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What is the purpose of a .THM file and and can I delete it?

The .THM file
After connecting your Canon's memory card into your computer, there's a chance you've seen files ending in .THM. In most cases, the THM is named something like MVI_1023.THM. If you have used your camera to shoot a number of videos, these can seem like unwanted pests clogging up your memory card. You may be tempted to just delete them, but learning from previous mistakes, you've probably decided to find out what the .THM file does and wonder if there any consequences.

THM is a contraction of thumbnail. Cameras are quite adept at creating thumbnails of photographs, but video files tend to be larger and tricker beasts. This is due to the compression techniques and the way the data is stored. You would have noticed that video files are many times larger than photo taken with the same camera. If you wanted to quickly preview all the video and photo files on a card, a camera would normally have to stop and process part of the video to display a thumbnail. This is a slow process and it gets slower with larger video files. Viewing the files on your camera would become very irritating as it would pause or freeze at each video in order to generate an image (Note: some cameras just display a generic video icon that gives no hint of the video's content, so you have to hit play each time to see what the video is about: another tedious process if you have a lot of video files).

The solution was to create a thumbnail and allow easy viewing of all your files. This is why the thumbnail has the same filename as the corresponding video (except for the extension). For example MVI_1023.MOV has a corresponding thumbnail file MVI_1023.THM. When using the camera, you are not aware you are seeing the THM files. The camera creates and displays them automatically, and then removes them when the video is deleted using the camera. Crucially, if you use a computer to delete or move the videos, the THM will remain on the card and need manually deletion too.

By now, you are probably thinking out that you can delete the the THM files off your computer. Before you jump in, there are a few important points you will need to consider.
  • Firstly the THM file is more than just a thumbnail - it contains EXIF data about the video (EXIF is extra data that contains information such as ISO, lens used and numerous camera settings). Unfortunately, you can't just double-click it to view the contents. But there is a really simple way to do it. Change the THM extension to a JPG. For example, MVI_1023.THM becomes MVI_1023.JPG. If you have Windows Vista or later, then right click on the JPG file, select Properties and click the Details tab. To view EXIF data in Window XP, right click and select Properties, then the summary tab, followed by the Advanced button.
  • Next up, be aware that there is a difference between viewing the files directly from the card and copying the files onto your computer and then using the computer to open, move or delete videos and their THM files. If you delete the THM file off your camera's memory card but leave the corresponding video file, you will no longer be able to view the video on the camera (but it will still be on the card). In effect, the video will be invisible to the camera. Plug the card into a computer and the video file will be visible. The result is that the video will continue to take up space on your card and you won't be able to do anything with it (this includes deleting or viewing the video) until it's plugged into a computer. Which answers a common question: why can't I see video files on my camera? The answer is that you have probably deleted the THM files, but left the viedos behind
  • That's all great, so what do you do? If you have moved the videos and THM files to your computer and don't want the EXIF data, you can delete the THM files from your computer. If you want to keep the video files on the camera, keep the corresponding THM files (as mentioned above, deleting a video by using the camera controls will automatically delete the corresponding THM file).
thm file
The EXIF data contained in the THM file.

This page: THM file - What is the purpose of a .thm file on a memory card and can I delete it? Plus, why can't I find my videos on my Canon camera?