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I have a Canon EOS 20D DSLR and shoot RAW+ Jpeg. On the CompactFlash card, the camera makes folders "327CANON", "328CANON"... etc in DCIM.

Why are these sub-folders generated, and what is the motivation behind that? Why are not all images simply in DCIM? Also, which event triggers the creation of these sub-folders?

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Each folder can only contain 100 images numbered between 0001 through 9999. When that limit is reached, the camera automatically generates a new folder and places the next images in it. When folder 328CANON has 100 images in it, the camera will create "329CANON" and place the next image in it. If the last image number in "328 CANON" is 'IMG-0051', then the first image in "329CANON" will be named 'IMG-0052'.

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Why are not all images simply in DCIM?

Slightly unhelpful answer: because that would violate the Design rule for Camera File System. Perhaps more helpfully, given that image files can have only 4 numbers in them (again due to the design rule), what would you expect to happen if you have more than 10000 images on one card - perhaps slightly unlikely for a 20D, but certainly feasible with today's cameras and card sizes.

Also, which event triggers the creation of these sub-folders?

According to the 20D manual (page 61), a new folder is created every 100 images. On more recent cameras, this tends to be a larger number (10000 on my T2i/550D); I suspect the lower limit on the 20D is due to problems that computers of the day would have had handling a folder with 10000 images in it (this is also another good reason why you wouldn't want every image directly in DCIM).

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