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.
In our testing, the E-M1's images were consistent with the ISO standard - ISO 100 = ISO 100.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The noise from the E-M1 is very similar to its peers (including the E-M5), both visually and numerically. This suggests that the use of some sensor pixels as phase detection elements for focusing has minimal impact on image quality. The JPEG engine does a good job of keeping noise at bay without sacrificing too much detail, and it's only at ISO 6400 and above that things start to fall apart. With our preferred settings, i.e. Noise Filter set to 'Off' and sharpening turned down, we'd expect a slightly noisier result but with greater detail retention. Overall, it's a good performance at all but the highest ISOs.
Noise Filter settings
Olympus's noise reduction setting is accessed from the menu (Custom Menu E: Noise Filter), with four options available (Off, Low, Standard and High). Confusingly there's also a 'Noise Reduction' control, but this only applies to long exposures. We've generally found Olympus's 'Standard' setting to be over-enthusiastic for our tastes, with a tendency to smear fine low-contrast detail at both intermediate and high ISOs (it seems optimised much more for print than on-screen viewing). Here you can see that turning the Noise Filter to 'Off' or 'Low' gives a bit more detail at ISOs up to 3200, without too much additional noise.
ACR Raw noise (ACR 8.2 noise reduction set to zero)
Here we look at the RAW files processed through Adobe Camera Raw (in this case version 8.2). Images are brightness matched and processed with all noise reduction options set to zero. Adobe does a degree of noise reduction even when the user-controlled NR is turned off.
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.
The EM1 again holds its own in this comparison, with if anything slightly lower measured noise than the E-M5. It's a very close match to the 24MP Nikon D7100 in this pixel-level analysis, but this means that the higher pixel count camera will give visually-cleaner results when images are displayed or printed at the same size. Overall though, it's again a good performance, with the Micro Four Thirds camera looking pretty competitive with its APS-C sensor counterparts.