You get far closer to comparison at equal exposure (i. e. at equal exposure index) if you move each dot on those curves horizontally back to the values 100, 200, 400 etc. indicated by the cameras' ISO sensitivity settings. That is, compare at the ISO exposure index values used by the camera, not the DxO raw file level saturation measurements.
I'm not sure what point you are trying to make here, BJL, unless your point is that the noise performance of two cameras used at equal shutter speed can vary to an even greater extent
than it would if different shutter speeds were used to compensate for different ISO sensitivies.
If two cameras have the same fundamental performance in terms of SNR and DR etc, but one of the cameras has a third of a stop lower sensitivity, as measured by DXO, then that camera with the lower sensitivity will require a third of a stop slower
shutter speed to achieve an equally correct or full exposure
, exhibiting the equal SNR and DR that it is potentially capable of.
However, graphs do have a vertical and horizontal axis, so it should be no problem to guess
what the difference in SNR and DR will be when two cameras are used at the same shutter speed. This gives one an appreciation of just how significant in practice, the different 'real' sensitivies of sensors may be.
For example, the DXO graphs for SNR, comparing the Sony RX-1 with the A99, indicate that using the same shutter speed
with the RX-1 and A99, at at their base ISOs of 100 for the RX-1(actually 81), and ISO 50 for the A99 (actually 48), would produce at least a 2dB worse
SNR on the A99. One can guess this value simply by moving one's eye vertically down from the best reading for the RX-1 (at ISO 81) to an imagined ISO 81 on the A99 graph.
However, if one uses whatever shutter speed is required to produce a full exposure
with each camera at its base ISO, the differences are less. SNR at 18% is a negligible 0.2dB
down for the A99, but DR is 1/3rd of a stop down, which is getting close to significant.
These results are roughly consistent with the fact that the A99 has a fixed, semi-transparent mirror which unavoidably redirects a small portion of the light. Consequently, at all manufacturer-nominated ISOs, one would expect the A99 to require a longer exposure to achieve, hopefully, the same or at least nearly the same performance.
The interesting point is, even with the longer exposure, the noise and DR performance of the A99 is not quite as good as one might expect. The third of a stop lower DR performance at the different base ISOs, is perhaps not significant, but the 2/3rds of a stop difference at the manufacturer-nominated ISOs of 800, is significant.
In other words, after giving the A99 a good 1/3rd of a stop more exposure
than the RX-1 at ISO 800, the RX-1 still retains a 2/3rds of a stop DR advantage.
If one were to use the same shutter speeds
with both cameras at the manufacturer-nominated ISOs of 800 (and same FL of lens at same aperture and same T/stop of course), the RX-1 would have a full stop DR advantage
, and as much as 3dB SNR-at-18% advantage
Such differences can be of practical significance when shooting in full manual mode, selecting a specific shutter speed required to freeze movement, and a specific F/stop for DoF and/or sharpness purposes. The RX-1 in these circumstances, according to DXO's measurements, should produce approximately (or as much as) a whole stop better DR and about 2/3rds of a stop better SNR, assuming lenses with the same T/stop are used. That's quite a significant improvement.
Tonal Range and Color Sensitivity will also be noticeably better in the RX-1 at equal shutter speeds and F/stops.
There's also another issue which still puzzles me a bit, which Bart has not yet clarified
The RX-1 has a fixed lens, and therefore any ISO sensitivity measurements by DXO must include the lens which will have a specific T/stop factor unavoidably included in the ISO sensitivity results. The T/stop factor is a transmission loss due to the opacity of the lens elements, which effectively reduces the size of the aperture regards exposure requirements. The lens on the RX-1 at F2 might effectively be F2.5 regards shutter speed requirements. I'm not aware of any lens that has a T/stop rating which is numerically smaller
than the actual F/stop rating. That would imply a light transmission gain
, instead of a loss.
When DXO test the A99 for ISO sensitivity, they do not
use a lens. However, the photographer who uses the A99 has
to use a lens, and that lens will inevitably introduce another factor which effectively
reduces the sensitivity of the sensor, by some degree, whether by 0.1 of a stop, O.25 of a stop, 0.67 of a stop, or sometimes even by a full stop.
So those figures I've extracted from the DXO graphs (and not from my arse, in case you are wondering), are probably very conservative. The best case scenario of a 0.2dB loss in SNR, and a 0.32EV loss in DR, comparing base ISOs at the appropriate shutter speed for an ETTR exposure, could only be reached if the lens used with the A99 had a T/stop equal to its F/stop, which is perhaps unrealistic.