On the other hand, when comparing noise level and DR at elevated exposure index, only SOS makes sense, because that compares at the same shutter speed when using the same aperture ratio in the same lighting. Pushing the DR and SNR 18% curves down to the left and thus down due to RAW files having more than 1/2 stop of highlight headroom (which DxO does) makes no sense when comparing low-light performance, so those graphs are best read by pushing the dots at ISO 200, 400, etc. back into alignment. Fortunately it seems that the "full SNR" curves as DxO are labelled with the camera's "ISO speed" settings or 200, 400, etc., so those can be compared without adjustment.
Not necessarily, BJL. Not all lenses used at the same aperture let pass the same amount of light. There's almost always some degree of light transmission loss which can vary by as much as the variance between the manufacturer's nominated ISO and the DXO-tested ISO.
For the purpose of testing ISO, DXO do not
use a lens, otherwise the results would be all over the place. This is why too much nit-picking attention directed to small differences in ISO serve little purpose for the practical photographer who has to use a lens.
For example, Camera A has 1/3rd of a stop less actual
sensitivity than camera B, but the lens used with Camera A may have 1/3rd of a stop lower transmission loss than the lens used with Camera B, which effectively cancels out any ISO differences.
On the other hand, if the lens used with Camera A has 1/3rd of a stop greater
transmission loss than the lens used with Camera B, then that difference in ISO sensitivity is effectively doubled, and becomes 2/3rds of a stop, which is significant.
This problem of differences between F/stop and T/stop first came to my attention when a I bought the Nikkor 14-24/F2.8 with Canon adapter. The wide-angle zoom I'd previously been using, mainly with my 5D, was the Sigma 15-30.
Naturally, the first thing I did when receiving the Nikkor lens was compare it with my Sigma 15-30, at equal FoV shooting equal scenes with equal lighting, and using the same camera body, the Canon 5D.
I was of course gratified to see that this copy of my Nikkor lens was indeed quite noticeably sharper than the Sigma 15-30. But what astounded me was the discovery that the same exposure at F2.8 with the Nikkor resulted in the same ETTR using my Sigma 15-30 at its maximum aperture of F3.5. There appeared to be as much as a whole stop of difference between the T/stop values of these two lenses.
Whether or not the magnitude of this difference is really so great, I'd like to find out, just out of curiosity. After all, I was using an adapter with the Nikkor lens which may have influenced the results. I've searched the internet for T/stop values for this lens, but cannot find any reference whatsoever. Nor can I find any test of this lens on the DXOMark website.
Maybe the T/stop is so bad there's a conspiracy to keep it a trade secret.
Of course, if I really wanted to waste my time exploring such issues, I could do more comparisons between the Nikkor zoom on my D700 and the Sigma zoom on my 5D, making allowances for the less-than-0.25 stop of difference in ISO sensitivity between the two cameras.
However, I see no practical purpose in such comparisons. The D700 has a base ISO of 200, at which ISO performance is at least equal to that of the 5D at ISO 100, and mostly better, especially DR which is a whole stop better. Problem solved! A difference of one whole stop of transmission loss is accommodated, and there's no way I would have gone back to using the Sigma zoom on my 5D in preference to the Nikkor zoom on my D700.