Photographic Dynamic Range (PDR) is a More Useful Measure than the DxOMark Landscape Score

Started Apr 17, 2015 | Discussions thread
Jack Hogan Veteran Member • Posts: 7,012
Black Level Without Optical Black Pixels

Jack Hogan wrote: Bill, I get very similar numbers to yours, within a few hundreds of a stop. The D7100 Dxomark Landscape score looks off, it should be much closer to the D7200's, which looks right. Say within 1/3 of a stop.

Here are the results I get from the files that you kindly provided. One question that comes to mind has to do with how to accurately determine Black Level when optical black pixels are not present. Those were quite useful because they allowed to compensate for capture environment biases such as temperature, dark current etc. How do raw converters estimate Black Level now? It can make quite a difference to read noise (not of course to fwc or gain). Here is data from your D750 captures for instance, assuming that shutter speed is fairly accurate it seems that it's 1.36 DN off from the standard 600 used by Raw Digger for instance. That's a lot:

The D750's green channels are well behaved as one can see. The same cannot be said for the B and R ones at base ISO, the subject of an older Jim Kasson thread . If one uses a Black Level of 600.0 instead of 601.36 read noise drops to 3.84e- from 4.55e-. I think the latter makes more sense. Here is the D610 which does have OB pixels and subtracts them before writing data to the raw file.

The D610's data was not of the highest quality overall, but the Black Level here is within about 1/8th of a DN, good enough for most applications I think. Here is the complete ISO 100 set, using average Black Level estimated from the two green channels only:

So how do we (and raw converters) deal with black level when there are no OB pixels?

Jack

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