Exposure basics, lesson two point one (& ISO)

Started Mar 19, 2013 | Discussions thread
bobn2 Forum Pro • Posts: 60,527
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.

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