The display contrast on my new iMac (the Retina 4k 21.5" model) is too high. Can it be lowered past the lowest setting?
Yes - by using display profiles, you can alter what absolute values the relative 100% and 0% contrast settings are. You don’t even need calibration hardware, but sometimes that helps you dial this in precisely.


Go to the "Color" tab of the "Display" panel of System Preferences
Select the display profiles, one by one to see which you prefer.


When I got to "SMPTE RP 431-2-2007-DCI (P3)," I hit PAYDIRT: contrast and saturation were down to something I could easily live with.