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Sigma AF 30mm f/2.8 EX DN (Sony E mount) - Review / Test Report - Analysis |
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Lens Reviews -
Sony NEX
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Page 2 of 3
Distortion
The Sigma lens produces only a marginal amount of barrel distortion (~0.3%) which is not objectionable in field conditions.
Vignetting
The light fall-off is visible at f/2.8 (~0.7 f-stops) albeit comparatively moderate. Stopping down results only in a minor improvement but this amount of vignetting is no longer disturbing.
MTF (resolution)
The Sigma AF 30mm f/2.8 EX DN is capable of delivering an outstanding center performance straight from f/2.8 - it even broke the existing record for the Sony NEX 7 as of the time of this review. The lens is basically "diffraction-limited" in the image center so stopping down has no effect on the center performance anymore. The outer image regions reach a good level at f/2.8 and improve to very good (just) figures at f/4 to f/8. The overall performance diminishes from f/11 onward - this is a typical diffraction effect and no problem of the lens.
The centering quality of the tested sample was very good and there's only a very slight degree of field curvature.
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)
The Sigma lens produces a relatively high amount of CA (color shadows at harsh contrast transitions) with an average pixel width of around ~2px at the image borders. This aspect is certainly a weakness.
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