Gambian sidling bush
- Feb 25, 2017
Photons to Photos's methods are documented. Since they're neither brand or sensor specific it doesn't appear this conclusion is correct. It might also be useful to review translation between DxOMark DNR and PDR. It's my impression cross-brand/cross-system comparison difficulties are more with DxOMark's scores for different sensor sizes. However, DxOMark does also change measurement scenes between versions of their protocols and that might affect the dynamic range values reported. It would be appropriate for DxOMark to more transparent about this and do basic things like indicating the protocol version of a set of measurements.I'm pretty sure that Photons to Photos DR-ISO chart should not be used between brands/systems. I'm not sure if even DxoMark should be used like that, but it gives more sensible results.
Photon shot noise is a function of pixel area so, if it was the only form of noise present on the IMX272 and IMX410 (the A7 III sensor) then we would naively expect a 1.7 stop difference due to the imaging areas and pixel counts of the sensors. That the measured difference between the G9 and A7 III is 1.2 stops suggests about half a stop of other stuff is going on (the E-M1 III shows some additional difference as its DNR is less than the G9's below ISO 3200). That half stop is presumably some combination of factors such as microlens efficiency, actual versus nominal photosite size, dark current, kTC noise, pattern noise, read noise, and probably other stuff I'm not immediately thinking to mention. Unless one has details of the image sensor design and knowledge of surrounding factors, such as the PSRR (power supply rejection ratio) of the ADCs and the quality of the camera body's circuit board design, it is difficult to reason about the relative importance of each mechanism's contribution. There is a tendency for newer bodies using a given sensor to have slightly higher DNR but it's similarly difficult to say how much that might be due to fab process tweaks on the sensor versus incremental improvements in the camera bodies.
So far as I know there's also been little effort expended on attempting to quantify body to body variation. It's nonzero due to fab process variations and tolerances on components such as power supply capacitors and it appears to be less of a thing than lens copy variation. My guess from looking through Photos to Photos is ±0.1 stop in DNR might not be unreasonable.