Sigma AF 20mm f/1.8 EX DG RF (FX) - Review / Test Report - Analysis
Lens Reviews - Nikon / Nikkor (full format)

Distortion

The lens shows a moderate amount of barrel distortion with a minor amount of mustache subtype towards the image corners. This can be visible in critical shots especially with straight lines near the image borders. However, the distortion is uniform and easy to correct in post processing. And for such a wide lens, the total amount can be considered as quite low.

The chart above has a real-world size of about 120x80cm.

Vignetting

The lens shows massive amount of light fall-off towards the corners at large apertures. Wide open, the corners darken more than 3 EV, which is well beyond our scale. As usual stopping down reduces the issue, but even at rather small apertures there is still more than half a stop of vignetting left.

We're performing our vignetting analysis based on (uncorrected) JPEGs straight from the camera. The JPG engine of the Nikon D3x features a rather flat gradation curve, thus has a moderate contrast characteristic, resulting in comparatively low vignetting figures - the corresponding Canon figures are roughly 40% higher due to the more aggressive default contrast setting.

MTF (resolution)

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. The borders recover to good values at f/5.6 and (just) very good resolution when stopped down to f/8. The extreme corners stay on a much lower level at large apertures and never get beyond good values, even when stopped down considerably.

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 Imatest Explanations

Chromatic Aberrations (CAs)

Chromatic aberrations (color shadows at harsh contrast transitions) are well under control for a wide angle lens and range from just below 1.3 to just above 1.6 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).

Bokeh

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 MFD comes handy here.

Bokeh Fringing

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 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.

Move the mouse cursor over the f-stop marks below to observe the respective LoCAs
f/1.8 f/2.8 f/4.0 f/5.6 f/8.0




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