My understanding was that for a given shutter speed/aperture/ISO the level of raw exposure reached by a back at ISO100 corresponds to the level of raw exposure reached by DSLRs at ISO30.
Bernard (and Erik),
Firstly some of that is the lower base ISO sensitivity of many digital,backs, due to tue absense of microlenses and such. Beyond that, the short answer seems to be that you (and DXO) are talking about measurements of numerical levels in raw files (relative to maximum level), which are basically the product of
- how much _exposure_ the sensor gets
by
- the conversion factor in "levels per electron",
The latter in turn depends on the degree of _amplification_ applied before ADC, measured by the gain in "volts per electron". The differences you are talking about are differences in the choice of _amplification_, not a different level of _exposure_.
Also, please note the very important difference between
- losing highlights due to overfilling electron wells (too much exposure), which I call "blown highlights"
and
- losing higlights from photosites that did not get overfilled, but whose signal was then amplified beyond the maximum voltage that the ADC can handle (over-amplification), which I call "clipped highlights".
Amplifying a bit less, but still enough that the noise introduced by the ADC is insignificant compared to the noise in the analog signal coming from the sensor, is an unequivocally good thing for image qualify (less risk of highlight clipping with no significant decrease in SNR), so it puzzles me why so many people wish to characterize as a defect or deception or falsificTionof the ISO speed with inaccurate talk about "underexposure". And let us be frank: the prefix "under-” is clearly pejorative, implying an error, which is not the case.
I agree with your characterization that this gives the appearance of more "highlight DR", and your inference that this is rather mythical entity, but if the practical consequence is that standard light metering is less likely to lead to highlight problems and with no downside, I do not underatand your cynical attitude to this approach. Can you point to any disadvantage that I am overlooking?