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. Cottontail rabbits, opossums and bullfrogs have pink eye shine. Never chase, flush or harass wildlife. This may cause them to use up valuable energy needed to breed or for survival. Avoid nesting sites and active dens and respect resting periods of wildlife.