SIGMA SD9 - Manual (Page 61)

DEPTH-OF-FIELD PREVIEW BUTTON
Normally, the lens’ aperture is always fully opened, except at the time the
picture is actually taken, for looking through the viewfinder under maximum
brightness conditions.
However, by pressing the depth-of-field preview
button, the aperture can be closed down to the
selected shooting aperture before the picture is
taken, enabling you to check depth of field
through the viewfinder before shooting.
DEPTH OF FIELD
When you set the focus for a particular subject, there is an area in front of and
behind your subject, which will also be in focus. This is called the
Depth-of-Field.
Depth of Field will increase when
Using a smaller aperture value (Larger F-number)
i.e. When you photograph the subject under the same conditions but change the
aperture value from F8 to F22, F22 will show more area in focus (more depth of field).
Using shorter focal length (wide-angle) lens.
i.e. When you photograph the subject under the same conditions, changing only focal
length from 50mm to 28mm, the 28mm lens will show more area in focus (more depth
of field).
Photographing the subject at greater distance
When you photograph the subject with same focal length lens and aperture value but
change the camera-to-subject distance, the subject at the greater distance shows
more area in focus (more depth of field) compared to subject at the shorter distance.
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