The following chart presents a comparison between basic parameters of 20 mm lenses as fast as f/2.0 or better aperture-wise, and designed for mirrorless full frame cameras.
It is clear that the Sigma is physically the smallest and the lightest lens in this group. Still the faster Sony model is only a tad heavier; the Sigma, along with the Tokina, stand out from the rest with the smallest filter thread diameter for a change.
In the photo below the tested lens is positioned between two other Sigma lenses: on the left the Sigma A 35 mm f/1.4 DG HSM, designed for full frame mirror cameras, and on the right the Sigma C 30 mm f/1.4 DC DN, designed for smaller sensors.
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The Sigma C 20 mm f/2 DG DN starts with a metal mount that surrounds contacts and a rear element, 26 mm in diameter. That element doesn't move and is positioned quite shallowly inside the barrel. All parts inside the barrel are properly blackened and matted so from this side everything looks as it should.
The proper body of the lens starts with a metal ring painted black. On this ring you can find a lot of information: the name, and parameters of the lens, the logotype of the producer, the 'C' letter meaning the lens belongs to the 'Contemporary' series, '022' meaning the model year of the instrument, the serial number of the lens, a white dot, making an alignment with a camera easier, and finally a focusing mechanism mode switch (AF/MF).
Then you see an aperture ring, 8 mm wide, with protruding ribbing on its sides to ensure you a firmer grip and better performance. The ring comes with aperture markings from f/2.0 to f/22 and also finer marks every 1/3 EV step; it also features an A value, allowing you to control aperture automatically from the camera level. The ring's performance is beyond reproach.
Next, you find an immobile part of the barrel with an inscription 'MADE IN JAPAN' and the focusing mechanism working range: '0.22M/0.724FEET–∞'.
The following ring is 17 mm wide and it helps you to set manual focus. It doesn't feature any markings and the majority of its surface is covered by ribbing that improves your grip. It is a focus-by-wire mechanism that moves very smoothly. Its focus throw amounts to an angle of over 400 degrees even if you turn it quite fast, a huge value, allowing you very precise settings.
The lens ends with an immobile, metal ring on which you can find a hood mount. The mount surrounds a non-rotating filter thread, 62 mm in diameter.
The front element is 33 mm in diameter, slightly convex and immobile. It is surrounded by black barrel without any inscriptions.
When it comes to optical construction you deal here with 13 elements positioned in 11 groups. Among them you can find one element made of low-dispersion FLD glass with properties similar to expensive fluorite, one element made of low-dispersion SLD glass, and as many as three aspherical elements. Inside there is also a round aperture with nine diaphragm blades that can be closed down to a value of f/22.
Buyers get in the box with the lens: a metal hood, a back cap and two front caps, one classic and made of plastics, the other made of metal and magnetic, a really nifty solution. C series instruments are sold without any kind of case.
