We’ve answered this question many times when it comes to technicalities, but there is so much more to photography than just being technical.
The folks over at Weekly Imogen recently published a video doing just that. They compare the Canon 50mm f1.8 and the Canon 85mm f1.8–both are great lenses. Their technical problems aside, there are lots of reasons why you’d use the 85mm lens over a 50mm lens when ti comes to using a full frame camera. 85mm lenses render less distortion, compress bulging parts and tend to throw a lot of the scene out of focus. That means that when you’re photographing a subject, you can make the scene really just focus on them.
Their video is after the jump, but if you’re interested in more then check out our comparison featuring the 50mm f1.4 vs the 85mm f1.8, Weekly Imogen’s own debate on the two, and our field test comparison of the Sigma versions of the 50mm vs 85mm lenses.