I've just bought a late 2015 iMac 5k (used, sadly!), and there seems to be a problem with the brightness. I noticed fairly quickly that the screen isn't as bright as either my Macbook Air or the Thunderbolt display I was using previously. The brightness control only makes any difference for the first five "blocks", after that any increase makes no difference. I am running 10.13.3. Here are all the things I've tried so far: Turning off auto-brightness (makes no difference). Night Shift is also off. Resetting the PRAM Resetting the SMC Turning it off and on again! Running diagnostics (no issues found). Nothing has made any difference. The screen is equally bright across all of it - no dark corners / sides. Can anyone help me work out what's going on?!