Sigma 30mm f/1.4

Focusing distance: 40cm / 15.7in
Magnification: 0.1x
Filter size: 62mm
Lens size: ø76.6mm x 64.1mm / ø30.2in x 25.2in
Weight: 410g / 14.5oz
Construction: 7 elements in 7 groups
Accessories: Hood, Case

Eddie's take on it

Unlike the 24mm f/1.8 and 50mm f/1.4, this is not a full frame, but an APS-C lens adapted to 4/3, and this is the best of Sigma's regular prime lenses. While it still suffers from sample variation and some back/front focus problems, it does so to a lesser degree.

This was, at the time, a cheap prime lens and a far cheaper option for a 30mm f/1.4 prime than any other manufacturer released for their cameras (it came out in all mounts, Canon, Nikon, Pentax, you name it). Thus it suffers from the usual array of problems of cheap primes of this era - soft corners, soft wide open, unreliable autofocus, abberations.

That said, if you need a fast prime at this focal length, your only other option is the Panasonic 25mm f/1.4, which is hard to find and going to be much more expensive. 

This lens is best stopped down. But if you stop if down, it becomes pointless, because the Olympus High Grade zooms are excellent wide open. For this reason, this was never a popular option with 4/3 users.

If you are using m4/3, there is no reason to buy this lens. The newer, native version is miles ahead.