I would not be thinking that there's a "sensor destruction" maximum time, the camera is smart enough to turn it off if it overheats (or should be). Max i've tried with my 7D is 12 minutes. (but i did do 4*3 minute, a 10 min, 12 min, 4 min, 30s, 10s, within the space of an hour, the 10 & 12 min were pretty much consecutive)


There's a difference between 'can' and 'should' though, I'd be more concerned about IQ, here's a100% crop of a shot i did with my 7D, iso100, 10 minutes. The noise is terrible (so is the diffraction and focus, but that's another thing). 40D is older, so having never used one, i'm still guessing it's got worse noise than the 7D for long exposures.


It's not all bad, I haven't used any noise-reduction, it's just the camera-JPG, i presume i could take out a lot of noise with RAWconversion, and dark-frame subtraction (the few shots i did in a row, the noise is in exactly the same spots), so it's no reason not to try... [H]


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