Instruction manual

12
For CAP purposes, we need to realize whether or not our photos have been
compressed. If they were compressed, we need to know by how much so we can
determine if they still meet our customers' needs.
1.4.1 Compression
Lossless compression is similar to what WinZip or PKZip does. For instance,
if you compress a document into a ZIP file and later extract and open the
document, the content will be identical to the original. No information is lost in the
process. TIF and RAW are photo formats that can be compressed in a lossless
way.
Lossy compression reduces the image size by discarding information and is
similar to summarizing a document. For example, you can summarize a large
document into two-page document that represents the original, but you cannot re-
create the original out of the summary because information was discarded during
summarization. JPEG is an photo format that is based on lossy compression.
The table shows how, on average, a 5-megapixel photo (2,560 x 1,920 pixels)
is compressed using the various photo formats.
Photo Format
Typical file
size
Comments
Uncompressed TIF
14.1MB
Reference
Uncompressed 12-bit
RAW
7.7 MB
Reference
Compressed TIF
6.0 MB
Lossless compression
Compressed 12-bit
RAW
4.3 MB
Lossless compression
100% Quality JPEG
2.3 MB
Hard to distinguish from
uncompressed
80% Quality JPEG
1.3 MB
Sufficient quality for 4" x 6"
prints
60% Quality JPEG
0.7 MB
Sufficient quality for viewing on
a computer and on the Web
20% Quality JPEG
0.2 MB
Very low photo quality
1.4.2 Common Photo Formats
A. JPEG (also JPG)
JPEG (Joint Photographic Experts Group) is the preferred photo format for
CAP purposes because the quality is almost the same as RAW but results in
much smaller-size files; this significantly reduces the time it takes to save,
convert, adjust, download or upload files. With JPEG you can shoot hundreds of
photos at a time and the files are ready for release with no further processing.
JPEG is not a file format, but rather a method of data encoding used to
reduce the size of a data file. Its compression can produce very small files for
speedy downloads, and it is supported by all popular digital cameras and Web