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

Leo wrote:

I have photographed xRite color checker 24 card with seven exposures stepping by +1/3EV:

0EV, +1/3EV, +2/3EV, +1.0EV. +1 1/3EV, +1 2/3EV, + 2.0EV

Then I have opened the images in LR and with expose slider adjusted the brightest Gray to 94.5 each image. Then I have verified the same color patch for all images (yellow). Its R, G and B reading were about the same up to +1.7EV. At the +2.0EV I had to adjust exposure control more negative. The adjustments were exactly:

0EV, -1/3EV, -2/3EV, -1.0EV. -1 1/3EV, -1 2/3EV.

The yellow patch R,G,B numbers were read as 90.7, 87.7, 37.6

The brightest gray patch showed that it is overexposed (red). At exposure -2.0EV the this patch reading were 97. 97, 97. I had to adjust exposure slider to -2.5EV to make the same reading = 94.5, however the whole image became little darker and the R, G and B for the yellow patch changed to: 87.7, 84.1 31.0. The overexposure red marking disappeared.

My conclusions from this test are:

1) I am underexposing routinely approximately -1.7EV.

2) The underexposure marking (red) is a good indicator and it well correlated with the RAW format.

Any suggestions?

A better way of verifying how you actually manage with exposure in this or that scenario is to use RawDigger, which immediately lets you see how far you got from the clipping point in each channel.

My practical advice is the same as before. Use the live-view highlight "blinkies" (with warning level set to 255). Dial in more exposure in steps of 1/3 EV until the first blinkies turn on in areas you want to preserve. This will work well in most cases but you may have to be a bit cautious in very cold or warm light or with very cold/warm subjects. Normally, the green channel is the first to clip but in these scenarios it might be red or blue in which case the blinkies might not give you proper warning. Note also that there may be scenes with such high DR that you are forced to give up the general strategy I propose (to expose such that the brightest highlights you care to preserve are at but not beyond the clipping point) since shadow noise becomes unbearable otherwise. In these cases, I usually bracket exposure and then select the best compromise at the PP stage or merge/align multiple exposures.

More about exposure in the post to which I link below, including some information on scenarios where the blinkies might not work as they should and how to handle that. This is for the E-M5 but everything probably applies to the E-PM2 as well since the sensor as well as camera generation is the same.

http://www.dpreview.com/forums/post/51169217