From Manufacturer
To maintain high optical performance from infinity to close-up 1:1 macro photos without accessories, the Sigma 50mm f/2.8 EX DG incorporates a "Floating Internal Focusing" (FIF) system. As a result, the lens also features a non-rotating front lens group which makes it easier to work with polarizing filters, and has a focus limiter to narrow down the broad focus range.
This lens is designed as a high-quality primary macro lens, good for film or digital SLR cameras.
The improved DG lens design corrects for various aberrations. This macro lens is specially coated to get the best color balance whilst cutting down on ghosting caused by reflections from the digital image sensor. The lens provides the utmost correction against lateral chromatic aberration which is a serious problem for digital SLR cameras.\
AF not supported by D3000, D3100, D5000, D40, D60 and other Nikon cameras with no AF motor in the body.