You would then learn that if indeed DxO claims that "most manufacturers exaggerate their ISO sensitivity",
then DxO is utterly wrong: instead, most manufacturers are following the current ISO standard, using measurements according to either "Standard Output Sensitivity" or "Recommended Exposure Index", as is in fact required of Japanese camera manufacturers. They are simply not using DxO's preferred choice of equating "more than the standard-specified _minimum_ amount of highlight headroom in raw files" with "overstating the sensitivity".
That's a very good point, BJL, upon which I think you should amplify. If DXOMark use the terminology, ISO, then the reader would naturally assume that DXO are abiding by the accepted ISO standard, whatever than may be.
If you have discovered that DXO are not abiding by that standard, and have in effect their own standard, and that this is the explanation for the variance between the manufacturer's nominated ISO and DXO measurements, then that's a good point to make.
In itself, the point may be of little practical relevance to the photographer using his camera, but may be of some general interest and could be the subject of a new topic.
If you do decide to raise a new topic on this issue, I have the following questions for points of dicussion, which I would raise in advance.
(1) If DXO are using a slightly different standard than the ISO, is it because they think that their standard is better
, or more appropriate within the context of their testing procedures, and/or in relation to the digital medium.
If this is the case, what are the relative merits of the two standards? My guess would be, despite my ignorance, that the true ISO standard is more conservative than DXO's standard, in order to accommodate in-camera jpeg processing which, in my experience, always tends to lose highlight detail.
(2) If DXO are in practice using a different sensitivity standard to ISO, is there perhaps a good reason for their not renaming their standard along the lines of, perhaps, SSSS (Sensor Saturation Sensitivity Standard)?
When placing the cursor over the ISO dot on the DXOMark graph, which produces two readings, the manufacturer-nominated reading of ISO 100 and the DXO measured reading of ISO 87, for example, would it be a good idea for the two readings to be ISO 100 and SSSS 87?
I get the impression that many folks don't go further than the overall, weighted scores for the camera, perhaps because they are a bit intimidated by decibels and EVs. Perhaps DXOMark have decided that such people would be even more intimidated by completely new terms such as SSSS, so they continue to use the widely recognised term ISO which is generally more meaningful.
(3) Does it really matter what standard is used provided that, whatever standard is used, it is consistent? Is that not the only legitimate criticism, a lack of consistency?
When I buy a camera after careful comparison of its specs, my main consideration may be that my choice of camera has a higher base ISO than another, without any compromise in performance with regard to noise and color sensitivity.
Whatever standard is used may be irrelevant as long as the standard is consistent. I don't care if someone gives me a meausrement in centimetres or inches. I can work it out.
Now I hope you can follow this up, BJL, so we can perhaps learn something useful and practical.