DxOMark's measured ISOs vs. manufacturer ISOs

Started Mar 13, 2013 | Discussions thread
Martin.au
Martin.au Forum Pro • Posts: 13,276
Re: DxOMark's measured ISOs vs. manufacturer ISOs
1

Jack Hogan wrote:

Mjankor wrote:

As you can see, both cameras meter about the same and record photos of similar exposure. If anything the E520 is a touch underexposed compared to the OM-D.

You are mixing several variables that are quite different and need to be kept apart: Metering/Recording, Input Exposure/ Output Brightness, Raw/rendered data. Plus you are forgetting an important one, in fact the only one that this thread is really about: IQ.

Metering: In theory for a given scene and ISO speed (S,going back to film days, therefore NOT the latest crappy definitions, but the old one) your camera's spot meter should provide the same parameters for ss and f/n (Exposure) as a hand held reflected light meter - for instance Sunny 16. The meter's variable is called the Reflected Light Meter constant K, which for CaNikon is 0.125. In theory all meters that have the same k will provide the same Exposure for a set S. This determines the input signal and has nothing to do with output brightness.

Recording: In theory a camera with a spot meter as above will meter off of and record a tone half way between perceived maximum diffuse white and black (called Middle Gray) in Raw values around 12.5% of full Scale, or 2.5 stops below saturation. Is this good? Can't say, it depends on the scene. Most modern cameras actually record Middle Gray 3-4 stops below full scale. This determines what gets recorded in the Raw data and it still has nothing to do with output brightness.

Rendered Image: your in-camera/in-computer rendering engine takes the raw data and applies a number of very severe linear and non linear transformations to the data in order to produce an image on the output medium of choice of pleasing brightness/contrast/saturation etc. This is what you see on your monitor: it has a lot to do with output brightness but it has very little to do with input Exposure.

IQ, for this thread SNR: SNR is really what ties together input Exposure and Brighness (ok, more than just brightness, bet let's simplify).

As you know, you may have accidentally severely underexposed a capture, but still produce a pleasingly bright output on your monitor by changing rendering parameters in PP - of course it may also be a tad noisier than desired...

Problem is, some DSC manufacturers use output image Brightness (instead of recording as a function of raw full scale) to help determine input Exposure by using it to determine S which affects metering, but as I hope you now see, the two are not really related - and IQ may suffer badly as a result. In fact, this horrible behavior is sanctioned in one of the least well thought out ISO non-standards around, the aforementioned 12232:2006. What is one to do? DxO to the rescue.

Conclusion:

The OM-D definitely did not require a shutter speed twice as slow as the E520 to take a similarly exposed image.

So you should now see that this means the reflected meter constants are similar.

Ergo the hypothesis that DXO ISO is comparable to the camera ISO is wrong.

So you should see that we can make any image look as bright/contrasty as another (in-camera or in-cpmputer), but that the real question for this thread is 'which one is cleaner'?

Cheers,
Jack

So, of the same scene, which images should we compare?

Should we shoot both cameras at the same settings, say f5, ISO200, 1/800, and compare those images, to see how the IQ stacks up? Both photos will be about the same brightness level, regardless of the DXO ISO (But dependant on the manufacturers sticking to the ISO standard).

Or are you suggesting we should shoot cameras based on their DXO ISO, with one camera being shot at f5, ISO 200 and 1/800 and the other being shot at f5, ISO 100, 1/800, and comparing images with a brightness level 1 stop apart. I presume we'd then jack up or down the brightness level of one of the images in post processing to achieve a same relative brightness of the final image.

Perhaps you could describe an experiment that would help me understand what it is you're trying to show. I have two cameras here, one of which has DXO ISO of 211, at ISO 200, and the other of which has DXOISO of 107 at ISO 200. What can I do, or what results should I expect, with those cameras?

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