Detailed analysis of the Canon Powershot A85 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. (And with an introductory price of only $59, it's hard to beat.)
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 A85:
Color Accuracy
For the most part, the Canon PowerShot A85 has pretty good hue accuracy, but like many consumer-level digicams, tends to oversaturate its colors somewhat. The oversaturation is shown by the extent to which the circles (camera color) are displaced outward (higher saturation) relative to the ideal values (squares). On average, color saturation of swatches on the MacBeth ColorChecker(tm) chart are 116.1% of their ideal values. (An average oversaturation of 16.1%.)
The modest oversaturation is pretty typical of consumer digicams, and not
a cause for concern. Possibly more problematic is the hue shift seen in cyan
colors, (the points numbered 6 and 18 in the chart above), which tends to
move shades of cyan more toward blue tones. Interestingly, I didn't see much
evidence of this in my shooting with the camera, I suspect the main effect
would be to produce richer blues in sky colors, something most consumers would
probably like.
Color Analysis
These images show the color behavior of the PowerShot A85 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.
Here, it seems that at least some of the oversaturation arises from the camera's
rather contrasty tone curve. It does show the effect of the hue shift in the
cyans and blues though, with the camera reproducing those colors as a little
"richer looking" than in real life.
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 PowerShot A85's noise levels are a little higher than
average across the board, but the noise spectrum is such that the noise is fairly
fine-grained.
This chart compares the PowerShot A85's noise performance over a range of ISOs
against that of other cameras. As you can see, the A85 is somewhat noisier than
average across the board, particularly so at ISO 400. - The positive side of
this though, is that the camera tends to hold onto detail in subject areas of
subtle contrast better than most competing models do.
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 851 line widths
per picture height in the horizontal direction (corresponding to the vertically-oriented
edge), and 1036 along the vertical axis (corresponding to the horizontally-oriented
edge), for a combined average of 943 LW/PH. Correcting to a "standardized"
sharpening with a one-pixel radius increases this number a fair bit, to an average
of 1117 LW/PH. This is a little below the results produced by the best 4-megapixel
cameras, but still not bad.
For the real techno-geeks, the two plots below show the actual edge response
of the A85, for horizontal and vertical edges:
A85 Test Images
A85 Imatest Results
A85 Specifications
A85 "Picky Details"
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