What is the difference between DG and DC lenses?

The designation DG indicates that the lens has been optimised for digital SLRs but is equally compatible with film SLR cameras. These will work on digital SLRs with a small (APS-C size) image sensor, digital SLRs with a full frame sensor and film SLRs. DC lenses are designed specifically for digital SLRs with the small, APS-C size image sensor. These are not suited for full frame digital or film SLRs as heavy vignetting will occur.