So I've been doing a lot of shooting with the IQ250, IQ180, and Achro+, and with all due respect to what Doug has said in an earlier thread, it is absolutely not the case that a dark frame is shot for every exposure, equal to the length of the exposure.
Two observations strike me as odd. Firstly, if you're shooting a long exposure on a tech camera with a manual wake-up, the dark frame exposure does not equate to the exposure time. It is the exposure time plus the time between wake-up and firing the shutter.
So for example, if I shoot a 1 second exposure, but take 3 seconds between waking up the back and taking the shot, the dark frame is exposed for 4 seconds.
Is that correct/expected behaviour? Is it because the back is actually "live" as soon as it is woken up, and so my 1 second exposure is not 1 second at all, but 4 seconds?
If that is the case, and it makes sense to subtract the noise from the dark frame over the full 4 seconds, then why - if my shutter speed is, say, 1/500th, and I still take 3 seconds between wake up and exposure, is there not a 3.002 second dark frame shot for subtraction?
I have shot thousands of frames now with the IQ250. Even on 5 second exposures, I would estimate that perhaps less than 1 in 20 exposures are accompanied by a dark frame.
This goes totally against what has been explained before. Is it an IQ250 thing?
Given my reasons for having the IQ250, I really would appreciate an explanation/understanding of what the logic is for firing off a dark frame.