[RickS (Rick)]

I see that Ray replied while I was writing this. I'll post it anyway in case it helps someone understand this apparently slippery subject...

Originally Posted by Peter Ward

I don't agree...as the argument falls apart with very low flux emitters, where you can't ignore shot noise.

The argument doesn't ignore shot noise. Shot noise is a function of the number of photons detected in a total integration but is indifferent to the length of the individual subs.

Originally Posted by Peter Ward

If you are only capturing say...one photon every five seconds...then a 1 second sub will likely fail to capture anything 4 or so times out of five.

So, if you take an "average" 10 second sub it will see 2 photons. If you take 10 x 1 second subs, on average two of them will see one photon and the rest will see none. You still get two photons in the same integration time.

Originally Posted by Peter Ward

This is assuming you have zero read noise and perfect QE....which is simply not available at present....the reality is you won't capture anything but noise.

Read noise has relevance as already discussed. QE has the same effect on long or short subs.

Originally Posted by Peter Ward

But if we go deeper...a five second exposure will capture that photon. A ten second exposure will capture two etc.

Doesn't matter because we're integrating the subs. It's the averaged total that's important.

Originally Posted by Peter Ward

Yes, you need exposure time, but reduced to is absurd conclusion, a million millisecond exposures is unlikely to produce anything useful.

Not with current sensors, but without read noise a million one millisecond exposures will produce the same result as one 1,000 second exposure or 100 x 10 second exposures. BTW, we're not suggesting you can ignore read noise as should be clear from the discussion above...

The maths is very clear. The problems arise when we use intuition to reason about signal and noise. Like conditional probability it seems like our brains aren't wired to grok it.