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That's not clear at all Peter. Shot noise increases with longer subs - it is the square root of the signal count and the longer the sub, the higher the count and the more shot noise you get. The read noise is fixed for a sub. With low signals from short subs, the read noise dominates. eg if you get 10 photoelectrons in a sub, the shot noise will be about 3e rms. That is much less than the read noise in an SBIG camera. You need about 100 photoelectrons to get up to the read noise level of such a camera.
When you stack, you get shot noise that is the square root of the total photoelectron signal - that is independent of the sub length and it doesn't matter at all which subs the photons appear in.
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