My copy of the Sigma-Z 28mm f2.8 has a 62mm filter thread; way larger than most other 28mm f2.8 lenses which have 52mm or 55mm, or even 49mm filter threads. The other YS Mount sister Sigma lens, XQ 35mm f2.8, has the same filter size of 62mm. So, the Sigma 50mm f1.4, having a 77mm filter size, is not that unusual; they started this trend decades ago.
Unfortunately, this lens has a dusty, almost hazy interior, which makes pictures taken wide open very low in contrast and hazy. The lens needs to be cleaned to perform at its designed optical quality. At f2.8, the lens is actually quite sharp, even the edges, on the APS-C sensor. I would like to have the lens cleaned and see what it can do. As the way it is, it's ok when strong, directional light is not present.
Stairs - NEX-6 & Sigma-Z Multi-coated 28mm f2.8 YS Mount @f2.8