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Sigma AF 18-50mm f/2.8 EX DC macro - Review / Lab Test Report - Analysis |
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Lens Reviews -
Nikon / Nikkor (APS-C)
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Page 2 of 2
Distortion
The 18-50mm EX shows the quite typical distortion characteristic for a lens in
this class. At 18mm there is very pronounced barrel distortion which eases
continuously towards the long end of the zoom range. At 35mm and 50mm the lens
is basically free of distortion.
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Move the mouse cursor over the focal length text marks below to observe the respective distortion
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| 18mm |
24mm |
35mm |
50mm |
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The chart above has a real-world size of about 120x80cm.
Vignetting
As mentioned the 18-50mm EX is a reduced image circle lens (APS-C) and typical for
such lenses the vignetting is more pronounced compared to classic full frame lenses.
Unsurprisingly the problem is most significant at f/2.8 at all focal lengths
with a peak of 1.26EV at 18mm f/2.8. Generally it is a good idea to stop down to
f/4 in situations where a high degree of vignetting could be disturbing.
MTF (resolution)
The Sigma AF 18-50mm f/2.8 EX DC macro exhibited a rather mixed performance
in the MTF lab. The center performance is very good to excellent at all
focal lengths. The weak spot of the lens is at 18mm where the extreme borders
are soft at f/4 and more so at f/2.8. At medium apertures the lens
is very sharp across the frame. At 24mm the extreme borders are better at
large aperture settings but don´t really improve significantly when stopping
down. At 35mm and 50mm the border resolution is generally very good.
The tested sample suffered a bit from a centering problem which probably
affected the performance at large aperture settings to some degree.
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 (CAs)
Chromatic aberrations (color shadows at harsh contrast transitions) can be a problem
at 18mm. The average CA pixel width at the image borders easily exceeds
1px with a peak at 18mm f/4 (~1.7px). This will be visible in some situations
unless you correct the issue via an imaging application. The situation eases
towards the long end of the zoom range and at 35mm and 50mm lateral CAs are very
low even by critical standards.
Verdict
Generally the Sigma AF 18-50mm f/2.8 EX DC macro delivered a solid performance
but without being any better than the competition. The resolution results are generally
very fine except at 18mm where the extreme border performance leaves something
to be desired below f/5.6. Distortions as well as CAs are about in line with
the rest of the gang. Typical for most dedicated APS-C lenses the vignetting
could be a little better at wide-open aperture. The build quality and handling
of the Sigma is fine. The Sigma offers lots of bangs for your bucks but the comparable
Tamron AF 17-50mm f/2.8 SP seems to be a better alternative.
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