What is the CTG file format?
A .ctg file is a catalog index file created automatically by Canon digital cameras. Canon cameras store this file in the hidden CANONMSC folder on the memory card - whether SD, SDHC, SDXC, or CompactFlash. Its role is purely internal: the camera firmware uses it as a table of contents to quickly locate and organize photos on the card, including .CR2 and .CR3 RAW photos, standard .JPG images, and .THM thumbnail files.
The .ctg format is binary and follows no published specification. Canon does not document the internal structure, and the format is specific to Canon's own firmware. Because the file is managed entirely by the camera, users should not edit or delete it manually - doing so can cause the camera to have trouble displaying images, or force it to rebuild the index the next time it scans the card. In most cases the camera can regenerate a missing or corrupted .ctg automatically.
When transferring photos to a computer, .ctg files can be safely ignored; only the actual image files need to be copied. No desktop software is required to open or work with .ctg files - they serve no purpose outside the camera. Canon EOS DSLRs, Canon EOS mirrorless cameras, and Canon PowerShot compact cameras all generate .ctg files as a normal part of operation.
Security & safety
RISK: LOWThe Canon catalog .ctg is harmless camera metadata; the main 'risk' is deleting it off the card, which can make the camera re-index or briefly misbehave (your photos stay safe in DCIM). Because '.ctg opener' searches are vague, avoid generic 'universal file viewer' downloads that bundle PUP/adware - there is nothing in the catalog to view anyway.
Format details
in a nutshell- ChessBase Opening Book - Tree of chess opening moves and statistics, usually paired with .ctb/.cto index files.
- BMC Software PATROL file - Obscure legacy data/cache file written by the BMC PATROL IT-monitoring product; not the common consumer meaning.
Programs that open CTG files
Windows1 app
No opener needed (camera index)
Built-in A CANONMSC .ctg is a camera index, not a viewable file. Ignore it; open your photos from the DCIM folder instead. Don't delete it on the card or the camera may rebuild/misbehave. 
macOS1 app
No opener needed (camera index)
Built-in Same as Windows: it's the camera's index file. View your images from the DCIM folder; the .ctg has nothing to display. Technical details
deep spec| Container type | Binary catalog index (proprietary) |
| Storage path on card | Hidden `CANONMSC` folder on the memory card |
| File role | Camera image-location index; maps photos stored on the card for fast retrieval |
| Created by | Canon camera firmware automatically on first write to the card |
| Related image types | `.cr2`, `.cr3`, `.jpg`, `.thm` |
| Editability | Not user-editable; manual changes corrupt the index |
| Regeneration | Camera rebuilds the file on next card scan if deleted or corrupted |
| Magic bytes | None published; identified by folder location (`CANONMSC`), not by byte signature |
| MIME type | `application/octet-stream` |
| Supported memory cards | SD, SDHC, SDXC, and CompactFlash (CF) cards |
| Compatible camera lines | Canon EOS DSLR, Canon EOS mirrorless, Canon PowerShot compacts |
| Spec status | Proprietary, undocumented; no public specification exists |
| Computer access needed | No - file serves no purpose outside the camera |
| Released | 2000s Canon digital cameras |
CTG conversions
Community Q&A
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