The higher the number the more amplified the hand movements which makes sighting objects more difficult. For example, it is easier to find a warbler high in a tree using 7X35 pair of binoculars than a 10X40. However, the image of the warbler through the 7X35 will appear smaller. The second number in the couplet refers to the size of the objective lens, the larger of the two lenses which point at the object of interest. The diameter of the objective lens is measured in millimeters.
Larger lenses let in more light.
The larger the second number, the easier it will be to view wildlife during evening hours or in deep shade. Most animals cannot detect red light. For night viewing, put red cellophane over a flashlight and cast the beam about in search of eye shine. Deer eyes shine bright green, while raccoon eyes shine yellow-gold. Both red and gray foxes have green or gold eye shine.