The following is an excerpt from Jeff Schertz’s (Lync MVP) blog and are the results of real tests: 

In the following graph, the labeled bars indicate the documented bandwidth range for each resolution contained within RTV (Microsoft Realtime Video codec for Lync), yet the darker colored-matched bars across the bottom show which resolution was negotiated when calling from an HD capable Lync (2010) client into a virtual meeting room set to each of the recorded Line Rates.