Question about DxOMark
memo90061 wrote:I've always wondered about their raitings for ISO. What does the number they post for the ISO actually mean?
Here's their description. In short, it's based on providing a specific amount of highlight headroom before sensor saturation. If I recall correctly, it's 1/2 stop.
As DxoMark notes at the end of that page:
"As tests show, the ISO settings reported by camera manufacturers can differ significantly from measured ISO in RAW. This difference stems from design choices, in particular the choice to keep some 'headroom' to avoid saturation in the higher exposures to make it possible to recover from blown highlights."
So to get the commonly-claimed 1-1/2 stops of highlight headroom in Raw, the camera will be using a higher indicated ISO setting — that's the setting that's used as the Exposure Index for the in-camera metering and auto-exposure system. So to get that extra stop of highlight headroom, what DxOMark calls ISO 200 might be called ISO 400 by the camera-maker.
And to be pedantic: DxOMark's numbers aren't "ISO" because they don't conform to any ISO standard. For modern digital cameras, the camera-maker's numbers are ISO because they conform to ISO 12232:2006 for sensitivity and ISO 2721:1982 for metering and auto-exposure. Older digital cameras only conformed to ISO 2721:1982, if that.
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