it collects is efficiently received by your eyes, so it is an ideal choice. But because the diameter of the eye pupil varies from person to person (for example, the pupil diameter of more than 40 year olds can only be expanded to 4 to 5mm), and the normal use of the telescope is mostly in the daytime, so the pupil is generally selected from 3 to 7mm.