As discussed extensively in [77Jump To The Next Citation Point, 80Jump To The Next Citation Point], this is not a global degeneracy, but arises because the lensed images tell you about the mass distribution in the annulus centred on the galaxy and with inner and outer radii defined by the inner and outer images. Kochanek [77Jump To The Next Citation Point] derives detailed expressions for the time delay in terms of the central underlying and controlling parameter, the surface density in this annulus [49Jump To The Next Citation Point].