Canon Xsi Instruction Manual page 66

3 Setting the Image-recording Quality
About 1
1 is the raw data before it is made into a 73 or other images.
Although 1 images require software like Digital Photo Professional
(provided) so they can be displayed on the computer, they also offer
flexibility for image adjustments possible only with 1. 1 is effective
when you want to precisely create your own art or shoot an important
subject.
For example, with a 1 image, you can use the provided software to
create various versions of the same image having a different Picture
Style. Also, if you took the picture with the wrong white balance
(explained in Chapter 4), you can change the white balance without
degrading the image quality.
Note that 1 images will not work with direct printing or print ordering
(DPOF).
About 1+73
1+73 records both a 1 and 73 image with a single shot. The
two images are saved to the card. Since the 73 image is recorded as a
processed image, you can view it as is on a personal computer without
the bundled software or print it as is. With 1+73, the two images are
saved with the same file number in the same folder. You can distinguish
between them with the image type or extension. The 1 image's
extension is "CR2," and the 73 image's extension is "JPG."
The Auto Lighting Optimizer (p.156) will not work with 1 and 1+73
images.
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1 image
73 image
0001.CR2
0001.JPG