This is another IXUS. This time the 255 HS instead of the 240 HS. What I was hoping for from the 255 HS was better low light handling, better resolution at and less vignetting in the corners as compared to the 240 HS. And yet, from the specs alone, there should be no advantage. They both have 1/2.3" (7.68 mm across the diagonal) Canon-designed and manufactured back-illuminated sensors (the "HS" indicates "high sensitivity" due to the back-illuminated sensors) but the 255 has a 12.1 Mpixel sensor and the 240 a 16.1 Mpixel sensor. They both have DIGIC 5 processors. The 240 has a 5x zoom with an f-stop range of f2.7-5.9 whereas the 255 has a 10x zoom with a lesser f3.0-6.9 aperture over its range. The 240 is a touch screen model, the 255 not. Looking at the specs, then over its 5x zoom range, you would expect the 240 HS to have the better performance due to its limited 5x zoom range and wider aperture. But improvements are being made all the time to the algorithms in the camera (that can be upgraded with the firmware version) and the design and manufacture of the lenses. These lenses have vacuum-molded elements, some with aspheric surfaces, and the design and manufacturing tolerances of these elements are improving all the time (lens elements should be accurate to within a quarter the wavelength of light). As for the lens raw image, this can be improved on by electronic correction of distortion and remapping the blue, green and red channels to reduce chromatic aberration effects and algorithms can improve chromatic aberration effects where these are unavoidable. So as time progresses, better images are possible through lens design and manufacture and through the electronics that correct the image.
And it works. There are improvements in the 255 images over the 240 images. Improvements in color balance and contrast are obvious on playback (though this is purely a feature of the viewing screen - those improvements are not there when downloaded to the computer). The chromatic aberration correction in the 255 is so good that CA is no longer an issue, even in difficult circumstances, although it was hardly an issue for the 240 either. Edge and corner resolution are improved but at the widest angle the corner resolution could be a lot better. The mucky looking corners and poor far edge resolution of the 240 are thankfully much less there for the 255, although the corners are still poor at the widest angle. The noise reduction is better than the 240 and is done so well that noise is no longer an issue and in many cases leaves room for further image sharpening if desired. The only obvious negative thing is the 255 uses too much image sharpening giving rise to edge artifacts. The 240 did not suffer from this fault. Usually, too much sharpening leads to more grain, but in this case the 255 has more sharpening with less grain. And there is no way to reduce this sharpening using camera settings, which is a shame. I doubt there will be a firmware upgrade to fix this problem. To be fair, these edge artifacts are only visible when viewing images at full size on a computer screen. If the image is printed or reduced in size for better on-screen viewing then the artifacts largely disappear. The image resolution is still noticeably poorer going out to the far edges as can be seen in the "Fielmann" photo.
It is mid-2014 while I write this. I have done some more careful testing of the lens resolution using a lens test chart. At the widest angle, the corners are not very sharp (reducing the aperture at the wide angle would help corner resolution but this camera does not have an adjustable aperture). But as you get to the 35mm equivalent of 40mm then things have improved a great deal in that respect. With its 10x zoom this is surely the best super-compact zoom camera on the market.