Taken from the Apple FAQ for Retina Displays http://support.apple.com/kb/ht5266
What happens when I use an external display with my MacBook Pro with Retina display?
Your MacBook Pro with Retina display automatically optimizes the resolution for your internal Retina display and any external displays that are attached. You can adjust resolutions for displays by choosing System Preferencesfrom the Apple () menu and then clicking Displays.
- If you have Mirror Displays enabled (under the Arrangement tab in Displays System Preferences), you will see the following under the Display tab on both internal and external displays:
This allows you to optimize for the internal or external display, or even scale the content on both.
- If Mirror Displays is not enabled, then you are using Extended Desktop mode. In this case, resolutions can be set individually for each display. Check the Display System Preferences window that has the description for the display you want to adjust on the top of the window (there will be a unique preferences window for each display). Adjust as necessary.
I need a resolution for my external display that is not appearing on the list. Is there a way to get more resolutions to appear?
Yes. Hold the Option key down while clicking the Scaled button to see more resolutions. This is only available in extended desktop mode.