Sigma AF 18-50mm f/3.5-5.6 DC (Nikon mount) - Review / Lab Test Report - Analysis
Lens Reviews - Nikon / Nikkor (APS-C)
Article Index
Introduction
Analysis

Distortion

At 18mm the Sigma shows a quite hefty degree of barrel distortion (3.1%) which eases continuously towards the long end of the zoom range. At 50mm there's only a marginal degree of barrel distortion.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
18mm 28mm 50mm

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

Vignetting

Typical for most true APS-C lens there's some vignetting at wide-open aperture. However, it is interesting that the level is a magnitude worse compared to the Canon variant - this does surely originate in the slightly smaller cropping factor (1.5x on Nikon vs 1.6x on Canon). At 18mm @ f/3.5 vignetting is very obvious and it is a good idea to stop down beyond f/5.6 at this setting. At 28mm the issue is quite well controlled whereas at 50mm vignetting is surprisingly high and stopping down doesn't really solve the situation here. It seems as if the lens is a little under-designed for 1.5x APS-C DSLRs in this respect.

MTF (resolution)

The lens produced surprisingly good resolution figures in the MTF lab. At 18mm the center performance is superb and the borders are also very good. The extreme borders are somewhat soft at f/3.5 so this setting should be avoided if possible. However, stopping down does improve the quality substantially here and at f/8 the figure are very good. The field curvature is quite pronounced at this focal length. At 28mm and 50mm the performance is more even - the center resolution decreases slightly whereas the borders and the extreme borders are generally very good.

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

The lens produced a rather extreme degree of chromatic aberrations (color shadows at harsh contrast transitions) at the image borders at 18mm. Stopping down helps to ease to problem a little here. At 28mm and 50mm the problem is well controlled.

Verdict

The Sigma AF 18-50mm f/3.5-5.6 DC is an budget lens which comes with a couple of weaknesses but also surprising strengths. At 18mm the level of distortions and CAs is quite extreme. Vignetting is also an issue at 18mm and 50mm - the latter is quite unusual especially because it doesn't really help much to stop down here. However, if you limit your aperture settings to f/5.6 and smaller the resolution figures are pretty impressive and comparable to much more pricey zooms out there. Regarding the extremely low price tag Sigma had to implement some compromises on the mechanical side like a rotating front element and a rotating focus ring in AF mode but other than that the build quality of this little lens is actually quite decent. Regarding its weaknesses the corresponding Nikkor AF-S 18-55mm f/3.5-5.6G ED DX may be a better choice though.

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