When I booted up my iMac yesterday, I was confused to find my login field had seemingly moved upwards, so that only the bottom half was showing. The lower half of the screen was just black. Is this a screen calibration problem? The display corrects itself once I log in to show a normal desktop. I've tried rebooting.
iMac 27", Late 2013
Running OS X El Capitan v.10.11.2
Screenshot of the problem is attached below: