How long can a DSLR shutter be left open for very long exposures such as 30, 45, 60... minutes? I remember hearing somewhere that sensors can heat up after a certain amount of time. With film I've gone well over an hour but there's no sensor heating up, cooling off, etc... On my D7000 I've gone about 15-20 minutes max with no problems. Is this even an issue? Was it ever an issue? I know you can stack photos/HDR for similar effects but there are times when I would like to shoot one very long exposure. Thanks.