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Sigma DP Merrill

Best Quality: ISO 100 or ISO 200?

ISO 200 is the native ISO of the Foveon sensor in the Sigma DP Merrill. Hence, the nominal recommendation which one will hear from Sigma and others is to shoot at ISO 200 instead of ISO 100 for increased dynamic range.

But after shooting 1500 or so images mostly at ISO 100, I developed a feeling that ISO 100 delivered solid image quality with highlight loss being rare, even the way I was inclined to shoot: carefully monitoring exposure, indeed even giving extra exposure (ETTR).

The working premise for the ISO 200 admonishment is that the non-native ISO 100 carries a risk of highlight degradation or hard-clipping whereas ISO 200 preserves a full stop of “headroom”. Why so? ISO 200 is calibrated so that about a stop of headroom is available for very bright highlights. Thus there is some margin for error in exposure before high-key detail “blows out”. At ISO 100, the exposure time must be doubled, and hence this safety margin is unavailable.

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