D4 vs 1Dx (Imaging Resource), RAW, ACR, 6400-12800

Started Sep 4, 2012 | Discussions thread
bobn2 Forum Pro • Posts: 59,031
Re: D4 vs 1Dx (Imaging Resource), RAW, ACR, 6400-12800

Jack Hogan wrote:

Lance B wrote:

At ISO6400, the D800 is actually ISO4211 and the D4 is actually iSO4388!

A difference of a whopping 0.06 of a stop. Easy to lose perspective when thinking linearly in a logarithmic world, n'est-ce pas?

Therefore, if you had a camera that was accurate at ISO6400 ...

I believe most cameras 'cheat' more or less the same, see for instance here http://www.dxomark.com/index.php/Publications/DxOMark-Insights/Pushed-ISO-Let-s-make-it-clear/RAW-ISO-measures-are-inferior-to-manufacturer-ISOs-is-this-a-problem

It's not a 'cheat' - it's DxO getting confused about what ISO is. All a lower DxO 'ISO' means is that the engineers have provided more headroom than they needed to. So, if the DxO ISO is half a stop low, it means you've an extra half a stop highlight headroom in raw compared with JPEG.
Testing the correct ISO in a camera means two things :-

testing that the meter gives the correct target exposure with the ISO standard light level.

Testing that the processing renders the standard exposure with the right brightness. DPR's ISO test does the second but not (I believe) the first. Howver they get confounded by the REI (Recommended Exposure Index) method of ISO. The trouble is that the whole point of multi-pattern metering is that it sets a non-standard exposure, depending on the scene, so it becomes very difficult to define an ISO standard to go with it. ISO gave up and REI says that the manufacturer sets it to give what they consider to be a good result. Mostly these will align for grey scenes but fo course diverge with any real scene. Now DPR adjusts the exposure to achieve the standard (grey card) levels with a real scene and therefore can end up with different exposures, because no grey level is guaranteed to be rendered at any particular level.

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