When you create a new project in iMovie, you’re asked to set an aspect ratio. This determines the final dimensions of the movie after you export it, but you can change the aspect ratio at any time before you export.
When you change a project’s aspect ratio, iMovie may zoom in or crop the footage a bit so that the video fills the screen without letterboxing (adding black bands to the top and bottom of the video) or pillarboxing (adding black bands to the sides of the video).
To change a project’s aspect ratio:
Click to select a project in the Project Library, or open a project so that it’s showing in the Project browser, and then choose File > Project Properties.
The File menu appears in a light gray bar across the top of your computer screen.
Choose an option from the Aspect Ratio pop-up menu.
Widescreen (16:9): Makes the movie appear much wider than it is high. Widescreen movies are optimized for viewing on widescreen computer monitors and high-definition TVs (HDTVs). When viewed on a standard-definition TV, widescreen movies appear with black areas above and below (sometimes referred to as “letterbox”).
Standard (4:3): Makes the movie more square in shape. When viewed on a standard-definition TV, these movies can fill the screen. When viewed on an HDTV or widescreen computer monitor, they appear with black areas on the left and right of the video (sometimes referred to as “pillarbox”).