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Sigma 65mm f/2 DG DN Contemporary Series I - Review / Test Report - Analysis |
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Lens Reviews -
Sony Alpha (Full Format)
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Page 2 of 3
Distortions
Enough raving about its build quality - let's go for the beef - image quality. If the Sigma 65mm f/2 DG DN Contemporary has a major weakness, it's image distortion in RAW images. A pincushion distortion of ~2.5% is certainly on the high side for a prime lens. Of course, image auto-correction comes to the rescue here so you won't notice this in real life unless you want to.
Vignetting
The Sigma is a comparatively slow lens by "50/55mm"-class prime standards, so vignetting is not a major issue. RAW images reveal a native vignetting of about 1.8 EV (f-stops) at f/2. That would be noticeable (and possibly even desirable for portraits). Stopping down to f/2.8 resolves most of this and at f/4, the issue is gone by real-world standards. Image auto-correction is primarily active at f/2 by reducing the peak.
MTF (resolution)
In terms of resolution, the lens is top-notch. The center quality is absolutely outstanding straight from f/2. It's safe to state that it exceeds the limits of the 42mp sensor (used for testing) by quite a margin. The near-center is almost as good. The outer image field is still on a very good level, with excellent quality around the f/4 mark. Diffraction effects are becoming more noticeable from f/11 onward.
The field curvature is low. The centering quality of the tested sample was good.
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 of sharpness.
If you want to know more about the MTF50 figures, you may check out the corresponding Imatest Explanations
Chromatic Aberrations (CAs)
Lateral CAs vary along the aperture range with a maximum pixel width of 0.75px at the image borders at f/2 - and a little less at smaller aperture settings. Once again, image auto-correction will take care of this without losing image quality.
Bokeh
Sharpness is one thing but if you are in the market for a fast prime lens, you want to use it for its shallow depth-of-field capabilities. We are pleased to report that the quality of the bokeh is just outstanding. Out-of-focus highlights are very smooth with no outlining as you can see below. This is almost surprising because the lens has two aspherical elements. Lower-quality aspherical elements tend to produce onion rings but Sigma's version doesn't fall into this category. The circular shape is mostly maintained at f/2.8 and the more edgy aperture shape creeps in at f/4 here.
The Sigma lens isn't immune to the deterioration of the highlight discs towards the borders/corners but the effect is much less pronounced than on most lenses with just a minimal tendency towards "cat eyes". Technically, stopping down helps to "fix" this. However, the results are still more pleasing at f/2 in our book.
The quality of the general blur in the focus transition zone is great in the background and still very good in the less critical foreground.
Bokeh Fringing / LoCA
Boheh fringing/LoCA is an axial color fringing effect with purplish halos in front of the focus point and greenish beyond.
The Sigma lens doesn't feature an APO design (like most lenses), so there's some color fringing present at f/2. It's reduced at f/2.8 and mostly gone from f/4.
This can't be easily corrected via post-processing (yet).
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