I have a Canon 450D Rebel XSi and have been taking some long exposure photos. I am finding that with the weather being sporadic lately I need to pack up my camera before the exposure has completed. Will it hurt my camera to turn it off during a photo while the shutter is still open? When I switch the camera to Off during the exposure it says that it's still recording. If turning the camera off is a bad idea, are there ways to end the exposure early so that it can start processing the photo while I put the cap back on and pack up?
Since you are talking in the context of long exposure shots, there are two possible scenarios when Long Exposure Noise Reduction (LENR) is enabled:
1. The shutter is still open and recording the scene. In this case, if you turn off the camera the shutter will close immediately and the exposure in progress will be recorded. No LENR dark frame will be recorded. There will be a short delay before the camera is completely powered down, but it will be completely safe to go ahead and move the camera and remove the lens during this time.
2. The shutter is closed and Long Exposure Noise Reduction (LENR) is recording a dark frame. In this case, if you turn off the power during the recording of the dark frame, the camera will complete the dark frame and save the image before powering down. It is completely safe to go ahead and move the camera and remove the lens during the recording of the dark frame, as the shutter is closed.
If Long Exposure Noise Reduction is not enabled, turning the camera off when the shutter is open will result in the shutter closing immediately and the image being saved before the camera is completely powered down. It is safe to go ahead and move the camera during this time.
Please note: In all cases the EXIF data will reflect the selected shutter speed and LENR option selected at the beginning of the exposure, not the actual time the shutter was open or whether a dark frame was actually generated and applied to the photo.
If you want to be able to store the data, you have to end the exposure and let the camera save the information before you turn it off, some cameras may do this automatically when you flip the switch, though pulling the battery wouldn't be able to finish. It shouldn't do any damage to just power it off, but without power, the sensor is going to discharge and any image data will be lost unless the camera saves the incomplete exposure.
You don't have to turn the camera off to move it either. If for some reason you can't end the exposure, simply putting on the lens cap and taking it with you while it is still exposing should be ok and won't contribute additional light to the image, though the image would be fairly dark since it wouldn't be fully exposed. You could stack multiple images together though to correct for this as long as the shots are the same.