Friday, January 1, 2010

Canon S90 Raw format distortion

As I wrote in the last post, Raw format is a very important feature of the S90. One of the first things you will notice is that the S90's raw captures have a significant amount of barrel distortion. Here is the raw capture of a photograph of some bricks, taken at the shortest focal length, 28mm (actually 6mm):

Brick wall, raw format before correction

The amount of distortion declines as the focal length increases, until there is almost no distortion at 105mm (22.5mm actual). The good thing is that it is quite easy to fix, although it is an extra step in post-processing that adds time.

With Photoshop Elements, use the Correct Camera Distortion tool (Filter -> Correct Camera Distortion...).

Here are the settings I use to fix distortion at various focal lengths, using the "correct distortion" slider:

  • 28mm (6mm actual): +25

  • 35mm (7.49mm actual): +14

  • 50mm (10.7mm actual): +7

  • 85mm (18.189 actual): +5

  • 105mm(22.5mm actual): +1
Bear in mind that you may also want to correct for virtical and horozontal perspective as well. Here is the result of the correction, before being cropped:

Brick wall, corrected for distortion, before cropping

The jpeg version of this image is processed inside the camera. The camera corrects distortion, but I have found it is not corrected fully. Here is the camera-processed jpeg, with a slight barrel distortion:

Brick wall, corrected in-camera for jpeg

. . . Rob Williams