(How) does gravitational lensing affect red shift? In all the measurements taken over the years, has anyone ever observed any effect of gravitational lensing on red shift?
So: if we would look at 2 galaxies at, say, 250 million lightyears away, where the light of one travels directly towards us while the light of the other passes another galaxy on its way and is thereby gravitationally lensed before it reaches our telescopes. Would we find any difference in red shift between the two? 
 A: In principle yes, in practice no.
When light passes through a gravitational potential it will experience a delay compared to a signal not passing through a potential. This effect is known as the Shapiro Delay. Consequently, the light passing through the potential will be stretched a little bit more due to the metric expansion of space. Hence the delayed light is redshifted a little bit more. Put differently, when see two gravitationally lensed images of the same distant galaxy, one will show the galaxy at an earlier time than the other, the scale factor at that time was also smaller, hence the older image is also more redshifted.
However, since the Shapiro delay is typically (on a cosmological scale) very small, this effect is almost certainly negligibly small, and absolutely insignificant compared to the accuracy with which you can measure the redshift.
A: Strong gravitational lensing does NOT change redshift of the background object. A gravitational lens is a potential; a photon will gain energy as it falls through (i.e. gets blueshifted) and has to give up the same amount as it gets out of that potential (i.e. gets redshifted). Hence, the energy of the photon is unaltered and the background galaxies will have its original redshift.
Light travels two different paths of different lengths for asymmetric lens systems, resulting in time delay. Since this effect is on the galactic scale, the cosmological expansion will not have a measurable effect on the two light. 
