Originally Posted by David Chapman
(Post 1444037)
From Daniel Browning on other forums:
When you select "ISO 320", what the camera actually does behind the scenes is set the analog ISO to 400, then reduce brightness digitally with a -1/3 stop pull. That makes it look as if you had used ISO 320, except that 1/3 stop of highlights are clipped. The upside is that noise is less.
It's like the opposite of HTP (highlight tone priority). HTP sets the analog ISO to one stop below whatever you pick, giving 1 extra stop of highlights, then increases the brightness digitally with nonlinear EC (to preserve highlights), which increases the visibility of noise. So HTP trades shadows for highlights.
The other tweener ISO settings (125, 250, 500, 1000) should be avoided, because they do a 1/3-stop push, but don't bother to preserve the highlights, so they increase noise for no benefit.
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• 100, 200, 400, 800, 1600: normal amount of highlight headroom.
• There is little difference from ISO 100 to 200, because most viewers can't tell the difference in photon shot noise, and the read noise drops by almost a full stop in ISO 200.
• 125, 250, 500, 1000: considered harmful. 1/3 stop less highlight headroom and 1/3 stop more shadow noise.
• 160, 320, 640, 1250: fine, as long as you are aware of the decreased highlight headroom. (Clips 1/3 highlights to get 1/3 more shadows.)
• HTP should *always* be enabled when the ISO is higher than 1600.
• There is a careful balance between increasing shadow detail through ISO (which clips highlights) or through HTP, the picture profile (e.g. contrast, tone curve), or Auto Lighting Optimizer. Generally, the higher you go in ISO, the more beneficial it is to use non-ISO methods to increase shadow detail. For example, ISO 3200+HTP (actually ISO 1600) is better than the real ISO 3200. ISO 800+HTP may be better than the real ISO 800, but it depends on how much highlight headroom you need.
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