Canon EF-S 10-22 mm f/3.5-4.5 USM review - Distortion

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Canon EF-S 10-22 mm f/3.5-4.5 USM

23 September 2007

Arkadiusz Olech

6. Distortion

Distortion is a real problem for all wide angle lenses. To understand the situation better we need to get acquainted with a few definitions connected with the projections used in lenses. Here we only say that there are two approaches. The first one, which applies to regular lenses, is projection usage, which maintains straight lines (also called gnomonic projection). The second approach, used in fish-eye lenses, doesn't require the straight lines to be straight after the picture is taken. The Canon's 10-22 mm designers decided to use the first approach. Thanks to their decision, in spite of using extreme view corners, the distortion (of which we only mean the bending of straight lines) is small even for a 10 mm focal length, in which we gain over a 107deg field of view. To show it in detail, for a 10 mm we noticed a barrel distortion of -2.4%. The problem is almost completely absent for higher values of focal lengths. Thus, for 17 mm there is a slight pincushion distortion of 0.1% and for 22 mm it is increasing at an almost invisible value of 0.2%.

Canon EF-S 10-22 mm f/3.5-4.5 USM - Distortion

If by distortion we understand all the other geometrical deformations (not only line bending), the situation is completely different. If you remember correctly, the world map made in the gnomonic projection from your geography lessons, the meridians and parallels are straight lines. Although the countries' and continents' surface deformation around the equator and the tropics was not very great, it was always weird to see the huge size of Greenland next to the North Pole, which looked completely different on the globe.

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The Canon 10-22 mm (especially for 22 mm) behaves in a similar way to the map. The lines are still straight but on the edges, the area of the planes which are not parallel to the projector is very distorted. To prove it, we will give an example. We set up the camera in front of a door and we took three pictures. Each time the camera was at the same distance from the wall. In the first case, the camera was situated exactly opposite the door, for the second picture we moved it a bit to the right, still maintaining the parallel plane of the projector and the door plane, in the third case we placed the camera at a slight angle to the door. The results are obvious. The door surface was gravely distorted. This is what is going to happen with any object situated far away from the center of the frame. So, claiming that the Canon EF-S 10-22 mm, in spite of its wide angle, does not cause geometric distortion, is a bit of an overstatement.