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Aberration correction using RAW and Digital Photo Pro 3.2

With the release of version 3.2 of their Digital Photo Pro RAW processing program, Canon quietly snuck in one hugely interesting feature, an automated lens aberration correction module. This is pretty well unique amongst the camera manufacturers' free software, in that it can correct for four specific aberrations - namely peripheral illumination (i.e. light fall-off), distortion, chromatic aberration, and colour blur. Essentially the program reads the RAW file's EXIF data to establish the camera/lens combination, then uses the focal length and distance data, presumably in combination with a look-up table of pre-determined corrections, to apply the relevant adjustments. One problem with this approach is that not all cameras and lenses are yet supported in version 3.2, however our test combination of the 18-55mm IS on the EOS 40D is one of those favoured few that made it onto the initial list, so we thought we'd investigate just how well it worked.

Chromatic aberration correction

Below are crops of checkerboard patterns taken from the extreme corner of our lens test chart shot at 18mm and F5.6, where the lens shows strong blue/yellow fringing due to chromatic aberration. The first was processed normally, and the second with chromatic aberration correction applied (with the blue slider set to maximum). DPP has done a very good job of reducing the effects of chromatic aberration, although it can't perhaps quite eliminate them altogether.

200% crops compared
(Photoshop 'nearest neighbour' upsizing)
Normally processed Chromatic aberration correction

Colour blur correction

Here we’re looking at Digital Photo Pro’s ability to correct for blue colour blur, one of this lens’s signature aberrations. Below are crops of checkerboard patterns taken from the centre of our lens test chart shot at 18mm and F3.5, where blue colour blur is most obvious. The first was processed normally, and the second with colour blur correction applied. Again DPP has done a pretty good job, and managed to turn the blue areas of the squares to something closer to black.

200% crops compared
(Photoshop 'nearest neighbour' upsizing)
a a
Normally processed Colour blur correction

Distortion and falloff correction

To round things off, here's a demonstration of Digital Photo Pro's falloff (peripheral illumination) and distortion correction capabilities, using a shot taken at 18mm, F3.5 and about 1.5m subject distance; conditions under which falloff and distortion will be almost as bad as they get. The first image file is processed normally, the next has falloff correction, and the third adds distortion correction. The results speak for themselves - DPP has done a pretty good job at correcting for light falloff, and the distortion correction has unbent the lines in the image near-perfectly.

EOS 40D, 18mm F3.5
Original Peripheral Illumination PI + Distortion

Taken together, these results show that Digital Photo Pro is really rather good at correcting the effects of chromatic aberration, colour blur, falloff and distortion in a pretty straightforward fashion, and therefore has the potential to improve noticeably the results obtainable from inexpensive and therefore less-well corrected lenses. By the nature of the process, you'll only benefit if your camera and lenses are on the list of those which are supported, but we fully expect that Canon has teams of boffins in a laboratory somewhere painstakingly working their way through every combination imaginable. And you'll still have to shoot RAW, but we expect it's only a matter of time before these kinds of corrections get written into cameras' internal JPEG processing routines.

Of course one thing no RAW converter can make up for is any inherent unsharpness of the lens, so don't expect to get quite top-end performance from a budget lens via software assistance.

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