Sigma AF 18-125mm f/3.5-5.6 DC - Review / Test Report - Analysis
Lens Reviews -
Canon EOS (APS-C)
Thursday, 27 December 2007 00:51
Page 2 of 3
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 rather extreme degree
of barrel distortion changing to very pronounced pincushion distortion from
35mm and up. If you´re looking for a lens for architecture photography this one
is clearly not for you.
The chart above has a real-world size of about 120x80cm.
Typical for most reduced image circle lenses the Sigma produces quite strong
vignetting. At 18mm the vignetting exceeds 1.2EV so stopping down to ~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.
The lens exhibited a baffling performance regarding resolution and more
so relative to its extreme zoom ratio. At 18mm the lens is capable to produce
an excellent center quality and very good borders and there isn´t even any
noticeable performance drop towards the extreme corners which is otherwise
quite typical for reduced image circle lenses. The situation remains as
positive at the 35mm setting. Beyond there´s a decrease in performance
but the center quality continues to stay on a very good level but the
borders suffer from a more pronounced drop specifically at max. aperture.
At 70mm it´s possible to lift the quality to very good figures by stopping
down a little whereas at 125mm the borders stay "just" on a good level.
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
The lens produced a relatively high degree of chromatic aberrations (color
shadows at harsh contrast transitions) at the image borders at the extreme
ends of the zoom range. However, the situation isn't quite as extreme as
with some other (even higher priced) lenses and actually pretty acceptable
for such an extreme zoom lens.