|
Sigma AF 18-50mm f/3.5-5.6 DC (Nikon mount) - Review / Lab Test Report - Analysis |
|
Lens Reviews -
Nikon / Nikkor (APS-C)
|
|
Page 2 of 2
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.
Please note that the MTF results are not directly comparable across the different systems!
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.
|
