Image Sharpness Results

The Canon SD500 images were resized to 5 MP for the same dimensions as the Canon S50. In Imatest, a 200x100 pixel region was selected, consistently in the same area of the image. Imatest then outputs all the calculations.

The Canon SD500 had consistently smaller 10 – 90 % rise distance and larger MTF50, but higher chromatic aberration than the Canon S50.

The ideal conditions are a short 10 – 90% rise distance, high MTF50, steep edge profile, broad frequency spectrum, and minimal chromatic aberration.

In the edge profile plots, the black line indicates the original edge profile for the image, while the red dotted line indicates the edge profile after standardized sharpening, to provide a normalization between different cameras. If the red and black lines are similar, it means the image processing of the camera did not try to over sharpen the image.

The MTF50 is defined as the spatial frequency where the image contrast drops to 50% of the maximum contrast. A higher cycle per pixel value means that the lens can resolve higher spatial frequencies and therefore will allow finer resolution in the image.

To quantify the chromatic aberration, the edge profiles are plotted separately for the R, G, and B channels. The chromatic aberration is then taken to be proportional to the area between the channels with the highest and lowest amplitudes. If the differences between the different channels is very large, there may be some “purple fringing” effects.

Below is a summary of image sharpness calculations for several images taken under similar conditions:

SD500	10-90% rise (pixels)	MTF50 (Cycles/Pixel)	Chromatic Aberration (area in pixels)
IMG_2890	1.12	0.416	0.197
IMG_2891	1.1	0.418	0.206
IMG_2892	1.06	0.429	0.193
IMG_2893 (3x zoom)	1.9	0.304	0.808
IMG_2894 (3x zoom)	1.8	0.317	0.851

S50	10-90% rise (pixels)	MTF50 (Cycles/Pixel)	Chromatic Aberration (area in pixels)
IMG_6681	1.35	0.334	0.0855
IMG_6682	1.33	0.334	0.165
IMG_6683	1.33	0.348	0.201
IMG_6684 (3x zoom)	1.92	0.28	0.124
IMG_6685 (3x zoom)	1.95	0.28	0.211

Image Sharpness Conclusions

We believe Imatest did a very good job at calculating the parameters to quantify the image sharpness. We did not write our own code, so there is no comparison to be made.

On a review site, www.dpreview.com, they suggested that the Canon S50 (5 megapixels) was more rushed to production, compared to the Canon S45 (4 megapixels), so although the sensor was upgraded, the benefits of the sensor in relation to the lens was not optimized. This resulted in images that are not as sharp, and with more chromatic aberration than expected. This agrees with our findings for the edge profile and MTF results. However, there is no way to confirm that it is due to un-optimized conditions.

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