A fellow photographer with an EOS 5D Mark III asked me for help to understand his apparently odd file numbering issue. After a bit of study, and some experiments I was able to explain why his numbering was getting reset, and provide a solution.
Odd file numbering behaviour… or is it?
The photographer was using an EOS 5D Mark III with an SD card and a CF card in the camera and the camera was set to put JPG images on the SD card and RAW images on the CF; what Canon terms Record Separately.
When he gets back from a shoot he takes out the CF card and downloads the RAW images to his computer then erases the card – either in his computer or by formatting in camera. The next image is created in a new folder and is once again numbered 0001. He was a bit perplexed since he expected (aka hoped) that it would just carry on from the last numbered image. He was convinced that even though he had made sure that File Numbering was set to Continuous, it still reset the file number.
What’s more his folders 100EOS5D, 101EOS5D were increasing each time even if they were not full. Normally the camera only increases the folder number once IMG_9999 has been written to the folder. He asked if there was something odd about his camera – it kept forgetting the last file number and in the case of shooting several projects in one day could create the problem with multiple IMG_0001 images being created.
The reason why
As both cards are being written to by the camera then the camera needs to ensure that the images can be safely written without overwriting an existing image. You can think of it like this; the camera remembers that the last file number it created was in folder 104EOS5D and was IMG_0670. Normally you would expect that the next image would be IMG_0671 in the same folder.
However since the camera has only one card with the 104EOS5D folder it needs to do something to create a folder on the empty CF card. What it does is create a new folder that is one number higher than the highest numbered folder it can find on either card. So it creates 105EOS5D on both cards and also the act of folder creation resets the file number to IMG_0001.
This apparent odd behaviour is due to the fact that the SD card was not also formatted or cleared at the same time as the CF card, so the solution is to format the SD card and the CF card once the images have been transferred to the computer. Then the file numbers follow on in the expected order.
Not just EOS 5D Mark III
This is a situation that occurs with several cameras that can record separately to multiple cards like the EOS-1D Mark III, EOS-1Ds Mark III, EOS-1D Mark IV, EOS-1D X and EOS-1D C. You could get a similar situation if you use a WiFi transmitter and connect external storage to it for some other camera too.
There’s a certain sense of satisfaction finding out the cause of these kinds of behaviours and then helping photographers understand why they happen, so if you have similar settings or deep Canon system questions that you seek answers for then get in touch.