How do I capture full 5K (5120 x 2880) iMac display?

So ive got a 2016 iMac full spec with 32Gb RAM and 4Gb GPU and this OBS app is nothing short of amazing but I can't get it to record full screen even though ive dialed the screen res in.

Please look at my Pref's and tell me if you can suggest anything to help? Thanks

I don't believe the iMacs have a 16:9 aspect ratio for the display. You would have to adjust the "rescale output" in the settings to be a resolution that equivalent to the aspect ratio of your iMac. This should hopefully fix your issue.

Did this work?