Is the EM5’s Miss-Stated ISO concerning to you?

Started Feb 27, 2013 | Discussions thread
Detail Man
Detail Man Forum Pro • Posts: 17,042
Re: Is the EM5’s Miss-Stated ISO concerning to you ?
4

mpgxsvcd wrote:

Just curious if anyone else noticed how much the Olympus EM5 ISO rating differs from every other camera’s? I know that it has been said that the EM5 and GH3 will produce very similar noise patterns at the same ISO.

However, if the exposures are totally different then that really is not a fair comparison.

It looks like DXO Mark’s tests show that he OMD E-M5 is exposing about 1 stop lower than every other camera in its class. In addition the Dpreview E-M5 comparison images are about 2/3s of a stop longer shutter duration than other cameras at the same ISO.

So is the E-M5 really about 1 stop behind the GH3?

DXO Mark Comparison

There are two issues here. The first issue relates to the distinct difference between DxO Labs' RAW-level "Saturation ISO" metric and how ISO values are measured and specified in relation to JPGs.

See this post and it's references: http://forums.dpreview.com/forums/thread/3387707

The second issue relates to the fact that the E-M5 (unlike the GH2, and probably also unlike the GH3) uses additional non-linearity (over and above the normal sRGB or Adobe RGB gamma-correction that is part of the process of mapping RGB image-data into these color-spaces) that affects the camera's (JPG, not RAW referenced) metering system (and, hence the "image brightness").

While the ratio of the peak levels (relative to the respective absolute maximum linear recording levels) between JPGs divided by RAWs in the GH2 equals 0.0 EV at all (peak) JPG Green channel levels, ...

... the ratio of the peak levels (relative to absolute maximum linear recording levels) between JPGs divided by RAWs in the E-M5 (Gradation=Normal) equals around -1.5 EV at all (peak) JPG Green channel levels where the (color-space mapped, gamma-corrected) JPG Green output channel level is less than (around) 208.

However (due to the non-linear tone-curve transfer-function applied in the E-M5 JPG-referenced metering), the ratio of the peak levels (relative to absolute maximum linear recording levels) between JPGs and RAWs in the E-M5 (Gradation=Normal) equals around -0.5 EV at (peak) JPG Green channel levels where the (color-space mapped, gamma-corrected) JPG Green output channel level is equal to the maximum value of 255.

As is the (general) case (on the RAW RGB-channel level) with (at least) the vast majority of RAW-recording cameras, the (RAW-level, pre white-balancing) Red and Blue channel levels exist at peak levels on the order of -1.0 EV lower than that of the RAW Green channel (subject to the particluar color-temperature of the previewed scene "luminance").

This effect occurs as a result of the approximately 1.0 EV of non-linear tone-curve transfer-function gain-reduction that is implemented in the E-M5's "Gradation=Normal" mode.

All of the other "Gradation" modes implement varying (but in all cases significant) amounts of this non-linear gain-reduction. From my reading of the E-M5 User Manual, there appears to be no way to disable these (JPG tone-curve transfer-function, "gradation" mode caused) results.

See the E-M5 "Dynamic range comparison" graph here:

http://www.dpreview.com/reviews/olympusem5/18

Where it comes to RAW-level processing, Adobe's Camera RAW and Lightroom post-scale the RAW image-data values when processing GH2 and E-M5 RAWs by -0.5 EV and +0.5 EV, respectively.

In the case of the GH3, Adobe applications do not appear to apply any RAW-level post-scaling

DM...

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