Canon A450 - Manual (Page 50)

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The images you shoot are automatically assigned file numbers. You
can select how the file number is assigned.
File Number Reset Function
Resetting the File Number
Shooting Mode
1
(Set up) Menu (File Numbering)
[Continuous]*/[Auto Reset].
See Menus and Settings (p. 19) .
* Default setting.
Continuous
A number one higher than the last one recorded is
assigned to the next image. This is convenient for
managing all your images together on a computer
because you avoid file name duplication when
changing folders or memory cards.*
* When a blank memory card is used. If a memory card with
recorded data is used, the 7-digit number for the last
recorded folder and image is compared with the last one on
the card, and the larger of the two is utilized as the basis for
new images.
Auto Reset
The image and folder number are reset to the starting
value (100-0001).* This is convenient for managing
images on a folder-by-folder basis.
* When a blank memory card is used. If a memory card with
recorded data is used, the number following the 7-digit
number for the last recorded folder and image on the card is
utilized as the basis for new images.