Mac OS X does not show you all the available screen resolutions. You can unlock more, so that your external Display its native resolution.
For more screen resolution on the Mac unlock
Mac: system monitors your Mac, you can only between the screen resolutions choose it for the screen as a useful considered. But you want to deliberately use a low resolution and choose help this Trick:- Open the control panel and choose "display".
 - Switch to the tab "monitors", if you are still not in this.
 - Click and hold the "ALT"key on the Checkbox "Scaled"
 - In the list are now more screen resolutions for the Mac, you can now freely choose.
 
Mac: Other resolutions unlock
                              By 
                              David Amoyal
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Wednesday, May 21, 2014
Mac OS X does not show you all the available screen resolutions. You can unlock more, so that your external Display its native resolution. ...