A CTG file is a catalog index file created by Canon digital cameras. It contains information about the number of images stored in each folder on a camera's memory card (typically up to 99). CTG files are stored in the CANONMSC folder and they correspond to other folders saved on the memory card.

You should not manually open or edit CTG files. The camera will automatically delete the CTG files when the corresponding images are erased. NOTE: CTG files are much smaller than .JPG files and do not contain any image data. Therefore they cannot be converted to JPEG files.