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 S100's measured sensitivities are about 1/3 stop lower than indicated (i.e. images are fractionally darker than expected for any given set of exposure values).
Noise and Noise Reduction (JPEG)
ISO range noise comparison
The combination of Canon's homegrown CMOS sensor and DIGIC 5 image processor does a good job of managing noise while maintaining a reasonable amount of detail. Noise is well-controlled throughout the ISO range, and JPEG noise reduction retains most of the fine detail up to ISO 800. After that point, more aggressive noise reduction begins to obscure low-contrast detail.
In general, the S100 out-performs both the Canon S95 and the Panasonic LX5 in terms of visible noise in JPEG. And although the noise at ISO 3200 appears similar between the S100 and S95 on the graphs, the retention of detail is much greater in the S100 (as you can see by comparing the faces from the samples tab).
Raw noise (ACR 6.6 noise reduction set to zero)
In regards to luminance noise, Canon's new 12MP CMOS sensor produces similar results when compared to its predecessor the S95, although chroma noise is marginally more prominent across the ISO range. However this is not the whole picture. The increase in resolution allows the S100 to retain more true image detail at a given ISO setting than the 10MP sensor in the S95.