(This is also of interest to sci.physics.research readers; hence the
crosspost.)

Alf P. Steinbach wrote:

Here is then my limited understanding, disregarding for now (until last
paragraph) the impenetrable paper.

Unfortunately, I don't have time to say very much, although if I could
draw some pictures on a whiteboard I think I could clear up the problem!
Since I can't do that in ASCII, let me recommend some excellent resources:

Probably my all-time four star scientific visualization site is this:

http://www.iam.ubc.ca/~newbury/lenses/lenses.html

You won't learn the math from this site (but there's some good
supplemental discussion there), but simply playing with the model may help
dispell some misconceptions--- plus it's a heck of a lot of fun.

To understand what you'll find at Newbury's site, probably all you need
can be found in the -third edition- of the excellent textbook by Stephani

author = {Hans Stephani},
title = {General Relativity: An Introduction of the Theory of the
Gravitational Field},
publisher = {Cambridge University Press},
note = {translated by {J}ohn {S}tewart and {M}artin {P}ollock},
year = 1990}

The current (third) edition has a nice concise discussion of the basics of
gravitational lensing, which should clear up your question. (Don't get
the second or first editions---if you can even find them--- since they
lack this section!)

For more detail, the Living Reviews article by Wambsganns is excellent:

http://www.emis.math.ca/EMIS/journal...-12/index.html

Both of these stick to pretty simple models, and are oriented toward gtr
students. That's probably fine for your purposes, but for a much more
complete discussion of possible models, aimed at serious astronomers, see
for example:

author = {P. Schneider and J. Ehlers and E.E. Falco},
title = {Gravitational Lenses},
series = {Astronomy and Astrophysics Library},
volume = 14,
publisher = {Springer-Verlag},
year = 1992}

"T. Essel" (hiding somewhere in cyberspace)