How do you measure how open or closed a lens is?
It might seem
simple, but complications arise because photographers use lenses of
different focal lengths. This means a 100-millimeter lens is twice as far
away from the film as a 50-millimeter lens when both are focused on the same
And, just as with a window, the further away the film is, the
less light will fall on it (fig 1-4). Is there any way to express how much
light is falling on the film regardless of the focal length?
How image brightness is affected by focal length
a. The way to know how much light is falling on the film is through the
use of the f/number system.
An f/number is the ratio of the lens' focal
length to the diameter of the lens' aperture (fig 1-5). For example, if a
lens of one-inch focal length also has an aperture diameter of one inch, its
f/number is 1; if a lens of two-inch focal length has a diameter of one
inch, the f/number is 2.
To make it clear that it is an f/number when
writing, it is written as f/1 or f/2.
The "f/" stands for "focal length
divided by:" and f/2 literally means "the lens focal length divided by 2"
The f/number of a lens