For an enthusiast, one of the S100's most useful features is likely to be its ability to shoot raw files. Although the S100's JPEG engine is quite capable of producing images that by default, have a good balance of detail and basic control over noise reduction, you have a lot more freedom if you shoot raw. With raw files you have the ability to really fine-tune noise reduction, adjust white balance, make lens corrections and tweak sharpening to a degree that would never be possible from editing a JPEG.
This becomes especially useful when shooting at the higher end of the S100's ISO range. As you can see in the samples below, the ISO 3200 raw image converted in ACR (on the right) retains more low-contrast detail than the JPEG on the left. Saturation and color rendition is more accurate as well. The wood in the converted raw image is much closer to its actual color, too, than the more reddish rendition from the JPEG.
There are some trade-offs when shooting in raw however. Raw file sizes are about 5 times larger (around 15MB instead of about 3MB) than JPEG, and shot to shot time increases slightly (becoming ~1 sec slower). Although, with a sufficiently large and fast SD card we would always opt to shoot with raw capture.
Automatic CA/Fringing Correction
Automatic fringing/CA correction in JPEGs is fast becoming a standard feature in compact cameras these days, and with the DIGIC 5 processor in the S100, Canon has caught up with the pack. The S100 corrects both fringing and distortion automatically in JPEG files.
Built-in ND Filter
The Canon S100 comes equipped with a built-in optical ND filter that can be engaged to allow you shoot with wider apertures in bright light or at slower shutter speeds to blur motion. The filter is equivalent to 3 stops of decreased brightness, meaning that if the camera was correctly metered at F5.6 @ 1/30s for a given ISO the ND filter would allow you to shoot at F2.0 @ 1/30s, or you could keep the aperture at f5.6 and decrease the shutter speed to 1/4s. This is especially handy if you need to make long exposures in bright light - for capturing traffic movement, moving water, and so on.
Overall Image Quality/Specifics
Canon's new purpose-built CMOS sensor in the S100 is capable of capturing images that are detailed, nicely saturated and very clean, especially at the lower end of the camera's ISO scale. The new CMOS sensor gives slightly better image quality than the previous-generation 10MP CCD - an improvement that is subtle but noticeable, especially at higher ISO settings. While the 2MP increase in sensor resolution over the S95 is modest in terms of the additional detail that the camera can capture, the 20% increase in total pixel count does help to offset the effect of noise at a given display size/magnification.
Although the S100 has a broader (24mm - 120mm) zoom range than its predecessor the S95, we've found that the new lens is not as uniformly sharp across the frame. We've tried no fewer than five sample cameras for our studio testing, and in the worst cases significant decentering of the lens causes one side of the frame to be noticeably out of focus in our studio scene (shot at a subject distance of approximately 1 meter). In all cases, moderate to strong softening occurs at the edges of the frame. This is very apparent in our studio comparison tool, where it is easy to spot any visual discrepancies at a pixel level. However, this does not tell the whole story, in our real-world sample shots decentering has been much less of an issue and most of our shots are not overtly blurred by the lens (take a look at the 'lens' page of this review for more detail).
A cause of genuine frustration though is that even though Canon has extended the range of ISO settings on the S100 up to 6400, when auto ISO is used, the ISO sensitivity span is capped at a maximum of ISO 1600. It is possible to adjust the auto maximum from ISO 400 to 1600, but it stops there. If you want to shoot in those higher ISO settings you'll need to select it manually in any of the PASM modes.