So to recap, my issue is that I can't get my MacBook Air to display in 16:10 FULL SCREEN (NOT letterboxed) with my Epson projector either through HDMI or Apple TV Screen Mirroring.

USING HDMI ('optimized for built-in display') - letterboxed all around

USING HDMI ('optimized for projector') - letterboxed on top and bottom I'm also having similar issues mirroring the screen using the Apple TV, but I'll leave those details out for now to avoid information overload. I marked the corners of the 'screen' in red so you can more easily see the letterboxing. So what gives? Is it not possible to fill a 16:10 projected image of a 16:10 MacBook Air using HDMI? See pictures below I took using a projector at home that is also 1280x800. the default setting of optimized for the Epson PJ), the projected image becomes 16:10 but is letterboxed now on all 4 sides. If I go to System Preferences > Displays and select to optimize the settings for the Built-in Display (vs. No matter what options I select on either the projector or the MacBook Air, I can't seem to make the darn image display at a full-screen 16:10 aspect ratio. However, when I connect the MacBook Air to the HDMI cable with a HDMI-to-Thunderbolt adapter, the projected image is 16:9, and thus is stretched and letterboxed at the top and bottom. I've routed this cable to a much more convenient access point than the wall-mounted VGA port, so I'd prefer to simply use it as my primary connection to connect the projector to either the Apple TV or the MacBook Air using an HDMI switch. I've recently added an HDMI cable to the setup to be able to use an Apple TV with the projector as well. Using the VGA connection, the image fills the screen perfectly, which of course makes sense since both the computer and projector have the same aspect ratio. The projector has a native resolution of 1280x800 (16:10) and my MacBook Air has a native resolution of 1440x900 (16:10). For years, the only connection I've used to connect my MacBook Air to my Epson projector has been a VGA cable with a VGA-to-Thunderbolt adapter.

Vga cable for macbook air to projector