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-M5's measured sensitivities are about 1/3 stop lower than indicated across the ISO range (i.e. images are fractionally darker than expected for any given set of exposure values). A discrepancy this small has little practical impact in real world use but means the camera will be slightly flattered in our tests.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The noise from the E-M5 is very similar to its peers, both visually and numerically - suggesting the 16MP sensor is much closer to contemporary quality standards. The JPEG engine does a good job of keeping noise at bay without sacrificing too much detail (giving it a distinct edge over the Panasonic G3). With, at our preferred settings, with Noise Filter set to off and sharpening turned down, we'd expect a slightly noisier result but with greater detail retention. Either way, it's a good performance at all but the highest ISOs.
Raw noise (ACR 6.7 Beta, noise reduction set to zero)
Again in Raw, the E-M5 is producing very similar results to its peers. To an extent this is because Adobe tries to equalize the output of different cameras but the fact that the Olympus is retaining both detail and contrast at least as well as its competitors suggests the underlying data is at least as clean. The Panasonic's result looks worse but this looks like it is as much to do with the profiling (and lower black-point) than the Olympus, we suspect if we raised the black point to reduce the magenta tinge, the results would be comparable to the E-M5.