OMD now on DPreview's studio scene comparison

Started Apr 25, 2012 | Discussions thread
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Anders W Forum Pro • Posts: 21,465
Re: OMD now on DPreview's studio scene comparison

Timur Born wrote:

Anders W wrote:

Timur Born wrote:

I just gave this another thought. DPR exposed the E-M5 2/3 longer vs. the GH2 while light level of the studio seemed to be about 0.8x EV lower for the E-M5.

Not sure how you arrive at the difference in studio light level. I have my own about how that might perhaps be deduced but let me hear what you say first so I don't need to take the trouble explaining things you have already realized.

Based on RickAstros' ISO 200 raw files of the GH2 vs. E-M5 I derived a difference of about 0.6 EV for their full scale histogram (also fits visually). This means that the E-M5 needs to be pushed by roughly +0.6 EV in order for the histograms to be matched. We know that according to DxO the GH2 uses a gain factor corresponding to its reports JPG ISO (209 vs. 200). My interpretation is that the difference of roughly 0.6 EV corresponds to the difference in analog gain between the cameras (aka E-M5 uses a gain corresponding to about ISO 130).

DPR exposed the E-M5 2/3 stops longer than the GH2, so at same aperture the E-M5 should have received more light/photons than the GH2, that is unless the studio light levels were lower on the E-M5. Since the 2/3 stops longer exposure compensates close to +-0 EV for the difference in analog gain any remaining difference between their exposures should be the difference in studio light levels.

And indeed, one has to push the E-M5 raw file by your calculated 0.85 EV in order to match their histograms (also visually). So the final calculation goes:

+0.66 EV (time) - 0.6 EV (gain) + 0.85 EV (manual compensation) = about 0.8 EV difference in studio light levels

OK. That would be the way I would deduce it too if I were to try to estimate the studio light levels. However some uncertainty still applies, I would say, to the exact figure for the saturation (or gain) difference when the two cameras are indeed given exactly the same amount of light on the sensor at the same ISO setting. You estimate about 0.6 EV and I have seen Detail Man's estimates of about 0.5 EV. I tried to experiment a bit last myself last night using RicksAstro's 200 ISO shots (there are two pairs of them, one in the first thread which was about comparisons at 3200 ISOs but where 200 ISO shots, at 1/4s, were also made available, and another pair in the second thread about comparisons at 200 ISO, shot at 1/6 s). I reached slightly different conclusions depending on which pair I looked at and how I tried to handle clipping, which ideally shouldn't be present at all for the purposes of estimating relative saturation/gain.

But until we have better data, I'd say about 0.5 EV is reasonable estimate, particularly since that fits rather well with the way Oly has calibrated this in the past ("actual" as opposed to "nominal" ISO, the way DxOMark measures them).

So it seems likely that the E-M5 received slightly less light (about 1/3 EV) than the GH2 in the DPR studio samples. What is puzzling here is that if Andy Westlake's report about DPR procedures and the behavior of the E-M5 in their ISO test is correct, we would have expected them to give the E-M5 1/3 EV more light than the GH2, not 1/3 less as our own estimates indicate.

Now the important part:

Does this not mean that the E-M5 did not get that much of a disadvantage as far as light exposure is concerned, but rather mostly suffered from the analog gain difference which had to be compensated digitally?

In what sense do you think the E-M5 "suffered" from the analog gain difference? If two cameras are shot so as to be given the same amount of light at the same ISO setting, in what way would the comparison be unfair or the one camera "suffer"?

I didn't mean that it would "suffer", but replied to DM's suggestion that the E-M5 had a disadvantage versus the GH2 in DPR's studio shots, because the E-M5 received less light. My argument is that the E-M5 did not suffer a 0.85 EV light disadvantage, because it received 0.66 EV longer exposure time. So the remaining difference would result in only about a 0.2 EV light disadvantage for the E-M5.

The note about possible a possible analog vs. digital gain disadvantage points to the analog gain difference that needs to be compensated digitally. How much of a visual difference that 0.6 EV analog vs. digital gain make, is another question.

Yes, we see eye to eye about this. The disadvantage suffered by one camera or the other due to them not having received exactly the same amount of light is minor (about 1/3 EV) rather than major.

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