Olympus OM-D (EM-5) comparison samples are now.. (continued)

Started Mar 17, 2012 | Discussions thread
Andrew Westlake Senior Member • Posts: 2,928
Re: Olympus OM-D (EM-5) comparison samples are now.. (continued)
1

panos_m wrote:

If DPReview changes the light intensity between cameras when testing in studio then what is the purpose of the studio shot comparison widget?

You seem to have entirely misunderstood what I've written. Let's try again.

We don't randomly change the light levels between cameras on a whim. But we have changed the lighting setup recently to one that gives us more control, and lets us dim the lights when testing cameras with very high ISO settings. For the purpose of the test, this simply doesn't matter - it doesn't have to be done at a fixed light level because, by a happy accident of design, cameras can compensate for differing light levels by using different shutter speeds.

The purpose of the studio comparison widget is to compare how cameras behave in terms of noise and noise reduction at different ISOs. To this end, the exposure is tightly controlled between cameras, based essentially on the SOS definition of ISO 12232:2006. This is the definition used used by camera manufacturers. DxOMark uses a different measure which may, or may not, give the same answer, depending on the camera.

For the purposes of our testing ISO defines the relationship betwen exposure (as a function of light level, shutter speed and aperture) and JPEG image brightness. If two cameras have the same SOS ISO calibration, and output JPEGs of the same brightness, they must have received the same exposure, regardless of the actual light level. Any differences in light levels can be compensated by changing the shutter speed. This is how cameras work. Our tests are based on setting the exposures so that specific reference gray patches are white-balanced and rendered at the same brighness from every camera (as far as possible).

Shutter speeds have absolutely minimal impact on noise. If you shoot the same scene at ISO100 1/125sec F16 and ISO100 1/2000sec F4 you really won't see any difference in the noise. To me this is so self evident from everyday shooting I'm actually quite surprised to find myself typing it here. Then again, I did spend several years of my life testing lenses, so I guess have a bit more experience of this than most.

DxOMark's ISO measurements are fine in their own way - they provide a logical framework for DxOmark's own particular method of RAW data comparison. They just bear no relation to our tests based on SOS ISO - the two simply don't intersect. You can't say our ISO tests are wrong because DxO's are different, and you can't reinterpret our data based on DxOMark's measurements. The two testing regimes just use entirely different methodologies.

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Andy Westlake
dpreview.com

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