Can you use a digital camera as a webcam?

ma camera is a canon powershot sd400

Yes, it is possible, some of my friends are doing this.
I have Cannon myself but never tried this thing.
Try to connect your camera to PC and through the webcam's proerties you will find out if it is possible with yours or not.

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Most can be. Check the software package which came with yours. If it has the ability to take a live picture, it probably can also be a webcam which can be used to capture video clips. Whether it can use live video like an instant messenger, is up to the software. Check your manual.