Quote Originally Posted by Daniel Browning


The short answer is to use the highest ISO you can without blowing important highlights. High ISO reduces noise.

Here's the long answer. ETTR (Expose To The Right) means to increase exposure (f-number, shutter speed, etc.) as much as possible without blowing the highlights you care about. ETTR is the best technique for reducing noise. A single stop of exposure (e.g. f/2.8 instead of f/4) can make more of a difference than 2 stops of ISO (e.g. 400 -> 1600). That doesn't mean ISO is unimportant, just that exposure is more important.

But there is a cost associated with high ISO. That cost is clipped highlights. For every doubling of ISO, one stop of highlights are lost. That is why I advise folks to use ITTR: ISO To The Right. That is, increase ISO as much as you can on every shot, without blowing important highlights. But don't go over 1600, as I said above, unless you need the convenience features.


This took me a long while to digest and interpret. What I finally figured out is that you're suggesting that we shoot in manual mode, starting at ISO 100 "for every shot", and choose "exposure" (using your definition of exposure) values such that the desired image is (hopefully) captured while also maximizing the light upon the sensor, without so much light that important highlights are lost. If it's impossible to achieve the desired image via aperture and shutter values (i.e. slowing the shutter will result in a blurry image, or opening the aperture will thin the DoF too much), it's best to increase the ISO to maximize the light upon the sensor, without so much light that important highlights are lost.


My translation of all of that is that it's best to achieve a right-justified histogram (using appropriate margins for the details we wish to keep) by prioritizing the widest artistically-reasonable aperture and the slowest artistically-reasonable shutter, and then the minimum amount of ISO increase. 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.