Files that contain the .ctg file extension are Catalog index files. These files are created by certain Canon digital cameras. When a Canon digital camera takes a photograph, it stores the photo in a folder on the camera's memory card.
The CTG file stores information about the number of pictures that are stored in each folder located on the camera's memory card. These files allow your computer to index the files and populate them to your computer. The CTG file does not contain any actual image data, rather it just contains an index of the data contained in the folder.
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