DxOMark's measured ISOs vs. manufacturer ISOs

Started Mar 13, 2013 | Discussions thread
GordonBGood Veteran Member • Posts: 6,308
Re: DxOMark's measured ISOs vs. manufacturer ISOs

ultimitsu wrote:

Just so we do not waste time beat around the bush. In iso 6400 test images, Pana G3 and OMD used the same lens (oly 50mm prime), both shot at F6.3, G3 1/2500, OMD 1/1600. that is a very significant 2/3 stop more light for OMD.

I know DPR insist that they cannot ensure lighting is the same for every shot. but I would guess the difference is in any case minimal. and the variation of exposure in DPR's images coincide with Imaging Resource too.

Let's assume that the light illumination was exactly the same and see where that takes us:

  1. I use the ISO 3200 results as I already have downloaded them and there is less chance of compulsory raw Noise Reduction (NR) being used at ISO 3200 than at ISO 6400 according to DxOMark.
  2. I think both of us will accept that there is still a significant difference in exposure, both at ISO 3200 and f/6.3 with 1/800 and 1/1300 seconds, respectively, for the E-M5 and the Panasonic G-3, for an extra exposure of about two thirds of a stop.
  3. Things are even worse than that, as the EM-5 has a peak green histgram of about 1400/4093 whereas the Panasonic G3 has a peak green histogram of 2400 for another required extra exposure of two thirds of a stop in order to make the raw histograms about equivalent for a total extra exposure of about one and one third more exposure of the Olympus E-M5.
  4. Now the comparison shows both the E-M5 and the G3 to have about the same brightness after raw conversion, which means that the raw converter has boosted the raw histogram by about two thirds of a stop by digital multiplication, meaning that the actual real sensor ISO sensitivity as used in the RGB comparison image is about ISO 5500 in camera reference terms.
  5. However, DxOMark tells us that the real raw sensitivity of the sensor is less than half of the camera relative ones at 1489 for ISO 3200, meaning that the ISO sensitivity as used in DxOMark terms is about ISO 2560.
  6. For the Panasonic G3, DxOMark tells us that a camera ISO of 3200 is really a raw referenced DxOMark sensitivity of about ISO 2950.
  7. Thus, a little bit of the good showing of the E-M5 versus the G3 is due to a slightly lower effective raw ISO, but it isn't huge and we can still fight about slight differences in colour rendering, contrast, brightness, and sharpening as applied by ACR to the different camera models.
  8. Once we have compensated for all those variations, we will come to a conclusion just as DxOMark:  on a per pixel basis, the E-M5 and the G3 have very similar image quality as to noise for a given real raw sensor sensitivity, other than the much greater noise of the G3 in the deep shadows = decreased Dynamic Range (DR) as can be seen in the deep shadows as revealed in the DPR 100% pixel for pixel views.

In summary, the differing exposures of the cameras means very little to the raw image quality/noise comparisons when both have been post processed to about the same output brightness levels other than if the required exposures started to get down to a tenth of a second or slower where differing amount of hot pixels would start to make a difference.

Again, the secondary conclusion is that much too much concern is being made about this and much more than it deserves.

Regards, GordonBGood

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