Basic Photography Course

PHC Ed Baily

302.269

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

contents

Basic Photography Course

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DOFMaster for Windows On-line Depth of Field Calculator DOFMaster for Mobile Devices On-line Depth of Field Table Hyperfocal Distance Chart Articles FAQ Recommended Books Support Contact Links Home As an Amazon Associate I earn from qualifying purchases.	contents <- previous page next page -> PHC Ed Baily 302.269 Figure 4-11.Use of selective focus. 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 contents Basic Photography Course <- previous page next page ->	As an Amazon Associate I earn from qualifying purchases.
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