The following chart presents a comparison between basic parameters of the new Sigma lens and other fast 20-21 mm instruments.
The Sigma A 20 f/1.4 mm DG DN sticks out in this group with the complexity of its optics as it features as many as 17 elements. It also offers an aperture with 11 blades. Compared to its reflex camera predecessor there is another difference - you get a normal filter thread, 82 mm in diameter.
In the photo below both new Sigma lenses are positioned next to the reflex camera Sigma A 35 mm f/1.4 DG HSM model.
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The lens starts with a metal mount that surrounds contacts and an immobile rear element, 26 mm in diameter. The element is hidden inside an inner tube less than 1 cm deep and is surrounded by a 29x23 mm frame for special, dedicated rear filters.
A black, immobile ring made of metal that increases its diameter very quickly is the first part of the proper barrel. The ring features a white dot, making an alignment with a camera easier, and the model year of the instrument, in this case 022, information that the lens was made in Japan, and an inscription with the minimum focusing distance :”0.23M/0.75FEET– ∞”.
An aperture ring, 12 mm wide, mostly covered by ribbing, is the next part of the lens; it features aperture marks ranging from f/16 to f/1.4 and an A value (meaning automatic settings). The ring moves every 1/3 EV stop but you can also declick it. Its performance is very comfortable and smooth, simply beyond reproach.
Then you see an immobile part of the lens covered by plastics with the logotype of the company, the name and parameters of the lens, its serial number, some fine ribbing, an 'A' letter that means the instrument is qualitifed as an Art line lens, and a whole array of switches. The first of them – AF/MF FOCUS – allows you to control the focusing mechanism working mode. The second one – AFL/MFL – recalls a specific autofocus position but you can allot it other functions too. Below you see a LOCK switch that locks the focus at a chosen point. The last switch, CLICK OFF/ON, sets the aperture ring mode. On the other side you can also find the aperture ring lock switch that can be set either at the A position or at any other point ranging from f/1.4 to f/16.
A manual focus ring, as wide as 38 mm and mostly covered by ribbing, is another part of the barrel of the lens. It is a focus-by-wire mechanism and you won't find any distance or DOF scale on it. Its performance we find very smooth and the ring remains properly damped at the same time. Its focus throw depends on the speed of your turning. When you move the ring faster it amounts to 180 degrees, when you turn it slower you can reach even over 300 degrees. Both these are values allow you very precise settings.
The barrel ends with a short, immobile ring that turns into a hood mount. The front element is 56 mm in diameter, convex, immobile, and surrounded by a non-rotated filter thread, 82 mm in diameter.
The lens is weather-sealed, splash proof, and dust proof. The front element is also protected against dirt, grease, and moisture retention by hydrophobic coatings that make it easier to clean.
When it comes to optical construction the lens features 17 elements positioned in 15 optical groups. Among them you can find two low-dispersion SLD glass elements, and three aspherical elements. Inside there's also a round aperture with 11 blades that can be closed down to a value of f/16.
Buyers get in the box with the lens: two caps, a petal-type hood, and a solid case with a dedicated strap.
