Changing aspect ratio with Keynote

The first thing we need to note here is that Keynote, however capable, is not an NLE. We should always use software dedicated to our specific cause, so before we change the aspect ratio of our video, we need to make sure we’re happy with it, and then export a master copy from iMovie. To recap, the master copy should have the following:

• The highest resolution possible (what you set the project resolution as)
• The highest quality possible (ProRes if you have the space, or slide the Custom quality slider fully to the right if not):

When we see Quality in iMovie, this refers to the video’s bit rate. More on that later.

Once that video has been exported, we can bring it into Keynote and change its aspect ratio. Here’s how to do that:

1. Open a new basic Keynote project (⌘ + N).
2. On the far right of the toolbar, click Document to open the Document sidebar.
3. Click the Slide Size drop-down menu and...