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). Note that these tests are based on the sRGB JPEG output of the cameras, in accordance with ISO 12232:2006, the standard used by camera manufacturers.
By our tests, the E-PM1's measured sensitivities are about 1/6 stop lower than indicated (i.e. images are fractionally darker than expected for any given set of exposure values), which is within the tolerance allowed by the ISO specification.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The E-PM1 uses the stalwart Panasonic 12MP LiveMOS Four Thirds sensor, and offers noise performance that is virtually identical to that of its big brother, the Olympus E-PL3. The JPEG processing of Olympus' PEN cameras has traditionally done a reasonably good job of balancing noise reduction and detail preservation. These results show that the PEN Mini is no exception with its noise reduction comparing favorably against its APS-C competitors up until ISO 1600, above which the image quality starts to deteriorate rapidly. ISO 6400 is not pretty at all and ISO 12800 is almost unusable. Looking at the graphs, it's clear that sensor and noise reduction system used in the E-PM1 are essentially identical to both the E-PL3 and E-P3.
As we point out, however, in the Photographic Tests section of this review, the default NF Standard setting is very aggressive. We find preservation of fine detail in real-world images to be more successful with the noise filter at its minimum setting or even turned off altogether for images at ISO 1600 and lower. For a noise graph comparison across the camera's four NR settings, refer to our review of the Olympus E-P3.
Raw noise (ACR 6.5 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.
As expected, with noise reduction turned down to zero in ACR all the cameras here show some signs of noise, even at base ISO. We can see that the E-PM1's sensor, while on par with its chief rival, the Panasonic Lumix DMC-GF3, produces noisier output than its APS-C competition. Comparison to the E-PL3 and E-P3 shows that the E-PM1 gives essentially identical output to its more expensive siblings.