Taking Klaus' advice for a small budget, the lens is a Vivitar 28mm 1:2.5 on m42 adapter, purchased cheap on e-Bay. I wanted something faster than the 18-50mm kit lens for doing portraits without flash in lower light conditions. (Though these two shots could have been done with the kit lens, too.)
congratulations to the VIVITAR. It is a very sharp and fast lens ... very nice to have.
This is mine with the Polish M42-adapter in the foreground. As I told you, it is optically wonderful (even at full aperture). Those who like to work manually will like it a lot. (...though automatic exposure in (A) mode perfectly works with all SIGMAs).
Here another low budget / high image quality recommendation:
REVUENON: 135mm / F:2.8
Nicely done ... I like pic 2 best! Just one thing:
Watch your colours. your white balance does not really hit the nail!
Colours should be balanced from too much yellow to more blue ... for my taste. Skin tones in particular say "thank you".