As part of my journey into digital imaging, I found myself writing CornerFix, which can be found on http://sourceforge.net/projects/cornerfix/. The image is a screen shot of the Mac version.

CornerFix corrects for color dependent vignetting in digital images, which shows as cyan colored corners, as in the image on the left hand side of CornerFix's main window in the screen shot. The image in the screen shot comes from an M8 with a CV12 lens and IR filter on it. All digital cameras are some extent subject to this; current generation sensors are highly IR sensitive, so there needs to be a IR filter somewhere. But the combination of sensors and IR filters also cuts into the red part of the spectrum, and do so in a way that depends on the angle through which the light bends as it travels to the sensor. So red gets cut more in the corners. Most DSLR's do this a bit - take a picture of a white wall and you will probably see it, although many cameras correct internally to a greater or lesser extent.

Leica's M8 has a particular problem in this regard. Historically, one of the advantages that rangefinder cameras had was that the back of a rangefinder lens can be a lot closer to the film surface than is the case for a film SLR - the SLR needs space for the mirror, which a rangefinder doesn't have. So in the film world, rangefinder lenses had a lot more design freedom that SLR lenses - wide-angle lenses that on a SLR had to be reverse telephoto designs could be normal designs on a rangefinder. Ironically, in the digital world, this turns into a disadvantage. Because the back of the lens can be a lot closer to the film, the angle is more acute, and you get a worse cyan corner problem than a DSLR would.

M8's can correct for this problem themselves, but there are two issues for users - firstly, your lenses must be coded, which means that it must be a Leica lens, either new enough to have been coded when it was manufactured, or one that has been sent to the factory to be upgraded. So anyone with a non-Leica lens or one that can't be upgraded is out of luck, unless they somehow code the lens themselves. The second problem is that the M8's correction is "one size fits all"; it's designed for average situations, and sometimes doesn't do well in unusual lighting.

CornerFix allows the cyan corners problem to be corrected for any lens, in more or less any lighting situation, by post processing the camera's image file. The right hand side of the main window in the screen shot shows the corrected image. CornerFix is available for the Mac and for Windows. It's free and open source, released under the GPL. Note however that image must be a DNG formatted file - CornerFix doesn't work with TIFF or JPEG files.

For those interested in the technical details, the core of Cornerfix, which is common to both the Mac and PC versions, is written in "pure" C++, and uses the Adobe DNG toolkit to decode files. The GUI of the Windows version is written in C++/.Net, and the GUI of the Mac version in Cocoa. For those REALLY interested in the technical details, as CornerFix is GPL, you can download all the source code from site above.