In the following chart you can find a comparison between basic parameters of the tested lens and other full frame f/2.8 instruments with a similar angle of view.
A quite simple optical system of the Tamron is surprising. Even old lenses as the Canon EF 2.8/20 or the Nikkor AF 2.8/20 featured more optical elements. Newer constructions of that type, like the Zeiss Milvus 2.8/21, might consist of as many as 16 elements.
The chart also emphasizes how filigree the Tamron 20 mm f/2.8 Di III OSD M 1:2 really is. It is the lightest device in the whole group, weighing even less than the very small Nikkor AF 20 mm f/2.8D.
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In the photo below the tested lens is positioned between the reflex camera Sigma A 35 mm f/1.4 DG HSM and the Nikkor Z 24 mm f/1.8 S, designed for full frame mirrorless bodies.
When you take the Tamron 2.8/20 out of its box for the first time that's what turns your attention, its low weight. Physical dimensions of the lens suggest that you might deal with something heavier. Still, it doesn't mean plastics are everywhere. The tested lens starts with a metal mount which surrounds an immobile rear element. The part of the casing around that element is black, and very well matted, with a 24×13 mm window, alarmingl small, taking into account the size of detector this lens is supposed to cooperate with.
The proper barrel is covered by black plastics; first you see a smooth ring which doesn't move with a silver stripe and a mark making an alignment with a camera easier. The ring increases its diameter very quickly and already on the enlargened part you find an inscription with the name and parameters of the lens and information that it was designed in Japan but produced in Vietnam.
A manual focus ring, 18 mm wide, completely covered by ribbing, is next. It doesn't feature any distance or depth of field scale. The ring is a focus-by-wire construction, it moves smoothly and is well damped. Even if you turn it quickly running through the whole scale needs an angle of almost 180 degrees and when you turn it slowly its focus throw might even reach 650 degrees. Both values allow you very precise settings.
Behind the manual focus ring you see a small part of the casing which turns smoothly into a hood mount for a petal-type hood you get with the lens in the box.
The convex front element is 28 mm in diameter and it moves when you focus. The element is positioned less than 0.5 cm below the edge of the barrel with the focus set at its minimum value; when you go toward infinity it hides about 2 cm deep. The element is surrounded by black casing and a non-rotating filter thread, 67 mm in diameter.
When it comes to optical construction you deal here with 10 elements positioned in 9 groups. Among them you can find three low dispersion LD glass elements and an aspherical one. Inside you can also find an aperture with seven diaphragm blades which can be closed down up to a value of f/22.
Buyers get in the box: both caps, and a petal-type hood.
