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Lens review

Sigma A 18-35 mm f/1.8 DC HSM

18 June 2013
Szymon Starczewski

6. Distortion

Zoom lenses designer for APS-C/DX sensors, which focal length starts from 17-18 mm, can have serious barrel distortion problems at wide angle even if they don’t qualify as megazooms. On the one hand the Sigma had it easier here because its focal range is not so wide. On the other hand such a great aperture fastness must create obstacles for optics specialists who have to think how to keep all optical aberrations on check.

Sigma didn’t follow the example of constructors of many mirrorless systems and didn’t give up on the correction of distortion. Quite the opposite in fact. You can say they managed to respond to that challenge quite nicely.

At the widest angle of view the barrel distortion is clearly visible, amounting to -2.46%. On the other hand such a value can be hardly called high if you take into account the fact that the lenses, mentioned by us at the beginning of this chapter, can have the distortion level of -3% or even -4%.

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The deformations decrease pretty quickly with the increase of the focal length. At 20 mm the “barrel” is just -1.58%, it changes the sign near 24 mm with the official result of 0.01%. Then the distortion starts to increase. At 28 mm it gets to a not very bothersome value of 0.88% and at 35 mm it amounts to 1.1% which is deemed by us to be an acceptable level.

Canon 50D, 18 mm
Sigma A 18-35 mm f/1.8 DC HSM  - Distortion
Canon 50D, 24 mm
Sigma A 18-35 mm f/1.8 DC HSM  - Distortion
Canon 50D, 35 mm
Sigma A 18-35 mm f/1.8 DC HSM  - Distortion