  The gravitational lens equation resulting from a single (non-linear) mass
concentration (the main lens) plus inhomogeneities of the large-scale structure
is shown to be strictly equivalent to the single-plane gravitational lens
equation without the cosmological perturbations. The deflection potential (and,
by applying the Poisson equation, also the mass distribution) of the equivalent
single-plane lens is derived. If the main lens is described by elliptical
isopotential curves plus a shear term, the equivalent single-plane lens will be
of the same form. Due to the equivalence shown, the determination of the Hubble
constant from time delay measurements is affected by the same mass-sheet
invariance transformation as for the single-plane lens. If the lens strength is
fixed (e.g., by measuring the velocity dispersion of stars in the main lens),
the determination of $H_0$ is affected by inhomogeneous matter between us and
the lens. The orientation of the mass distribution relative to the image
positions is the same for the cosmological lens situation and the single-plane
case. In particular this implies that cosmic shear cannot account for a
misalignment of the observed galaxy orientation relative to the best-fitting
lens model.
