The following chart presents a a comparison between basic parameters of the tested lens and other universal zoom instruments with large f/2.8 aperture which are designed for full frame mirrorless cameras. The dimensions and weight of the new Sigma don't stick out when compared to Canon, Nikon, and Sony models. Evidently the Tamron remains physically the lightest and the smallest construction but you have to keep in mind its slightly different focal range. The Canon is optically the most complex instrument here and also the only one which features an optical image stabilization system. The Sigma stands out with its shortest minimum focusing distance and the highest number of aperture blades.
In the following photo the Sigma A 24–70 mm f/2.8 DG DN is positioned between the mirrorless Sigma C 45 mm f/2.8 DG DN and the Sigma A 35 mm f/1.4 DG DC designed for reflex cameras.
The tested lens starts with a metal mount surrounding contacts and a rear element which doesn't move. Around the rear element you see black, well matted interior of the tube and a 29×22 mm frame. The element itself is 31 mm in diameter. From this side everything looks very nice indeed – the lens is completely weather-sealed and properly blackened.
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The first part of the proper lens barrel consists of a smooth, black, metal ring which quickly but smoothly increases its diameter. On the ring you can find a white dot, making an alignment with a camera easier, the „019” numbers meaning they model year of the instrument, an inscription „MADE IN JAPAN”, and another one, „MIN 0.18–0.38M/0.92–1.25FEET”, indicating the minimum focusing distance.
The next part is a zoom ring, 26 mm wide, mostly covered by rubber ribbing; below you can find focal length markings at 24, 35, 18, 50, and 70 mm. The ring moves smoothly and is properly damped.
Further on you see a part covered by plastics which doesn't move; on it you can find the logo of the company, the name and parameters of the lens, its serial number, fine ribbing, the „A” letter which means the lens was classified as a part of Art line, and two controlling devices. The first of them, the AF/MF FOCUS switch, allows you to choose the focusing mechanism working mode; the other one, the AFL button, allows you to lock and recall a specific autofocus position but other functions can be attributed to it as well. Then you see the LOCK switch which blocks the lens at 24 mm, preventing lens creep. Still it should be said the lens we tested didn't have any tendencies to extend uncontrollably due to its own weight and the switch releases the block when you move the zoom ring.
The next part of the lens's body is a manual focus ring, 27 mm wide, completely covered by fine ribs. You won't see any distance or DOF scale next to it and it's a focus-by-wire construction. The rings turns very smoothly remaining properly damped at the same time. The working range depends on the speed of your turning. If you turn the ring quickly you can cover the whole range with an angle of 180 degrees, if you turn it slower you might even need 300-320 degrees. These are very high values, allowing you precise manual settings.
A uniform tube made of metal is extended when you turn the ring and change the focal length from 24 to 70 mm; at its end you can find a front element system. The length of the lens increases by 35 mm as a result.
The front element is 69 mm in diameter, surrounded by a non-rotating filter thread, 82 mm in diameter, and a hood mount.
When it comes to optical construction you deal here with 19 elements positioned in 15 groups and the number of special elements is truly impressive. You get as many as six low dispersion FLD elements made of glass with properties similar to fluorite, two low dispersion SLD glass elements,and three aspherical ones . Additionally, the rounded aperture of the lens features as many as 11 diaphragm blades and it can be closed down to a value of f/22 at the maximum.
The lens's casing is supposed to be dust- and splash proof, with water- and oil-repellent coating on the front element which also ensures more comfort in taking photos in difficult weather conditions and easier cleaning. It's worth mentioning that usual antireflection coatings this time are paired with the new NPC technology (Nano Porous Coating) which contains porous nano-particles of silica in the coating material.
Buyers get in the box both caps, a petal-type hood, and a pouch.
