![]() Distortion: -2.39īarrel distortion is of a fairly low order and it’s very uniform and easy to correct, when necessary. There’s only fairly minor lateral chromatic aberration, which only tends to be visible towards the extreme corners of the frame. The tests also measure distortion and color fringing (chromatic aberration). We use Imatest SFR (spatial frequency response) charts and analysis software to plot lens resolution at the centre of the image frame, corners and mid-point distances, across the range of aperture settings and, with zoom lenses, at four different focal lengths. Photos of test charts are taken across the range of apertures and zooms (where available), then analyzed for sharpness, distortion and chromatic aberrations. We run a range of lab tests under controlled conditions, using the Imatest Master testing suite. Contrast remains very good even when shooting wide-open, and colour rendition is excellent. Lateral chromatic aberration and barrel distortion are of a low order, and there’s good resistance to ghosting and flare. Overall performance is highly impressive. Reduce the aperture by a single f/stop and the fringing virtually disappears. Even so, the Sigma isn’t an underachiever in this respect, matching or beating many other fast primes including the Canon EF-M 32mm f/1.4. Unlike lateral chromatic aberration, which generally only occurs towards the edges and corners of the frame, axial chromatic aberration can occur anywhere in the frame. Also referred to as ‘bokeh fringing’, this shows up as purple and green fringes around high-contrast transitions, in front of or behind the point of focus. The only downside is that axial or ‘longitudinal’ chromatic aberration is sometimes visible when shooting at very wide apertures. Bokeh (the quality of defocused areas) is pleasantly smooth and remains so when stopping down a little, helped by a well-rounded 9-blade diaphragm. At this aperture, you can get a really tight depth of field when shooting at fairly short focus distances. ![]() Sharpness is impressive even when shooting wide-open at f/1.4. This quarter-frame crop shows the slight purple and green axial chromatic aberration that can occur when shooting wide-open (Image credit: Matthew Richards) ![]()
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