The following chart presents a comparison between parameters of the new lens and other ultra wide angle Tamron devices; we also included older lenses of other producers with a similar focal range. You can notice that the construction of the new model has been simplified – instead, they put inside as many newest technical achievements and special glass elements as they could. What's more, in case of ultra wide angle systems its design is simplified a bit by a shorter distance between the mount and the sensor, a characteristic trait for mirrorless lenses. As a result, you can construct a device which is small and also exceptionary lightweight. The new lens, compared to older constructions, impresses also with its minimum focusing distance.
In the photo below the Tamron 20-40 mm f/2.8 Di III VXD 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 with contacts and a 29x22 mm frame. Inside that frame you see a rear element of the lens, 20 mm in diameter. The element moves – at 20 mm its edge practically touches the frame but at 40 mm it hides about 2 cm deep inside the inner tube. The tube is well blackened and ribbed but below you can spot a green plate with electronic components. Of course, with such a view, you can't speak about the lens being fully weather-sealed.
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The proper body is covered by black plastic and it starts with an immobile, smooth ring with a silver stripe and a mark, making an alignment with a camera easier. There is also information that the lens was produced in Vietnam. The ring increases its diameter very quickly and on its enlargened part you see an inscription with the name and parameters of the lens, the filter diameter, amounting to 67 mm, and information that the lens was designed in Japan. The lens also features a connector port (the USB type C), through wich you can calibrate it using the Tamron Lens Utility software package. Such a port is perhaps not a good idea...you can start wondering how such a solution will work in the long run, especially after using the lens in different weather conditions – after all it constitutes an unprotected access to the interior of the lens...
The next part is a zoom ring, 22 mm wide, equipped with a comfortable rubber ribbing and also focal length markings at 20, 24, 28, 35, and 40 mm.
Further on you see a smooth, immobile part of the casing, 8 mm wide, behind which starts a manual focus ring. The ring is completely covered by ribbing, without any distance or DOF scale. It is a focus-by-wire construction that moves smoothly, and is properly damped. Unfortunately its range can be called only symbolic, amounting to just 30-40 degrees. In fact, you barely touch it and it can jump to the middle of the scale, hardly a functional feature you have to admit...
The homogeneous tube made of plastics, ended with a front element system, extends as you turn the zoom ring. During that operation the length of the lens increases by about 1 cm.
The front element is convex, 42 mm in diamenter, surrounded by a non-rotating fitler thread, 62 mm in diameter, and a mount for the lens's hood.
When it comes to the optical construction you deal here with 12 elements positioned in 11 groups. The producers weren't skimpy when it comes to special elements – three of them are made of low dispersion LD glass, three others are aspherical ones, and among them you can find two made of molded glass and one hybrid element. Inside the construction you can also find a circular aperture with nine blades that can be closed down to a value of f/22.
Buyers get in the box: both caps, and a petal-type hood.
