Minolta Dynax 7xi Instruction Manual page 87

For any given f-number and subject distance, an increase in focal length will
reduce the depth of field and a decrease in focal length will have the opposite
effect. For example, if a 28-105mm zoom lens is set at 50mm, f/8 and the
subject is 12 ft. away, changing the focal length to 90mm without altering the
exposure settings or subject-to-camera distance will noticeably shorten the
depth of field. Setting the lens to 28mm, however, extends the range which
will appear in focus.
Depth of field also depends on subject distance. Without changing lens
aperture or focal length in the above case, if you move to 6 ft. from your
subject, there will be less depth of field in the final image.
The depth index provides a qualitative method for you to understand the
relationship between the above three factors and background sharpness. It
not only shows roughly how in-focus the background of your scene will
appear, but the indicator will also change position to reflect changes in
aperture, focal length, and subject distance which will also effect depth.
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