SIGMA SD1 - Manual (Page 82)

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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.
FLASH PHOTOGRAPHY
ELECTRONIC FLASH EF-610 DG SUPER SA-STTL (SOLD SEPARATELY)
ELECTRONIC FLASH EF-610 DG ST SA-STTL
(SOLD SEPARATELY)
Optional Sigma Electronic Flashes EF-610 DG SUPER SA-STTL and EF-610 DG
ST SA-STTL with the STTL system allow you to use features such as Auto Flash
with the SD1. The EF-610 DG SUPER SA-STTL lets you do even more, such as
use Multi Flash and other advanced features.