Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
Anders W Forum Pro • Posts: 21,468
Re: It's about useful comparisons - not which is better

I additionally know, and can prove, that the DPR studio scene samples offer no set of camera ISOs such as to meet your preferred criterion. However, they do offer one set of camera ISOs such as to meet another criterion that I find acceptable, namely one in which the cameras are given the same signal (as I defined the word signal above) although the mean raw values are not the same.

I don't think that they do do that. DPR exposes as I understand for a fix point in the sRGB output - that is that 18% (though they never specify 18%) on their step wedge gives the appropriate value in the JPEG file. You have to relate that back to the effective (meter ignored) ISO value that they discovered (and do not always report) in their ISO 'sensitivity' test.

In general you are right that you have to check the outcome of the DPR ISO accuracy test reported in the "noise and noise reduction" section of each review. If a camera if off by a certain amount in that test, DPR procedures imply that the signal (exposure) will be off by the same amount when the studio scene samples are shot.

Speaking specifically about the two cameras at issue here, the E-M5 and the G3, the latter passed the ISO accuracy test without discrepany whereas the E-M5 was reported to be 1/3 EV off in the direction of "too dark". Consequently, one would expect the E-M5 to have enjoyed the extra benefit of 1/3 EV more exposure when the studio scene samples were shot. However, I have now, for one reason or another, taken the trouble to check for the actual presence of that extra 1/3 EV in comparison with three other cameras (GH2, D800, GH3), the first two of which known to pass the DPR ISO accuracy test without any discrepancy and the third not being reported yet, without finding any trace of it. You find the latest such comparison, which include a description of the methods used, here:


Yes, I saw that. It puts an element of doubt there, doesn't it? If DPReview do as they say, the E-M5 should have had 1/3 more exposure, yet you found that it didn't. So either they are not doing as they say, or there is sufficient error in your method that 1/3rd of a stop is not registered. I have to say, I have more confidence in you than the other party.

Yes, I agree that this "missing" 1/3 EV adds an element of doubt with regard to the accuracy of DPR procedures, although I wouldn't call it a major one.

A main point here, as I see it, is that I haven't yet encountered any case where the comparisons provided by the DPR studio scene samples are seriously off in the sense that one camera has been given significantly more exposure than another at the same camera ISO setting.

Another main point is that we don't have to speculate about the matter. We can just check the RAWs by means of RawDigger as I did and then relate that information to the DxO measured ISOs in the manner I did in the post I linked to. By that means, we can know whether two cameras got the same exposure at the same camera ISO regardless of what DPR says about the matter.

The method I use rests on the assumption that we can trust the DxO "measured ISOs". But in the end, we don't have to do that either. If we want, we can just test the ADU response of two cameras to the same exposure at the same camera ISO ourselves. We are not really in the hands of other parties here.

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