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.
Olympus XZ-1 Noise /Noise Reduction
Unlike most of its peers, the XZ-1 has only its default noise reduction setting - there's no way of adjusting it other than shooting in Raw mode and applying your own choice of noise reduction in processing.
Noise and the effects of noise reduction are minimal until ISO 400 but above this point the image quality begins to drop off quite quickly. ISO 800 and 1600 lose a lot of fine detail to noise reduction but retain a reasonable amount of color integrity, meaning the images are still usable as moderately-sized prints. However, the highest two settings lose much of their detail and much semblance of color accuracy, making them increasingly difficult to use.
Although not visible in these samples, significant blotchy chroma noise starts to creep into the images from ISO 1600 upwards. Of course it's worth remembering that the XZ-1 has the brightest zoom lens on any current compact camera, meaning you can generally keep the ISOs down in many situations. The Samsung EX-1 hits F2.4 at its full 72mm extension, the Panasonic LX5 reaches F3.3 at its 90mm limit while the XZ-1 is F2.2 across this range. It then continues out to 112mm equiv at F2.5, making it almost two stops brighter than the Canon S95 when it gives up at 105mm equiv.
Raw files processed with a beta version of Adobe Camera Raw and may not represent final image quality. These will be updated when the finished software is available.
The XZ-1 looks both more detailed and more noisy than its peers at most settings (it's particularly noticeable at ISO 1600). This suggests a lower lever of noise reduction but our concern is this may mean Adobe hasn't yet completed its calibration of the processing. We don't want to draw too many conclusions while using a beta version of Adobe Camera Raw.