Tokina AT-X 116 PRO DX AF 11-16 mm f/2.8 review - Distortion

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Tokina AT-X 116 PRO DX AF 11-16 mm f/2.8

25 May 2009

Arkadiusz Olech

6. Distortion

Distortion in such instruments is an interesting problem, which may be hard to comprehend for those less familiar with the subject. How are we supposed to understand the fact that fisheye lenses have focal lengths of 10-10,5 mm and a field of view of 180 degrees, while Tokina, at 11 mm, gives us “only” 104 degrees? We must remember one, very important thing. The field of view depends not only on focal length, but also on the type of projection used in our lens.

Tokina 11-16 and the likes of it use a type of projection that favors the reproduction of straight lines (rectilinear projection), trying not to curve them (hence their field of view is more narrow than that of a fisheye) and at the same time, it causes significant distortions in the corners. Just revisit the times of your geography classes at primary school. Remember the world map, drawn in gnomonic projection, where all parallels and meridians are shown as straight lines? While around the equator and the tropics , the distortion of the surface of continents and countries are insignificant, the dimensions of Greenland (not at all large on a globe) are strikingly monstrous. The same goes for such lenses.

Most ultra-wide lenses tested on small sensors, at the widest possible angle would produce a barrel distortion of about -2%, decreasing to less than 1% for bigger focal lengths. A glorious exception was the Sigma with just -0,88% at 10 mm, as opposed to Tamron reaching -4.0%. Unfortunately, Tokina 11-16 mm gets closer to the shameful result rather than the good one. At 11 mm, we measured a distortion of -3,24%. Even midway through the range, it remains visible, amounting -1,74%. No sooner than at 16 mm does it fall to the decent level of -0,95%.

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