Canon eos rebel t1i - Manual (Page 132)

Handy Features
132
The file number is like the frame number on a roll of film. The captured
images are assigned a sequential file number from 0001 to 9999 and
saved in one folder. You can change how the file number is assigned.
The file number will appear on a computer in this format:
IMG_0001.JPG.
Under the [5] tab, select [File
numbering], then press <0>. Follow
the steps below to select a file
numbering method, then press <0>.
[Continuous]: The file numbering continues in sequence even
after the card is replaced.
Even after you replace the card, the file numbering continues in
sequence up to 9999. This is convenient when you want to save the
images numbered anywhere between 0001 to 9999 from multiple
cards into one folder in your computer.
If the replacement card contains images recorded previously, the file
numbering of the new images might continue from the file numbering
of the existing images in the card. If you want to use continuous file
numbering, you should use a newly-formatted card each time.
3 File Numbering Methods
0051 0052
Card-1 Card-2
File numbering after replacing the card
Next sequential file number