Chapter 1
✦
Exploring and Setting Up the EOS Rebel XS/1000D
15
If you edit JPEG images in an
editing program, image data
continues to be discarded each
time you save the fi le. Over time,
the loss of data can become vis-
ible, so I recommend download-
ing JPEG fi les to the computer,
and then saving them as TIFF
(Tagged Image File Format)
before you begin editing them.
TIFF is a lossless fi le format that
does not discard image data.
As the compression ratio increases, more of
the original image data is discarded, and the
image quality degrades. Compression also
introduces defects, referred to as artifacts,
that can create a blocky, jagged look, blur-
ring, and diminished color fi delity in the
image. At low compression ratios, artifacts
are minimal, but as the ratio increases, they
become more noticeable and objectionable.
Image Quality
Approximate
File Sizes in
Megabytes
(MB)
Image Size in Pixels (approximate
number of images stored on a 2GB
SD/SDHC card)
S (Small) JPEG (Jagged
quarter-circle icon)
0.7MB
1936
×
1288 (999)
RAW
9.8MB
3888
×
2592 (122)
RAW +
L (Large) (Solid
quarter-circle
icon)
9.8MB
3888
×
2592 for each image (180)
Avoid Losing Images
When the camera’s red access light — located on the back of the camera — is blink-
ing, it means that the camera is recording or erasing image data. When the access
light is blinking, do not open the SD/SDHC card slot cover, do not attempt to remove
the SD/SDHC card, and do not remove the camera battery. Any of these actions can
result in a loss of images and damage to the media card and camera. There is an
audible warning to let you know that images are being written to the card, but make
it a habit to watch for the access light anyway and don’t open the media card slot
cover or turn off the camera.
JPEG format
JPEG, which stands for Joint Photographic
Experts Group, is a popular image fi le for-
mat that enjoys not only smaller fi le sizes
than the RAW format, but also off ers the
advantage of being able to display your
images straight from the camera on any
computer, on the Web, and in e-mail mes-
sages. To achieve the small fi le size, JPEG
discards some data from the image during
compression — usually data that you would
not easily see anyway. This characteristic
gains JPEG its lossy moniker because it
“loses” image data during compression.
There are diff erent levels of JPEG compres-
sion. High compression levels discard more
image data than low ratios. The higher the
compression level, the smaller the fi le and
the more images that you can put on the
media card.
