I've been struggling with the resolution avec my external display with my mbp 2012. Couldn't understand why the image was so poor, when I could plug in the pc laptop and image would be great.I had tried all possible settings, still bad. I stumbled on this link, and the trick works perfectly. Just thought I would share this link here, as it works on El Cap. If ever it helps anyone.
http://www.ireckon.net/2013/03/forc...ix-the-picture-quality-of-an-external-monitor
http://www.ireckon.net/2013/03/forc...ix-the-picture-quality-of-an-external-monitor