Canon EOS-3 Instructions Manual page 102

Basic Terminology 4
● Depth of Field
This is the range in front of and behind the plane of optimum focus
where acceptable focus can be achieved. The smaller the aperture (the
larger the F-number), the greater the depth of field will be. And the
larger the aperture (the smaller the F-number), the shallower the depth
of field will be.
The depth of field is affected as described below:
(1) A smaller aperture (a larger F-number) increases the depth of field.
(2) A lens with a shorter focal length increases the depth of field.
... A wide-angle lens obtains a greater depth of field than a telephoto
lens.
(3) A longer distance between the camera and subject increases the
depth of field.
(4) The depth of field behind the plane of optimum focus is twice as
long as the depth of field in front of the plane of optimum focus.
Taken with an f/22 aperture.
102
Range of acceptable focus
Point of optimum focus
1
Taken with an f/2 aperture.
2