Open any camera card and you find files like IMG_0001.CR3, DSC00001.ARW or
P1000001.MOV. The name isn't unique to your shoot, or even to your camera. Import
two cards into one folder, and sooner or later two different pictures want the same name. This is
how camera numbering works and what to do about it.
The short version: cameras number files from 0001 to 9999 and often restart after a format, and two bodies of the same model write the same names. Give each card its own folder by date and camera, set continuous numbering, and never use the file name alone to decide whether you already have a file.
How cameras number files
Most cameras follow the DCF standard (Design rule for Camera File system). It puts pictures under a
DCIM folder, in subfolders whose names start with a number from 100 to 999 followed
by five characters that often identify the maker, such as 100CANON or 100MSDCF.
File names start with four characters and end in a number from 0001 to 9999. A folder holds up to 9999 files;
when the counter passes that, the camera opens a new folder and starts again at 0001, so after
DSC09999 comes DSC00001 in the next folder, not DSC10000.
Some makers deviate: Panasonic, for instance, numbers 0001 to 0999.
Why names repeat
- Formatting. Many cameras restart the counter after you format the card, so a freshly formatted card begins at 0001 again.
- Camera settings. Cameras usually let you choose between a continuous counter and one that resets whenever you insert a new card. The wording varies by brand.
- More than one body. Two bodies of the same model both write
IMG_0001. On a two-camera shoot, name collisions are normal. - Re-importing. Import a card twice and the same file arrives under the same name again.
The risky moment: dragging two cards into one folder in Finder. Finder asks whether to Replace or Keep Both. Replace overwrites the first file with the second, and if you are tired, or the dialog is for 400 files, that is an easy click to get wrong.
How to keep them apart
- Put each card in its own place. A folder per date and camera means
A7S_0001from Tuesday never meets the one from Wednesday. This is the single most effective habit; see our folder structure guide. - Set the camera to continuous numbering if it offers the option, so the counter doesn't restart each time you format or swap cards.
- Keep the original name until the copy is verified. Rename on the copy, not on the card, and put the meaning in the folders and in a prefix, as in our naming rules.
- Don't use the name to tell whether you already have a file. Two shots can share a name, and one shot can arrive twice. A name plus the size, capture date and camera model is a much safer test.
How ShotGrab handles it
ShotGrab decides whether a file was already imported from four things together: name, size,
capture date and camera model, per project. An IMG_0001 from another day or another
camera is a new file, and a card you plug in twice is skipped the second time. If a different file
already occupies the destination path, the new one gets a numeric suffix such as -1;
nothing is overwritten. The original files on the card are never deleted.
The key doesn't read file contents: hashing a whole card to decide what to skip would be too slow. The SHA-256 checksum is used afterwards, to verify each copy.
Frequently asked questions
Why does my camera start again at IMG_0001?
Camera file numbers come from a counter inside the camera, and many cameras reset it when you format the card or switch cards, depending on a setting such as continuous numbering. The DCF standard also limits a folder to 9999 numbered files, after which the camera starts a new folder.
What happens if two files have the same name in a folder on a Mac?
Finder asks whether to replace the existing file or keep both. Choosing Replace overwrites the first file with the second one, so check before you confirm.
Does a duplicate filename mean a duplicate photo?
No. A name only tells you which counter value the camera used. Two different shots can share a name, and the same shot can reach you under the same name twice. Size, capture date and camera model are better clues.
What we couldn't confirm
The DCF numbering rules above come from the standard as summarised on Wikipedia. Camera makers add their own variations and menu names, so check your camera's manual for how its counter behaves after a format.