Nikon FM2 Instruction Manual page 28

-CONTROLS IN DETAIL-continued---------
Depth-of-fleld preview lever ®
What depth of field is
When you focus on your subject at a certain aperture, you will
find that not only is the subject itself in focus but objects in a
certain distance range both in front of and behind it appear
sharp. Objects increasingly out of this range become in-
creasingly out of focus. This " in-focus zone" is known as "depth
of field ." When this zone of sharpness is large, the depth of
field is said to be deep; when it is small, the depth of field is
said to be shallow.
The following can be said of depth of field , given the same
focusing distance :
1) The smaller the taking aperture (i.e., the higher the flstop
number), the deeper the depth of field , and the larger the
aperture, the shallower the depth of field.
2) The farther away the subject is from the lens, the deeper the
depth of field becomes ; the closer to the lens, the shallower
the depth of field .
3) There 's greater depth of field behind the main subject than
in front of it.
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Note, too, that in the case of lenses with different focal lengths
but of the same maximum flnumber, the shorter the focal length
of the lens, the larger the depth of field , and vice versa. The
depth of field at the taking aperture is indicated by the color·
coded lines having the same color as that of the aperture set
and which correspond to the distance scale on the focusing
ring . The range is indicated by the distance between the lines.
Shown in the photo is an example of the difference in depth of
field with a 50mm f/1.4 lens when using different aperture set·
tings at a set focusing distance of 2 m. Control of depth of field
enables selective blurring of the background elements of a
picture either to let the major subject stand out or for overall
sharpness, so that all elements in the entire picture field appear
sharp. This will give your picture its own character, thus making
it different from other pictures.
Getting the subject in focus is only one aspect of photography.
When you take pictures, be sure to consider depth of field, too.