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The size of an image file and the quality of the picture it contains
depend in part on the number of pixels in the image and the amount of
compression used to store it.
Compression
To make large image files smaller and more manageable, digital cameras
store images in a format called JPEG after its developer, the Joint
Photographic Experts Group and pronounced "jay-peg." This file format not
only compresses images, it also allows you to specify how much they are
compressed. This is a useful feature because there is a trade-off between
compression and image quality. Less compression, sometimes called Fine mode,
gives you better images so you can make larger prints, but you can't store
as many images. More compression, in modes such as Normal or Basic, lets you
store more images and makes the images better for making smaller prints,
posting on a Web page, or sending as e-mail attachments. The only problem is
that your prints won't be quite as good. For the highest resolution, some
cameras offer an uncompressed format.
Here, two versions of the same image have been enlarged. The image on the
left is uncompressed. The one on the right is a compressed JPEG file.
Image size
In addition to offering two compression modes, many cameras let you also
change image size as a way of controlling the size of image files. Because
you can squeeze more 640 x 480 (VGA) images into a storage device than you
can squeeze 1600 x 1200 images, there may be times when you'll want to
switch to a smaller size and sacrifice quality for quantity.
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The left top
image of lips is a detail blown up from the image (left) shot in a low
res mode at a small size. The bottom right lips were shot at a high res
mode at a larger size. These are the widest extremes you'll find with
most digital cameras.
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How To: Selecting a Quality Mode
Look in your camera manual for a section on image quality,
image size, or compression. |
The shutter keeps light out of the camera except during an exposure, when
it opens to let light strike the image sensor. The length of time the
shutter is open affects both the exposure of the image and how motion is
portrayed in it.
The shutter and exposure
Slower shutter speeds let more light strike the image sensor making an
image lighter. Faster shutter speeds let less strike it and make the image
darker.
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In these
pictures, the shutter was left open longer for the image on the left
than for the one on the right. It's this longer exposure time that has
made the image lighter.
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The Way It Was: Early Shutter Designs
The shutter, used to control the amount of time that light exposes
the image sensor, has changed considerably over the years. The earliest
cameras, using materials that might take minutes to be properly exposed,
came with a lens cap that the photographer removed to begin the exposure
and then replaced to end it. As film became more sensitive to light and
exposure times became shorter, faster shutters were needed. One kind
used a swinging plate while another design used a guillotine-like blade.
As the blade moved past the lens opening, a hole in the blade allowed
light to reach the film. |
The shutter and motion
In addition to controlling exposure (the amount of light that reaches the
image sensor), the shutter speed is the most important control you have over
how motion is captured in a photograph. Understanding shutter speeds is
vital if you want to anticipate if a moving subject will appear in your
image sharp or blurred. The longer the shutter is open, the more a moving
subject will be blurred in the picture Also, the longer it's open the more
likely you are to cause blur by moving the camera slightly.
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A fast shutter
speed (left) opens and closes the shutter so quickly a moving subject
doesn't move very far during the exposure, a slow speed (right) can
allow moving objects to move sufficiently to blur their image on the
image sensor. |
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Katie turned a
little just as the shutter opened causing unwanted blur in the image. |
Understanding shutter speed settings
Although digital cameras can select any fraction of a second for an
exposure, there are a series of settings that have traditionally been used
when you set it yourself (which you can't do on many digital cameras). These
shutter speed settings are arranged in a sequence so that each setting lets
in half as much light as the next slowest setting and twice as much as the
next fastest. The traditional shutter speeds (listed from the fastest to the
slowest speeds) include 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30, 1/15, 1/8,
1/4, 1/2, and 1 second. Although speeds faster than 1 second are fractions
of a second most cameras display them without the numerator. For example,
1/2 second is displayed as 2.
The Decisive Moment
Henri Cartier-Bresson is famous for his photographs that capture that
"decisive moment" when random actions unfold into a single instant that
makes an interesting photograph. His eye-hand coordination is
unrivalled, and he was able to get the results he did because he was
always ready. There was never any fumbling with controls and lost
opportunities. Most digital cameras have an automatic exposure system
that frees you from the worry about controls. However, these cameras
have other problems that make decisive moments hard to capture.
There is a delay between the pressing the shutter release and the
actual taking of the picture. This is because when you first press the
button, the camera quickly performs a number of tasks. It first clears
the CCD, corrects white balance to correct for colour, meters and sets
the exposure, focuses (on auto focus cameras) the image, and finally
fires the flash (if needed) and takes the picture. All of these
processing steps take time and the action may have passed its peak by
the time the picture is actually taken.
There is an even longer delay between pictures because the captured
image must first be stored in the camera's memory. Because the image
must first be compressed, a lot of processing is required and this can
take a number of seconds, an eternity in action photography because you
can't take another picture until the first is compressed and saved. |
How To: Selecting a Shutter Speed
Look in your camera manual for a section on shutter preferred
or shutter priority mode, or shutter speeds. |
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