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As
with modern "point and shoot" film cameras, most
digital cameras have auto exposure settings. The cameras sense
the brightness of the light in the direction the lens is pointing,
and automatically adjust for optimum image quality as determined
by the camera. For most images, the photographer can rely
upon the camera's auto adjust feature for excellent images.
However, in certain instances manual exposure adjustments
would provide better image quality.
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For
example, when images are being collected for use in a panorama,
some areas of the panorama will be brighter than other areas.
If the auto exposure is set, some images may appear darker
or lighter than others, which will produce stitching lines
between the images when stitched. If the camera has a manual
adjustment, these stitching lines can be eliminated.
The
image above shows two images which make up a panorama. The
image on the left includes a daylight sky. The camera automatically
adjusted for this, which made the walls of the landform look
dark. When the shot on the right was taken, the light was
not as intense, so the camera adjusted the exposure and lightened
up the picture. Therefore, the rock wall on the right looks
brighter than the same rock wall on the left. It would have
been better to set the camera exposure instead of using auto
exposure.
What
determines the image exposure? In other words, what influences
the amount of light entering the lens during imaging?
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The
Effect of Shutter Speed
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Compare
the images obtained for various camera shutter speed settings.
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Setting
= 1
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Setting
= 2
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Setting
= 4
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Setting
= 8
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Setting
= 15
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Setting
= 30
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longer the shutter is open, the brighter the resulting image.
What is the relationship between the camera setting and the
time the shutter is open? The movie below was taken by a video
camera that records images at a rate of 30 frames per second.
Step through the movie using the arrow keys to determine how
many frames the shutter is open at the various settings. Compare
the camera setting to the time the shutter is open (time =
number of frames/30). |
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The
inverse of the camera shutter speed settings represent the
time period which the shutter is open. For example, a setting
of 4 means that the shutter is open 1/4th of a second; and
for a shutter setting of 60 means that the shutter is open
for 1/60th of a second. A tripod is best used when the shutter
speed 1/125th of a second or longer.
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The
Effect of Shutter Aperture
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For
some cameras, the size of the shutter opening can be adjusted.
This is usually done with a device call a diaphragm on a
Single Lens Reflex camera. If the shutter opening, or aperture,
is large, more light can come into the camera for a given
shutter speed. The object movie shown below illustrates
the changes in the shutter opening using a lens from a SLR
camera.
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The
aperture of a lens is usually referred to in terms of an
f-number (focal ratio number). A large aperture has
a small f-number (such as 1.8). A very small aperture
would have a larger f-number (such as 22).
Digital
cameras that allow for manual exposure settings may enable
the photographer to change either the shutter speed or the
aperture f-number or both. Sometimes the camera will
use icons selected by the photographer which set both the
shutter speed and f-stop based on what the icon represents.
This is demonstrated below, looking at the display screen
of the Sony Cybershot DSC-30.
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Twilight
Mode:
Suppresses
the washed-out color of a bright subject in a dark place
so that you can record the subject without losing the
dark atmosphere of the surroundings.
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Twilight
plus mode:
Increases
the effectiveness of the twilight mode function.
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Landscape
mode:
Focuses
only on a distant subject to record landscapes, etc.
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Panfocus
mode:
Changes
the focus quickly and simply from a close subject to
a distant subject.
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Spot light-metering mode:
Select this mode when there is
backlight or when there is strong contrast between the
subject and the background, etc. Position the point
you want to record on the spot light-metering cross
hair.
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Aperture priority mode:
Makes the subject stand out against
an unclear background or makes both the subject and
the background stand out clearly.
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Shutter speed priority mode:
Records a sharp picture of a
fast moving subject or the flow of motion of a moving
subject.
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