Detailed analysis of the Canon PowerShot S2 IS images, from Imatest(tm)
I've recently begun using Norman Koren's excellent "Imatest" analysis program for quantitative, thoroughly objective analysis of digicam test images. I highly commend it to our technically-oriented readers, as it's far and away the best, most comprehensive analysis program I've found to date.
My comments below are just brief observations of what I see in the Imatest results. A full discussion of all the data Imatest produces is really beyond the scope of this review: Visit the Imatest web site for a full discussion of what the program measures, how it performs its computations, and how to interpret its output.
Here's some of the results produced by Imatest for the Canon PowerShot S2 IS:
The S2 IS generally has pretty good color accuracy, and for the most part, color shifts are such as to produce more pleasing images in typical situations. Like most digital cameras I test, strong reds are oversaturated, but less so with the S2 IS than is commonly the case. Like many cameras and particularly most Canon models, cyans are shifted toward pure blues a fair bit, a tactic that produces more appealing sky colors. At the same time, purple-blues are shifted away from purples, to avoid the tendency of many cameras to render blues with tinges of purple in them. Saturation in greens is pumped up slightly, producing nice-looking foliage in landscape shots, and yellows are slightly undersaturated. Overall, color that will be very pleasing to a majority of consumers. Average saturation was 109% (oversaturated by 9%, mostly in the reds, less so in the greens and blues), average "delta-E" color error was 5.4.
These images show the color behavior of the S2 IS directly. In each color swatch, the outer perimeter shows the color as actually captured by the camera, the inner square shows the color after correcting for the luminance of the photographed chart (as determined by a 2nd-order curve fit to the values of the gray swatches), and the small rectangle inside the inner square shows what the color should actually be, based on perfect rendering to the sRGB color space.
Gray Patch Tone and Noise Analysis
There's a lot in this particular graph, a lot more than I have room to go into here. Bottom line, the Canon S2 IS's noise levels are a bit higher than average, but the noise spectrum is such that the noise is has a bit finer-grained character to it at low ISOs than some of its competitors. At high ISOs, the grain pattern becomes coarser, while at the same time, more subject detail is lost.
This chart compares the Canon S2 IS's noise performance over a range of ISOs against that of other cameras. As you can see, the S2 IS's noise levels are a little high relative to those of most of its competitors across the ISO range, and particularly at higher ISO settings. What isn't shown here though, is the "grain" structure of the camera nor the extent to which it trades away fine detail as the ISO is increased. The S2 IS's noise starts out finer-grained at low ISO levels, but becomes rather coarse-grained at ISO 400, while also trading away some subtle subject detail there.
The chart above shows consolidated results from spatial frequency response measurements in both the horizontal and vertical axes. The "MTF 50" numbers tend to correlate best with visual perceptions of sharpness, so those are what I focus on here. The uncorrected resolution figures are 1280 line widths per picture height in the horizontal direction (corresponding to the vertically-oriented edge), and 1131 along the vertical axis (corresponding to the horizontally-oriented edge), for a combined average of 1206 LW/PH. Correcting to a "standardized" sharpening with a one-pixel radius increases this number a lot, to an average of 1369 LW/PH. Both uncorrected and corrected numbers are fairly good for a 5-megapixel camera. (This highlights the fact that you'll get the best resolution from the S2 IS by shooting with the camera's sharpening set to its lowest value, and then sharpening the images post-capture in Photoshop or some other imaging program.)
For the real techno-geeks, the two plots below show the actual edge response of the Canon S2 IS, for horizontal and vertical edges. (Here, you can see that there's actually relatively little overshoot caused by the camera's internal sharpening algorithm, but that the "bump" caused by the sharpening extends fairly far from the edge itself, contributing to the slight coarseness in the S2 IS's images.)
S2IS Test Images
S2IS Imatest Results
S2IS "Picky Details"
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