Apple Macintosh PhotoFlash - Manual (Page 114)

Using JPEG compression
JPEG is a standard method for still image compression established by the
Joint Photographic Experts Group. It is designed to compress and decompress
digitized images while preserving maximum image quality. You can use JPEG
compression with a variety of file formats in addition to the standard JPEG
format. Many kinds of computers can send, receive, and display image files
saved in the standard JPEG format.
The JPEG method of compression systematically omits redundant
information from the image, deleting the portions of the image that aren’t
critical to human visual perception, and then applies a highly effective
compression technique to the remaining data. Although JPEG dramatically
reduces the size of the image file, it doesn’t necessarily change the quality of
the image when the file is decompressed. The effect on the image depends on
the level of JPEG compression you apply. At High quality settings, the change
is usually imperceptible. At progressively lower settings, the image quality
may degrade.
The JPEG method of compression specifies a mathematical formula, not a
specific program. Thus results can vary when using different programs to
apply JPEG compression. The PhotoFlash implementation of JPEG
compression is extremely accurate. If you use PhotoFlash to compress and
decompress images several times, the image may deteriorate in quality the
first few times, but after that the quality remains the same no matter how
many times you recompress it.
The way you use JPEG compression depends on your needs and work habits.
In general, it’s worth compressing all your images. Any loss in image quality
at the Excellent quality setting is usually imperceptible. If you don’t know
what an image will be used for in the future, use the Excellent setting. If an
image is 300 dpi or higher and you intend to print it, the High quality setting
is usually sufficient.
104 Chapter 6