Thanks, but I must admit I am still confused.
Let's take an exposure during 1 minute with a 16 bit camera with the magical QE of 100 %.
During this time it is hit with 65 000 photons. The same number as its FWC. That would mean a full 16 bit dynamic range (not taking read noise into consideration).
If the camera had a QE of 50 %. Then only 32 500 photons would be registered and transfered. That would imply a 15 bit dynamic range. Or?
I am sure that I am missing some thing here and would be happy to be "enlighted".
You are probably conflating dynamic range with SNR. Dynamic range is agnostic of any amount of signal, it just has to do with the CAPACITY for signal, and the MINIMUM VIABLE signal. Dynamic range is really a hardware trait that tells you, for a given amount of read noise, how many discrete levels of useful information could the camera represent?
SNR, on the other hand, is inextricably related to signal. SNR has to do with the ACTUAL signal, and the TOTAL NOISE in that signal. While SNR and DR are similar in that they are both ratios, they are otherwise very different things. If you have a camera with a 50ke- FWC and 5e- rad noise, your dynamic range is 50,000/5, or 10,000 discrete steps, 10000:1. It is also 80dB of dynamic range, or 13.33 stops of dynamic range. DR describes the capabilities of the camera. DR is:
DRsteps = FWC/RN
DRdb = 20 * log(FWC/RN)
DRstops = 20 * log(FWC/RN) / 6
Now, using this hypothetical camera, lets say we expose such that we build up a signal that meets the 10xRN^2 criteria. This means we want a background signal that is at least 10 * 5e-^2, or 250e-. So you expose for a while and build up a signal that meets this criteria, and your SNR comes out to:
SNR10x = 250/SQRT(250 + 5^2) = 250/SQRT(275) = 250/16.6 = 15.1:1
While this is a good SNR, it is also very different from the 10000:1 ratio we get for dynamic rage. Even though it is with the same sensor.
Also note how bit depth doesn't even come into play here. Bit depth ultimately becomes another noise term, an error that is folded into all the other noise terms. If this error is large, it could be a problem...so, if you have a sensor with 13-14 stops of dynamic range but you use only 10 or 12 bits, then that is probably not optimal. You still have the same hardware dynamic range, but then you kind of stuff that dynamic range into a more limited number of output steps. You lose some of the original fineness of the information. If your dynamic range is more limited, say 12, 11.5, 10.5 stops, then 12 bits should be plenty sufficient to represent the information you are actually capable of using.
As for quantum efficiency...that is just another ratio. It is plain and simply the rate at which photons convert into electrons. There isn't much more to it than that. You "see" 50,000 photons, with 50% Q.E. you "get" 25,000 electrons. But that is just the conversion ratio of photons to electrons...it doesn't have anything to do with dynamic range. The only relationship it has with SNR is that with lower Q.E. it takes longer to reach a given SNR.
Edited by Jon Rista, 19 July 2019 - 06:02 PM.