In El Capitan, under System Preferences > Displays, for the built-in screen, if you OPTION-click the word Scaled, can you get a LIST of resolution numbers, instead of the usual row of thumbnails? (A way to get more choices.) I was trying this at a (non-Apple) store today, and that trick worked on the retina iMacs but not the retina MacBook Pros. Yet I know it used to work, at least prior to El Capitan. (It's important because this lets you use the FULL native res of the display at 1x: super tiny text, but useful for some of my development projects.) TIA!