Sigma AF 18-125mm f/3.5-5.6 DC (Pentax K) - Review / Lab Test Report - Analysis
Lens Reviews - Pentax
Article Index
Introduction
Analysis

Distortion

A major weakness of most long range zoom lenses is the level of distortion and the Sigma is no exception. At 18mm the lens produces a very pronounced degree of barrel distortion changing to pronounced pincushion distortion from 35mm onwards. If you´re looking for a lens for architecture photography this one is clearly not for you.

Move the mouse cursor over the focal length text marks below to observe the respective distortion
18mm 35mm 70mm 125mm

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

Vignetting

Typical for most dedicated APS-C lenses the Sigma produces quite strong vignetting. At 18mm the vignetting exceeds 1.3EV so stopping down to at least ~f/5.6 is generally a good idea here. The problem isn't quite as pronounced at the other tested focal lengths but you have to be careful towards the long end of the zoom range where the vignetting increases again to more than 1EV @ 125mm f/5.6.

MTF (resolution)

The Sigma has already been tested with surprisingly good resolution results within the Canon scope . However, the 10 megapixel Sony sensor used in the Pentax K10D is slightly bigger thus pushing the borders a bit more towards the end of the supported image circle of the lens - this is not without impacts specifically in the extreme corners of the image field.

At 18mm the center and border performance is very high throughout the tested aperture range. However, the extreme corners are soft at f/3.5 although they recover nicely at f/5.6 and more so at f/8. The situation is fairly similar at 35mm. At 70mm and 125mm there's a slight drop in performance - less so in the center which remains on a very good level even at wide-open aperture - but the border performance drops down to fair or good (just) levels. Stopping down to f/8 or f/11 boosts the borders quality to pretty high levels though.

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)

It may be surprising but the Sigma actually manages to produces a comparatively low degree of lateral chromatic aberrations (color shadows at harsh contrast transitions) with a peak around 1px on the average at the image borders at 18mm and 125mm and even less in between.

Sample Images

I've published some sample images in the review of the Canon variant of this lens.

Verdict

The Sigma AF 18-125mm f/3.5-5.6 DC is naturally a bit of compromise solution but the overall package is actually quite sound especially when taking the low price tag into account. The lens is pretty sharp when stopped down a little and CAs are surprisingly low for such an extreme zoom. However, the vignetting could be a little better at wide-open aperture specifically at the 18mm and 125mm setting and the level of distortions is very high throughout the zoom range. Nonetheless these are viable compromises for a compact 7x zoom. The Canon variant of the lens showed a miserable AF performance on the EOS 350D but the Pentax K10D has obviously a superior AF module which can handle the task quite well although the AF speed diminishes significantly in low light situations - this may be very different on smaller or older Pentax DSLR with a less capable AF system so if you own one of these better check this aspect if you're interested in this lens.

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