Quote Originally Posted by Jon Ruyle
The camera also "overexposes" the picture by 2/3 of a stop as compared to an ISO 3200 shot. Thus you lose highlight headroom.

To be clear, here is the whole story behind ISO 2000:



  • Sets the camera to ISO 1600 (but displays "2000")
  • Adds a hidden -1/3 stop EC to the autoexposure meter.
  • After the photo is taken, it increases brightness of the raw file by 1/3 stop (with a linear digital push)

<div>The net result is that you lose 1/3 stop of highlight headroom, just as you said. You can simulate ISO 2000 by setting ISO 1600, -1/3 EC, and then increasing brightness in post. The only difference is that when you do it yourself, you can increase brightness in a way that doesn't clip highlights.</div>
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<div>It's the same story with all the "plus 1/3" ISO settings: 125, 250, 500, 1000, 2000, 4000.</div>