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The lens shows a fair amount of barrel distortion on a DX camera. The total amount is not unusual for such a wide lens (in its native scope), but it's quite surprising that the lens shows slightly more distortion on DX than on FX. This is an indication that most of the distortion happens towards the center of the frame and is evened out on the FX frame by a little pincushion style distortion. On a DX camera, this part of the image (and thus the distortion) is cut off.
The chart above has a real-world size of about 120x80cm.
The lens shows quite pronounced vignetting wide open. As usual, stopping down reduces the issue. At f/2.8, the light fall-off towards the borders is already down to just half a stop, at smaller apertures it's no longer an issue at all.
The lens shows very good resolution in the image center wide open, which increases to excellent levels by stopping down.
The borders and corners are a very different story, though. Both are poor wide open and and when stopped down to f/2.8. However, from f/5.6 onwards both areas show significantly increased values and the lens is able to deliver very good resolution across the whole DX frame.
Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Chromatic Aberrations (CAs)
Chromatic aberrations (color shadows at harsh contrast transitions) are well under control for a lens with such a focal length and range from just above 1 to just around 1.1 pixels at the image borders throughout the tested aperture range.
CAs can easily be corrected in software or by the camera itself (most modern Nikon DSLRs remove CAs themselves if you shoot JPGs).
One of the primary usage scenarios for a large aperture lens is to separate the main subject from the background. In such an image the quality of the bokeh (out-of-focus blur) is of major significance.
At large apertures, the EX 20 shows quite smooth bokeh in front of the focal plane, but some signs of nervousness behind it, especially in the transition zone.
Background highlights also look quite nervous and in addition show pronounced outlining. Towards the image borders, they are cut off at large aperture due to mechanical vignetting.
Both the general bokeh quality as well as background highlights improve in quality by stopping down. However, with a wide angle lens one needs to get quite close to the main subject to still get a significant amount of background blur. The lens' very short minimum focus distance comes handy here.
Bokeh fringing (non-coinciding focal planes of the various colors) is a common issue with relatively fast glass. As you can
notice below, the halos have different colors - magenta (red + blue) in front the focus point
and green beyond. Truly "apochromatic" lenses don't show this kind of fringing but these lenses are very rare - especially
below 100mm. Unlike lateral CAs, bokeh fringing cannot easily be fixed in post processing.
Typical for most fast primes, the EX 20 shows some amount of fringing at large aperture settings, which can of course be reduced by stopping down.
You can find some sample images taken with the Nikon D3x in our Nikon FX review of the lens.
VerdictJust like its slightly longer sister lenses, the Sigma AF 20mm f/1.8 EX DG leaves us with mixed feelings. The center resolution of the lens is very good to excellent, while the borders and corners disappoint at large apertures. Stopped down, the resolution is very good across the DX frame, though.
Vignetting is very pronounced wide open, even on a DX camera. Distortion is moderate and CAs are well under control, especially for such a wide lens (in its native scope).
The bokeh is somewhat nervous at large apertures, but improves considerably when stopped down a little.
Unlike the 24mm and 28mm lenses, the EX 20/1.8 does not feature a moving inner lens tube and consequently doesn't suffer from wobbling. In fact, the lens is very solid and the build quality leaves nothing to complain about. Unfortunately the lens has no internal motor, which means it doesn't autofocus on Nikon's entry-level DX cameras.
So, just like its longer sister lenses, it's not an overly attractive choice for DX shooters, but because of its very short minimum focus distance it's an interesting lens to shoot close-ups with wide perspectives.
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