Sony a (alpha) SLT-A55VL - Manual (Page 70)

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Shooting an image the way you want it
(Exposure mode)
With an Interchangeable Lens Digital Camera, you can adjust the shutter
speed (how long the shutter is open) and aperture (the range that is in focus:
depth of field) to enjoy a variety of photographic expressions.
Adjusting the shutter speed and aperture not only creates the photographic
effects of movement and focus, but also determines the brightness of the
image by controlling the amount of exposure (the amount of light the
camera takes in), which is the most important factor in photo shooting.
Changing the brightness of the picture by the amount of exposure
When using a faster shutter speed, the camera opens the shutter for a
shorter time. This means less time for the camera to take in light, resulting
in a darker picture. To take a brighter picture, you can open the aperture
(the hole through which the light passes) to some extent in order to adjust
the amount of light the camera takes in at one time.
The brightness of the picture adjusted by the shutter speed and aperture is
called “exposure.”
This section will show you how to adjust the exposure and enjoy various
photo expressions by the use of movement, focus, and light.
Amount of
exposure low high