I regularly import photos from my phone or the SD card of my brand new mirrorless camera (a Sony A7) to my home computer in order to archive the best shots. On the computer, a homegrown program sorts them into a folder structure that creates a separate directory for each year, month, and day. After importing, the images usually remain on the card or phone. Of course, I don't want the importer to recopy previously imported images the next time it is called, but instead pick up where it left off the last time. If several SD cards are used, it is important to keep track of them because they sometimes use conflicting file names.

The photos on the SD card are files with a name format of DSC<number>.JPG. On the phone, they have a different file name, say, IMG_<number>.JPG. Cameras and photo apps increment the consecutive number of newly taken photos by one for each shot. This process is described in the Design rule for Camera File system (DCF) [1] specification. The DCF specification defines the format of the file names along with their counters and specifies what happens if a counter overflows or the camera detects that the user has used other SD cards with separate counters in the meantime.

Figure 1 shows the typical, DCF-compliant file layout on the card. On a freshly formatted card, the camera saves the first images as DSC00001.JPG, DSC00002.JPG, and so on in the 100MSDCF/ subdirectory; this, in turn, is located in the DCIM folder. Now, it's unlikely for anyone to store 99,999 pictures on a card, but if a crazy photographer actually shot that many photos, the camera would create a new directory named 101MSDCF/ and, after the next shot, would simply start again at DSC00001.JPG.

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