The following chart presents a comparison between basic parameters of the tested Sigma and other lenses of this class. As you can notice the Sigma remains the lightest and, along with the Canon, the smallest device in this group.
In the photo below the Sigma C 28-70 mm f/2.8 DG DN is positioned between the Voigtlander Apo-Lanthar 2/50 and the reflex camera Sigma A 35 mm f/1.4 DG HSM.
The tested lens starts with a metal mount that surrounds contacts and a rear element, about 30 mm in diameter that doesn't move. The element is situated about several millimeters deep inside the barrel and above it you see a black 31x22 mm frame with rounded corners made of plastics. The inner tube is properly blackened, ribbed, and matted so from this side everything looks very well.
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A black, smooth ring made of metal is the first part of the proper lens. It increases its diameter smoothly and pretty fast. On the ring you can find a white dot, making an alignment with a camera easier, '021' number meaning the model year of the instrument, and an inscription 'MADE IN JAPAN' along with the focusing mechanism working range „0.19-0.38M/0.62-1.25FEET”.
The next part is a zoom ring, as wide as 28 mm, most of its surface covered by rubber ribbing. Under that you find focal length markings at 28, 35, 50, and 70 mm. The ring works properly well, is smooth and nicely damped across all its range.
Further on you find a part that doesn't move and is covered by plastics with the logo of the company, the name and parameters of the lens, its serial number, the C mark meaning that the lens belongs to the Contemporary line, some fine ribbing and the AF/MF switch that allows you to control the focusing mechanism mode.
A manual focus ring, almost completely covered by rubber ribbing and as wide as 20 mm, is the next part of the proper barrel. This ring is a focus-by-wire mechanism and its focus throw weekly depends on the speed of your turning; with a speedy turning it can amount to about 200 deg and with slower movements it can reach almost 360 deg. These are significant values, allowing you very precise settings.
The Sigma C 28-70 mm f/2.8 ends with a hood flange inside which you can find a non-rotating filter thread, 67 mm in diameter. The front element is relatively flat, 54 mm in diameter. With the increase of the focal length the whole front optical system is pulled out on a homogeneous plastic tube. As a result the length of the lens increases by 24 mm.
You should emphasize the fact that even though the tested Sigma belongs to the C line considered to be lower than the top-of-the-range Art instruments, it doesn't influence the build quality which remains simply brilliant. In case of C line you can find some compromises but they concern optical system solutions, weight, and dimensions, not build quality or the country of production. You still deal with a high quality instrument produced in Japan and you can feel it by moving its different parts and rings.
When it comes to the optical construction of the lens there are overall 16 elements positioned in 12 optical groups. Among them you can find two FLD elements with properties similar to fluorite, two elements made of low dispersion SLD glass and as many as three aspherical elements. Even though you deal here with a low-budget construction belonging to the Contemporary line there are a lot of special elements inside. There is also a round aperture with as many as nine blades that can be closed down to a value of f/22 at the maximum.
Buyers get in the set: both caps, and a petal-type hood.
