I am using an Asus external monitor with 1600 x 900 native resolution connected to my Macbook Air (Yosemite) using a mini display port to VGA adapter. When I try to set the display resolution in System Preferences > Displays, I click "options" and "Scaled" to see a list of additional resolutions ranging from 800 x 600 through 1920 x 1080. For some reason 1600 x 900, the native resolution of the external display is not listed. 1920 x 1080 is the closest match to the native resolution, but it looks awful. How can I set the resolution to 1600 x 900?