Olympus E-PM2 Actual ISO vs IS0 set in camera?

Leo wrote:

I am sorry to start the well beaten subject...

I have two 4/3 cameras: Olympus E-PM2 and Panasonic GX1. On ISO graph by dxomark.com for these two cameras the E-PM2 actual ISO is about half of what it set in camera (for ISO set to1600 the actual ISO is 800), while the actual and set in camera ISO for GX1 are equal. All the above measurements apply to RAW settings as DXO test the sensor.

As I have two cameras one, which perfectly match the set and actual ISO (GX1) and one with the actual ISO only half of the set ISO, I decided to test if there is any difference in the exposure between these two camera.

I have set the cameras to the same ISO, aperture, expose compensation and exposure metering and photographed the xRite White Card filling the viewfinder. The two cameras showed identical shutter speed. I have opened the images in the Lightroom and the RGB readings for both images were the same within few percent points difference.

I do trust DXO, however … my test does not follows the DXO statement.

See my reply to you in this other thread.

http://www.dpreview.com/forums/post/54220550

The in-camera ISO and the DxO "measured ISO" are simply two entirely different things.

The in-camera ISO is a measure of how much light it takes to achieve a certain brightness in the OOC jpegs. It also regulates the metering. As a rule, this ISO should be the same for all cameras since it corresponds to a norm issued by ISO that the manufacturers have agreed to follow. How well a camera conforms to this norm is tested by DPR and reported in the "noise and noise reduction" setting of each review. Very few cameras deviate significantly from the norm. One exception, however, are the Fuji X cameras, where the OOC jpegs come out about half a stop too dark, and which are therefore unduly favored (by given more exposure) in the DPR studio scene samples.

The DxO "measured ISO" is simply a measure of how much light it takes to reach the clipping point in the RAW file at a certain camera ISO. The more the camera ISO exceeds the "measured ISO", the more highlight headroom the camera reserves. The manufacturers are free to set the clipping point in the RAW file as they see fit and there is no cheating involved in setting it this rather than that way.