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 tests we found that measured ISOs from the E-M10 were accurate, meaning ISO 100 indicated is equal to ISO 100 measured.
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The E-M10 keeps up with its APS-C and MFT peers with retention of fine detail until ISO 6400 when things take a turn for the worse. 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. 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. NR High applies a great deal of noise suppression, and at ISO 800 starts to overpower any fine detail left in the scene.
ACR Raw noise (ACR 8.4 noise reduction set to zero)
Here we look at the RAW files processed through Adobe Camera Raw (in this case version 8.4). 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 E-M10 matches the E-M5's Raw noise performance closely, both of them looking competitive with their APS-C counterparts. The 24 megapixel D5300 shows less Raw noise throughout the ISO range, but considering the difference in sensor size and pixel count, the E-M10 isn't far behind. Bear in mind though that the higher pixel counts of the T5i and D5300 will give those cameras an advantage when images are displayed or printed at the same size as the E-M10's.