CTG files are only used in one capacity -- to index the digital images taken on Canon cameras. The files do not hold the images themselves, but are necessary for the camera to calculate how many images have been taken. CTG files also help a computer populate images during a data transfer.
CTG files are officially known as catalog index files. These files are generated automatically when you capture an image on certain Canon brand digital cameras. CTG files do not contain an actual image, but rather information that is correlated to a folder containing photographs you took on the Canon digital camera.
Canon digital cameras save image data on SD cards, which are inserted into the side of the camera and can be removed when photos need to be transferred. CTG files are stored on the SD card, in a separate folder than where image files are saved. The CTG files correlate to an image folder. For example, file 123.CTG would correspond to the camera images held in folder 123CANON.
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Opening CTG Files
Canon CTG files are not designed to be opened on a computer, either Mac or PC. The files facilitate the indexing of your camera images by your computer, and allow your computer to populate the image files during a transfer. Once on your computer, an image file can be opened by a variety of photo editing programs. The correlating CTG files, however cannot be opened or edited. When you delete an image file from your camera, the corresponding CTG file is also erased.
A CTG file cannot be converted into an image file, such as a JPEG. Because the files contain no image data, they will not successfully be converted to an image of any sort. Attempting to alter a CTG file may interfere negatively with your computer's ability to index the images transferred from your Canon digital camera to the computer.