DxOMark's measured ISOs vs. manufacturer ISOs

Started Mar 13, 2013 | Discussions thread
Jack Hogan Veteran Member • Posts: 6,988
Re: DxOMark's measured ISOs vs. manufacturer ISOs

Mjankor wrote:

Jack Hogan: So our hapless EM5 owner, having evaluated his scene, sets ISO in both cameras up so that their sensors will both saturate at the same Hsat = 0.0525 lx-s, which just so happens to correspond to an in-camera setting of ISO 3200 for the EM5 and ISO 1600 for the G3 (or Ssat of 1489 and 1481 respectively). And this is the response he gets:

Noise performance when in-camera ISO is chosen so that both *sensors/ADCs* will provide the same mean outputl when presented with the same Exposure

Whoa, un-wow, right? The G3 is not so bad after all

Can you please include some hypothetical shutter, iso, etc numbers for these examples.

For example, the first two at ISO 3200 would be:

OMD - 1/100, ISO 3200, f5

G3 1/100, ISO 3200, f5.

And for the second

OMD - 1/100, ISO3200, f5

G3 - 1/50, ISO 1600, f5

In my simplified example above both cameras use the same lens and both sensors are set up* to clip the same scene highlights at the same Exposure (Hsat). Since DSC sensor performance below saturation is linear, that means that the mean Raw value produced by either sensor/ADC as a fraction of full scale will be about the same for any scene Exposure below this value - resulting in an apples to apples, objective comparison of the foundation of IQ: 'for a given amount of light (Exposure) from the scene, camera A produces a SNR of x and camera B a SNR of y'.

Exposure is fully defined by shutter speed and f/number only. So to make sensor Exposure the same one can use the same shutter speed and f/number in both cameras, or any other combination that keeps the Exposure Value constant. In the apples to apples image above with the same lens, the settings were

EM5 f/6.3 1/800s
G3 f/6.3 1/640s

The two shutter speeds should be identical but they are in fact 0.3 stops apart because of differences in metering - see the PS of my previous message.

*In-camera ISO settings do not matter for this comparison and are simply a consequence of having set up both cameras' sensors/ADCs to respond similarly to scene light, so that they will provide the same mean output level for a given input. If you are curious, that meant 3200 for the EM5 vs 1600 for the G3.


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