I have found a new favorite wide angle prime lens. The Tamron 28mm f/2.5 is a handy little lens, about the same size and weight as the zuiko 50mm f/1.4 and sigma 24mm f/2.8 that I have. The f/2.5 aperature is a bit of a wierd size, but it is 1/3rd stop faster then a 2.8 lens and it does aid in view finder brightness and focusing as well as that extra little bit of speed for availible light photography.
The lens focusing is smooth with the inner barrel protrouding from the sorrounding lens barrel. This does pose a problem for using lens hoods with this lens as most rubber lens hoods project behind the barrel of the lens. I can use my rubber lens hood okay with the lens, but I cannot tighten it down 100% or else I lose infinity focus because the lens can’t retract fully (I have to leave it about 1 full rotation lose). The lens takes a very small 49mm filter. The lens is multicoated and is composed of 7 elements in 7 groups.
The lens also has a minimum aperature of f/32, which is incredibly tiny, though diffraction at that tiny of an aperature would likely have a large impact on image quality. The lens has a minimum focusing distance of 9.8″, weighs 6.3oz and is approximately 1.6″ with the olympus OM adaptor on the lens. The lens has clear clicks for each half an f-stop. This specific lens is part of the Adaptall-2 system of interchangable lens mounts.
I ended up getting this lens for about $40 shipped off Ebay. The diagram of the lens elements, lens weight and minimum focusing distance are from http://www.adaptall-2.com/