How I would look with a massive black hole halfway between us. Note that there is an inverted image of me inside the ring formed by my glasses. If my face is about the size it would look if I were standing 20 feet away, these distortions would be about ten degrees across. This would require a black hole halfway between us of about sixty times the earth's mass (more than Saturn), with a Schwarzschild radius (event horizon, green circle) of about 1 degree. At 10 feet, it would pull us in with a force of more than a trillion times the earth's gravity, and we would have little time to admire the effects (less than a microsecond).
Because it is much farther away, gravitational lensing by our sun creates angular distortions of about 1/1000 degree, and the corresponding displacement of background stars was first observed in 1919 during a solar eclipse, as a striking confirmation of general relativity.
The angular distortions by other stars in our galaxy are much smaller (only about a billionth of a degree), so we are not likely to observe them directly for some time. The Macho project has, however, detected the brightening of stars in nearby galaxies due to lensing by stars in our own galaxy.
Galaxies, on the other hand, are very massive,
and despite their enormous distances, they produce observable
gravitational distortions of about 1/3600 degree.
Now, close to 100 cases of
galaxy-galaxy lensing
have been discovered.
Dense clusters of galaxies can produce distortions
larger than 1/60 degree, and a few dozen of such cases are now known.
This image was produced using Pete Kernan's "Lens an Astrophysicist!" applet.