User Manual

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Chapter 12: Taking Great Pictures
Microsoft Digital Image Standard User’s Manual
Compression
You’ve seen one way to reduce picture file size: lower the picture resolution.
You can also reduce file size by having the camera compress the picture file
as it saves the file to the camera’s memory. Compression consolidates similar
information in the picture, and discards some information. With JPEG com
-
pression, for example, series of similarly colored pixels are grouped together
and considered to be the same color. In the code that makes up the file, the
color information for these grouped pixels only has to be listed once instead
of hundreds, or even thousands, of times. This shortcut can reduce file size
considerably. Taken to extremes, a picture in highly compressed JPEG format
might be 95 percent smaller than the same photo in an uncompressed format.
Although compression does reduce file size, it also reduces visual quality.
Slight to moderate compression might not noticeably reduce picture quality, but
high compression produces visible areas of splotchy color called artifacts. Even
with slight compression, artifacts become more pronounced each time the photo
is resaved and compression is applied.
The left photo has been saved with lossless compression, so all of the image quality is
retained. The photo on the right was saved with heavy JPEG compression—which is not
lossless—and the compression significantly reduced the image quality.
For important pictures that you want to print, it’s good practice to use little
or no compression. Or, if your camera offers it, you can take your photos in a
format, such as TIFF, that offers lossless compression. Lossless compression
reduces file size, but retains all of the photo quality once the photo is restored.
File size and
picture quality
Lowering resolution and
increasing compression
both reduce file size
and picture quality.
Bearing this in mind,
you can use file size
as a rough way to
judge the overall
visual quality.