Black bars on log in screen (not hardware problem)

I get these weird thick black bars on my log in screen, and they disappear when i log in.
Picture: http://i.imgur.com/pCnnnsP.png

Are the black bars (or borders) you are referring to on the bottom & right side of the photo?

What model Apple computer is this? Is this a builtin display or an external display?

What OS version is installed?

- Nick

Yes they are on the bottom and right side. It is a 2015 macbook pro 13" with el capitan (v. 10.11.4)

First up check your User Account and make sure you are logged in as Administrator and not Guest. To fnd this System Preferences > Users & Groups > Password.

Don't see the black bars, however if no joy checking Admistrator, if it is under warranty make an appointment at the Genius Bar or Apple Store.

I am logged in as admin. If there isn't an easy fix for this i'll just leave it - after all it doesn't really matter

If I'm following along correctly, the "black bars" you are referring to are the blank areas along the right edge and bottom edge. To me, this seems to indicate an issue with the resolution of the screen prior to login.

Try this: Power off the Mac. Immediately after powering it back on, press and hold Command, Option, P, and R until you hear the startup chime for the second time.