Sigma's TC-2011 Teleconverter is specifically designed for use with Sigma L-Mount lenses.
When mounted between your lens and camera body, this teleconverter will increase your focal length of your lens by 2x.
It is constructions from 8 elements in 4 groups, is splash and dust-proof as well as being able to maintain effective autofocus across your entire focal range and through all apertures.