I have been shooting the 5D3 with the eyecup since April. About a month ago, I took the eyecup off, as I did not like how it bent, etc... when I put it in/out of my shoulder bag - which I use most of the time these days.
One of the issues was my nose and cheek were now becoming great friends with the LCD - what a mess. I was carrying a cloth just to keep it clean.
But the worst part, I somehow was changing the diopter setting. That really screwed things up, as I have been experimenting with manual focus quite a bit and this slowed me down considerably. I also wear glasses and manual focus is a bear. (thats another topic)
Yesterday, I decided to put the eyecup back on. It is funny how a small change can make such a difference. No more messy LCD - the eyecup keeps my nose and cheek off the LCD. And the diopter is mysteriously staying put.

