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When most digital photographers stray from Auto
or Program exposure modes, it's usually to the Shutter-Preferred
mode to better stop action. As in conventional photography, too
few apply the Aperture-Preferred mode to achieve the control of
Depth-of-Field.
Without going all scientific, depth-of-field is
an optical characteristic of lenses which can provide more
apparent image sharpness by using smaller apertures. Roughly
speaking, depth-of-field is a zone approximately one third in
front of, and two thirds behind, the point of focus.
The rules of depth-of-field are as follows. The
smaller the aperture, the greater the zone of focus. Conversely,
the wider apertures produce less depth-of-field.
Depth-of-field is greater in shorter focal
length (wide angle) lenses, less in telephotos. As the subject
(point of focus) gets closer to the camera, depth of field
decreases. Here's an example.
I set up to miniature figures from my ephemera
collection on a tabletop. The Guardsman is about three inches
high and roughly five inches from the lens. The armored knight
is roughly five inches high and placed about 13 inches behind
the Guardsman.
I used a tripod and manually focused on the
Guardsman, set my CP990 to Aperture mode and opened the lens to
its widest: f:2.5. As can be seen, the Guardsman looks pretty
sharp (I find it difficult to use the Nikon's manual focus) but
the knight is a blur on the background.
The second shot was made with the lens set at
its smallest aperture of f:9 and no change in focus.
Depth-of-field takes over and brings the knight into much
sharper focus and even reveals more of the background detail.
Hand holding the CP990 outdoors in its Macro
mode, I focused upon the nutmeg blossom with the macro setting
at its optimum focal length (about equivalent of 50mm). Since it
was a bright day, the Auto exposure mode stopped the lens down
to its smallest aperture. As the shot above shows, even a macro
photo a lot of background detail due to depth-of-field.
Switching to the Aperture-Preferred mode, I
opened the lens up wide (f:2.5) and repeated the macro shot. The
combination of close focus and decreased depth-of-field throws
the background into a blur. Since too much background detail is
a complain of digital photographers about macro shots, using
depth-of-field to blur the background is a handy technique.
Next week, we'll examine some of the ways
depth-of-field can be put to use creatively. |