Samsung Galaxy S5 — Samsung S5 Camera view with split screen?

Today I turned on my camera and it had split the screen in half (viewfinder on device had 2 seperate halves of the normal picture it would show) - when i held the phone still for a second or 2 it settled down and it was hard to notice that the screen was split in the middle - but it was still split.

When i moved my phone again it was obvious it was split. It was like the inner circle of an old rangefinder camera.

Was this a focussing aid or what did I do to let this happen?

Thanks for any help,

Matt.