PHC Ed Baily
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the contrast brightness of two images: one reflected
from a fixed mirror, the other from a movable mirror.
This system works on the theory that the sharpest images
have the highest contrast. When maximum contrast is
reached, an electronic device converts the contrast
brightness information into impulses. These impulses
start a motor that moves the lens to the point of sharp
focus. This type of autofocus system does not perform
effectively when the subject is all one color or does not
contain much contrast.
Another type of autofocus camera uses sonar or
infrared These systems emit either a sonar or infrared
signal to determine subject distance. The distance is
determined by the amount of time it takes the
transmitted energy to reflect back from the subject to a
sensor on the camera. This information is then sent to a
motor that moves the lens to the point of sharp focus.
The sonar autofocus system has a disadvantage. You
cannot photograph subjects through glass. The sonar
reflects off the glass and not the subject.
SELECTIVE FOCUS
You do not always want everything in your
photographs to be in sharp focus. By using selective
focus, you can emphasize the main subject and draw
attention to it. "Selective focus" means the use of a
shallow depth of field to isolate or emphasize the subject
(fig. 4-11). Selective focus is the control of the zone of
sharpness, or depth of field, in your photographs.
Once the lens has been focused on the main subject
of the picture, using a progressively larger aperture
(f/stop) will reduce the zone in front of and behind the
subject that is in focus. Long-focal-length lenses are
more effective for selective focusing because of their
larger real apertures. Wide-angle or short-focal-length
lenses are not as effective for selective focus because of
the great depth of field they provide at most apertures.
The following factors provide the maximum selective
focus control by minimizing depth of field:
Working close-up
Using a wide aperture
4-9

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