Wildlife are both captive and in the wild.
This amount of light is the result of a complex relationship between the size of the lens opening, the distance of the lens from the film, the amount of light entering the lens, and other factors. Imagine if you had to work out this relationship every time you tried to figure out the best exposure. It would be nearly impossible. The advantage of the f-number system is that it enables you to compare apples with apples. The following are full f-stops which you may wish to memorize.