The actual sensitivity of each indicated ISO is measured using the same shots as are used to measure ISO noise levels, we simply compare the exposure for each shot to the metered light level (using a calibrated Sekonic L-358), middle gray matched. We estimate the accuracy of these results to be +/- 1/6 EV (the margin of error given in the ISO specifications). We found that measured ISO from the E-PL2 is roughly identical with the indicated values. Only at the very highest ISO setting (6400) indicated ISO is about a 1/3 of a stop lower than indicated - so ISO 6400 = ISO 5000 (approx). This test is performed in manual mode without reference to the camera's metering system.
Noise and Noise Reduction (JPEG)
The Olympus E-PL2 offers four noise filter settings - Off, Low, Standard (default) and High, with noise reduction being applied at all ISO levels. The 'Off' setting still applies some chroma noise reduction but with the Noise filter set to a minimum the shadow areas can be quite grainy, even at low sensitivities. Depending on your taste the 'Low' or 'Standard' settings arguably provide the best balance between retention of detail and a clean image. Just stay away from the 'High' setting, as it smears fine and low contrast detail even at base ISO.
The E-PL2 captures its images, like the Panasonic GF2, on a Four Thirds sensor. From ISO 1600 upwards it becomes pretty obvious that these two cameras cannot quite keep up with the APS-C competition and produce more noise and/or detail smearing at higher sensitivities. Having said that the E-PL2's JPEG engine manages to render more pleasing results than the GF2, especially at the very highest ISO settings. There is a very visible loss of fine detail, but on the upside both luminance and chroma noise are kept under control.
RAW noise (ACR 6.3 noise reduction set to zero)
The amount of NR applied 'under the hood' is not high, but it does vary by camera (Adobe is attempting to normalize output across different sensors), so inevitably we are still looking at a balance of noise and noise reduction, rather than pure noise levels. However, the use of the most popular third-party RAW converter is intended to give a photographically relevant result, rather than simply comparing sensor performance in an abstract manner.
Please note that the Adobe ACR plugin used to process the E-PL2 RAW files for this test was a very early beta version and that there might be differences in the final release. If that's the case we will replace the files in this comparison accordingly.
With noise reduction turned down to zero in ACR all the cameras here show some signs of noise, even at base ISO. Having said that, the Micro Four Thirds cameras show more of both luminance and chroma noise at all sensitivities and the gap to the APS-C sensor competition widens as you go up the ISO scale. However, the Olympus E-PL2 performs again better than the Panasonic GF2 and produces cleaner output across the ISO range. The measured results in the graphs confirm what we can see in the sample shots.