Isos Hmm this is concerning.... (top DSLR's)

Started Sep 8, 2009 | Discussions thread
Andrew Westlake Senior Member • Posts: 2,928
DxO and ISO

The difference here is pretty straightforward - the issue is that there are several definitions and methods of measuring ISO, and the one DxO uses is different from everyone else's.

The relevant standard, ISO 12232:2006 has five possible definitions; the ones used by camera manufacturers are about image brightness (called standard output specification and recommended exposure index, SOS and REI). In the simplest fashion, according to SOS if you point the camera at a grey card, meter and shoot it should be rendered as middle-grey (50% luminosity). This makes sense - it's the way most photographers think about exposure. REI extends this to take into account the fact that pattern metering of high contrast scenes becomes a matter of interpretation rather than 'right' or 'wrong'.

DxO's method is based on something different - highlight clipping. This is useful for comparing raw sensor performance but doesn't reflect how cameras will render the image. Expose cameras from various manufacturers based on DxO's measured ISO and they'll all clip whites at the same point, but give images of different brightness.

The difference between DxO's measured ISO and the manufacturer's recommended ISO gives an indication of what we call the highlight dynamic range of the camera. Ironically, the greater the difference (i.e. the lower DxO's ISO relative to the indicated), the larger the highlight dynamic range, resulting in smoother rendition and rolloff of highlight detail.

So it's not all one big con after all.

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Andy Westlake

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