Quote Originally Posted by Daniel Browning



Quote Originally Posted by peety3
My initial interpretation was that I should set my camera to ISO 1600 and lock that in, which I don't think is the desired interpretation.
Correct. ISO 1600 always has less noise for a fixed exposure, but that doesn't mean we should always use it. High ISO has a price: clipped highlights. That price is always far too high to pay, except when we are in low light, and we no longer need the normal amount of highlight headroom.



OK now, the lights are starting to turn on for me. Daniel, I think I was not comprehending what you meant by 'highlight headroom'. Would the following statement be correct?


Highlight headroom is roughly analogous to available dynamic range. In the case of a low light shot, we slow the shutter as much as is acceptable, and open the aperture as much as is acceptable. Then when there is still not enough light for our needs, we increase the ISO. The trade off here is that by increasing the ISO we are sacrificing dynamic range (losing highlight headroom), in order to decrease read noise and (in the case of digital) amplify the signal on the sensor. This works in low light because there is less dynamic range in the scene, so we can use the higher ISO.