# Angle deflection from gravitational lenses

Hi all

The expression for the angle deflection on the following page: http://relativity.livingreviews.org/open?pubNo=lrr-1998-12&page=node2.html [Broken]

Is the impact parameter r in the expression the distance to the surface of the objects that deflects the light or the the center of mass of the object? Unfortunately they do not specify this.

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