One of the great things about AstroBin is the amount of knowledge here.  A friend asked me about gravitational lensing, and I wondered if there were any examples I could capture.  A quick search for "gravitational lensing" on AstroBin and I found several captured by others. 

This one is pretty easy to find, as it is near NGC 3079, the edge-on galaxy in the center of this frame.  There are quite a few faint galaxies in this field, but to the right and below is a pair of smudges that represent QSO 0957+561 (I've drawn a box around it and added an inset with a magnified view).  In 1979, astronomers searching at optical wavelengths for counterparts of radio sources found these two objects just 6 arc seconds apart.  Subsequent observations of spectra confirmed that they were in fact the same quasar, 8 billion light years away, lensed by a cluster of galaxies including YGKOW G1, a huge elliptical galaxy four billion light years away on a nearly direct line.

Einstein predicted this effect in a short paper written in 1936, as a consequence of the bending of a light ray's path in a gravitational field.  If two stars lie on a central line from an observer, then the light from the more distant star will be bent by the intervening star, and we will see two images from the light passing on each side (if they are perfectly aligned, we would see an arc or ring around the intervening star).  Einstein provided the equations to calculate the angle of deviation, but he thought it was just a curiosity, writing "Of course, there is no hope of observing this phenomenon directly.  First, we shall scarcely ever approach closely enough to such a central line.  Second, the angle...will defy the resolving power of our instruments."

Einstein was thinking about lensing by stars when he wrote this (I don't think we understood how many galaxies there are in 1936) but I still think it is cool that we can observe something Einstein thought was purely theoretical with amateur equipment.  This image was taken through a 115mm refractor from my back yard, 90x120 seconds luminance only.