G Dan Mitchell Photography

Earlier this weekend I read a forum thread about the Canon EF 50mm f/1.4 lens, of which I have a copy. The thread lamented the barrel distortion of this lens (which I don’t really find to be a significant issue at all) and continued with posts suggesting alternatives including software correction is post-processing. A response to this proposal was that it would cause the image to deteriorate and would therefore be unacceptable.

I decide to test this idea. Using an old photograph taken with this lens, I cropped a small section from one of the far corners – the worst part of the frame and, according to some, subject to a lot of softness and distortion on full-frame bodies like my Canon 5D. To make things a bit more challenging I used a photograph that included a bunch of dried california grasses – full of very fine details and high contrast.

I converted the original RAW file using ACR with no sharpening. In Photoshop I cropped to a reasonable size, using that section from one of the corners of the image. Then I made a duplicate of the cropped image.

In one of the two versions of the crop I used the LensfixCI plugin to correct for the slight barrel distortion of the EF 50mm f/1.4 lens. This $29 plugin includes a database of many lenses, and also keeps a smaller databases of your lenses. It uses EXIF data to identify the lens (and focal length with zooms) used to take the photo and automatically applies optimum adjustments from its database. It takes me about 10 seconds to select the plugin and apply its changes.

Next I used my normal sharpening methods on both images, inspecting the results and making adjustments as I applied them. In the end, as would typically be the case, I used very slightly different sharpening settings for the two images – but that reflects the normal way of operating. Finally, I took the two images and placed them side by side in the single high quality jpg file that follows.

I have a darned hard time seeing any difference in sharpness, contrast, or color that might have been introduced by the correction process. If a difference is visible a) it is almost impossible to say which version is better, and b) the difference is almost certainly completely insignificant in an actual print. (Keep in mind that these are 100% crops of the worst part of the frame in the far corner – and that the area shown here would be a very tiny section of a full print.)

After doing this test, I’m not very concerned at all about any negative effects of using this method of correcting lens distortions – though I still would only apply the corrections in situations where the lens distortions actually create a noticeable problem with the photo.

(Anyone care to guess which half contains the “corrected” version of the crop? Feel free to post a comment and an explanation of what you (think you) see… ;-)

G Dan Mitchell :: Sep.16.2007
Equipment, Software, Technique, Tests
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