DxOMark's measured ISOs vs. manufacturer ISOs

Started Mar 13, 2013 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
Re: DxOMark's measured ISOs vs. manufacturer ISOs

Jeff Charles wrote:
I have read a number of Andy Westlake's posts, and my understanding from them is that DPR adjusts output brightness by varying shutter speed, i.e. exposure. Here's what he wrote in a post about a year ago:
The ISO testing uses critically-controlled lighting. Once you know the results of the that, nothing else needs to be shot under 'critically' controlled lighting (i.e. to a fraction of an EV) , it just needs to be shot to a controlled output brightness. Then, by the very definition of ISO, it doesn't matter whether the light level is slightly different and you compensate by changing the shutter speed - if the grey patches are rendered at the correct output brightness, you've got the same exposure.
He's right that changing shutter speed is a valid way to adjust for different light levels, but he's not right that "if the grey patches are rendered at the correct output brightness, you've got the same exposure." Output brightness is a function of processing, not exposure.

Jeff, there seems to be some clarification needed as to how output brightness is used for purposes of adjust raw conversions; you are correct that it enters into the evaluation equations. For instance, for the purposes of comparisons of the E-M5 to any other camera, if brightness compensations are used to brighten the raw conversion output than the effect is to increase the effective ISO sensitivity.

Also, I haven't read a post by any DPR staff member that says they adjust output brightness in ACR. Check this post from Richard Butler, in which he dismisses my suggestion that they do so.

The problem with the use of Adobe Camera Raw (ACR) as a raw conversion "standard" is that it is known to automatically apply differing conversion parameters by model so that comparison of the results is essentially useless.  For instance, we know that the raw histograms from the E-M5 will have about half the width of more typical raw histograms, yet when passed through ACR using default settings (even without automatic exposure compensation), the outputs come to to look to have the same brightness - ACR's defaults settings are so as to produce about the same output brightness as for Out Of Camera (OOC) JPEG's.

There are almost certainly other differences in processing applied by camera model or errors in setting equivalent exposures.  For instance, in the E-M5 review where raw noise is compared to that of the Sony NEX-7, there must be a reason for why the grey noise is plotted to be so much higher than that of the EM-5 which isn't there according to DxOMark (which I trust), or for that matter differences in grey noise between the Nikon D7000 and the Pentax K-5 as to raw grey noise which we know should be almost identical for a given ISO sensitivity.

For these discrepancies, I find that I can't really use DPR reviews for comparisons of raw noise.

Regards, GordonBGood

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