For me, the great value of the DXOMark test results is not their over all scores but the side-by-side comparisons one is able to make amongst 3 different models of cameras, of specific sensor performance characteristics presented on graphs on the same page, at various measured ISO settings which are sometimes quite at variance with the manufacturers' claimed ISOs.
I imagine it might be offputting for some people looking at such graphs if they are not familiar with the units of measurement used with the graphs, so it might help to translate a few of the units, although I'm not necessarily the most qualified person to do this, but I'll try.
We all know the effects on noise of underexposing. It's why the ETTR concept has been the subject of so many threads on the forum. Underexpose just one stop (that is, give one stop less exposure than one needs to, without adverse consequences such as camera or subject movement) and one will usually notice an increase in noise somewhere in the image. Skin tones might not be as smooth as they otherwise would be and, if the image has deep shadows, there may be unacceptable noise there, causing one to blacken the shadows more than one might otherwise do during post processing, or use heavier noise reduction which tends to reduce resolution.
Under the DXOMark Measurements tab, there are a number of headings; ISO Sensitivity, SNR at 18% Grey, Dynamic Range, Tonal Range, and Color Sensitivity.
The unit of measurement used for SNR (Signal-to-noise ratio) is the decibel. As I understand, at the luminosity level of 18% grey, a difference of 3dB respresents a difference of one F\stop in exposure. In other words, if Camera A has 3dB better SNR at 18% grey than Camera B, it means one could underexpose an image from camera A by one full stop and still get skin tones (or other smooth surfaces) with about the same amount of noise as Camera B.
The unit of measurement for Dynamic Range is the Exposure Value, or EV. One EV is equivalent to one F/stop. As I understand, DR refers to noise in the deep shadows and should be distinguished from noise at 18% grey. If Camera A has 2 EV greater DR than Camera B, but only 3dB higher SNR at 18% grey, and one underexposes an image from Camera A by two stops, and compares with the image from Camera B which has been correctly exposed, then the noise in the deep shadows should be about the same in both images, but the noise at 18% grey, generally representative of skin tones, should actually be one stop
better in the image from Camera B.
A difference on the graphs of less than 0.5EV in DR is of little consequence and probably not noticeable except at extreme pixel-peeping levels.
Tonal Range refers to the number of distinguishable grey levels the sensor can record, and the units used refer to the number of bits required to encode those levels. I believe a difference of less than 1 bit would not (or hardly) be noticeable.
Likewise, Color Sensitivity is also measured in bits, and the number of bits indicates what is required to encode all the subtle nuances of different shades of color the sensor is capable of capturing. Again, any differences in the test results smaller than 1 bit are of little consequence.
The above comments refer to comparisons of normalised or equal-size images. Whilst the 'print' mode on DXO graphs represents a particularly small size of 8"x12" at 300ppi, to avoid having to uprez images from the cameras with a low pixel count, which introduces other problems, the results for comparison purposes apply to whatever normalized size one may choose which doesn't involve interpolation. Whilst the absolute values may change with changing image size, the relative values won't.
In other words, if one compares the D800 with the Phase IQ180, the relative differences shown on the DXO graphs at 8"x12" and 300ppi would also apply if the D800 image were displayed or printed at 16"x24" and 300ppi, and the IQ180 image downsized to 16"x24" at 300ppi.
Now I can't guarantee that everything I've written here is absolutely correct, but it's as I understand it.
