  We present a formal derivation of the key equations governing gravitational
lensing in arbitrary space-times, starting from the basic properties of Jacobi
fields and their expressions in terms of the exponential map. A careful
analysis of Jacobi fields and Jacobi classes near the origin of a light beam
determines the nature of the singular behavior of the optical deformation
matrix. We also show that potential problems that could arise from this
singularity do not invalidate the conclusions of the original argument
presented by Seitz, Schneider & Ehlers (1994).
