ISO AccuracyThe 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 the actual ISO to be roughly 1/3 to 1/2 stop lower than indicated. Therefore, ISO 200 is really ISO 160 (or lower).
Noise and Noise Reduction (JPEG)
The X-M1 exhibits very similar noise levels to the bigger brothers, the X-Pro1 and X-E1. And, while this isn't the lowest measured value amongst its peers, it does a good job of maintaining a good balance between noise and noise reduction. Up to quite high settings, the X-M1 does a good job of retaining fine image detail. Interestingly there's a significant drop-off in image quality above ISO 6400 - the highest setting at which the camera will record Raw files.
Noise Reduction Settings Compared (JPEG)The X-M1 has five noise reduction settings, ranging from -2 to +2. To our tastes, the default setting strikes a good balance between noise and noise reduction. Turn it up to the +2 setting and fine detail can be scrubbed away. Equally, at the lower NR settings, there's not much more detail to be had and the extra noise can become a little obtrusive.
RAW noise (ACR 8.2 RC NR set to zero)
As with the the X-Pro1 Adobe Camera Raw's processing of the X-M1's files exhibit remarkably low noise figures, when compared with its Bayer sensor rivals. Chroma noise is strikingly low, and detail retention is impressively high - very much like the camera's JPEGs, in fact. Because of this, direct comparisons have to be treated with a degree of caution - it's best to assume that the de-mosaicing process of the X-Trans CMOS sensor behaves as though it's doing substantial chroma noise reduction relative to ACR's standard treatment of Bayer sensors.