2.
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
distance.
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?
Figure 1-4.
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"
(fig 1-6).
Figure 1-5.
The f/number of a lens
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