Why do shots with boosted ISO have more noise?
Let’s assume that we are keeping the EV constant, so the final output is the same brightness in a high ISO vs. a low ISO shot against which we are comparing. We’ll also assume that we’re doing analog amplification. First, we’ll consider the case where the ISO is boosted, but the shutter speed is kept constant, which implies that we have used a smaller f/stop. Since the integration time is constant, dark current accumulation will be the same in both cases, as will be readout noise and reset noise. Since less light was striking the sensor during the exposure, photon shot noise will be lower in absolute terms – since it is proportional to the square root of the signal and the signal is lower. However, the signal to noise ratio (SNR) will be worse overall since other noise source have remained constant, or shrunk more slowly than the signal. When this signal is fed through the amplifier, this has the effect of multiplying both the signal and noise by a constant. Since the SNR was worse tha
