Originally posted by biz-engineer Pentax K-1 vs Sony A7RIV: Comparing test charts (curves) from photonstophotos.net, dxomark and side-side comparison of raw image noise at ISO800 and ISO1600.
- According to photonstophotos PDR curve, above ISO 320 camera setting, the Sony A7RIV should be superior to Pentax K1, thanks to the dual gain sensor of the Sony A7RIV that the Pentax K1 doesn't have...
- If I look at DXO curves ("print" option selected) at same measured ISOs, dynamic range curves are basically on top of each other. Where I see a bump of "performance" on the photonstophotos PDR curve for the A7RIV, I don't see it in DXOmark (measured ISO).
- Now, if I go to DPR studio comparison tool, select Pentax K1 and Sony A7RIV RAW ISO800 or RAW ISO1600, comparison mode. I check camera settings in both cases (ISO, lens aperture and shutter speeds are the same for both K1 and A7RIV). A visual side-side comparison, same image size for both cameras, RAW format, show that the Pentax K1 has slightly less noise than the Sony.
So why photonstophotos PDR curve shows that the Sony A7RIV is "superior" to K1 above ISO320 setting (presumably thanks to dual gain technology)?
You have to read the charts a bit more carefully. First, a difference of 1/3EV is really hard to see and it doesn't all have to come from the bottom end (noise) but can also come from the top-end (highlight clipping). This depends on the choice of designated ISO, at least for the sites that don't measure the ISO according to their own definition, but use the camera's designated ISO. It's a useful diagram from photographer's point of view for each individual camera (you control nominal, not 'real' ISO), but of course opens the door form 'beautifying' test results by overstating ISO, as Fuji was reported to do.
What I can see in the photonstophotos PDR curve is that the A7RIV's PDR declines more rapidly that the K-1's in the 'low gain' range. The gain switch stops that, so that the curves of both cameras become basically parallel over a large range, with a mere 0.4EV difference. Now compare the DXO curve: Note that the first two A7RIV points are taken at nominal ISO50 and ISO200 - first point in the 'non-native' low ISO range! If you connect the ISO 50, 200, 400 points only on the photonstophotos PDR curve, you could not see a jump either. Instead, you see a very similar shape as on the DXO curve, especially if you move the ISO50 up to the measured 72. No contradiction here from a 'shape' perspective. Why the K-1 data is lower at base gain, conveniently comparable in
DxOMark Derived Photographic Dynamic Range versus ISO Setting, I don't know. Note that this comparison is at nominal ISO. Taking the measured ISO of DXO into account, we find that at, say, measured ISO200 we have less than 0.1EV difference at the interpolated data in the DXO diagrams - which already benefit a bit from the 'lift' at ISO400 in the A7RIV case.
The visual appearance can be quite different even at similarly measured noise amplitude (or mean square averaged - RMS ... or whatever they define PDR as). I noticed that when I compared pixel-shift K-1 and 645Z test pictures on dpreview's test shots: Head to head at medium ISO, the 645Z data looks quite a lot cleaner at low ISO, even when giving the K-1 a one stop ISO advantage. The photographic dynamic range curves don't tell that story, both cameras should equally improve. Noise in the 645Z pictures is higher in amplitude, but less correlated that in the K-1 (at one stop lower ISO), leaving a much nicer rendition of fine detail on the 645Z. This isn't taken into account in the PDR definition, but can be observed visually.
On dpreview's test shots, I also notice that the dark areas render with a bit more contrast on the A7RIV compared to the K-1. Lens differences may be at play. This again make a difference in visual impression of shadow detail.