By default, iMovie makes all the transitions in your movie the same length. Standard transitions are one-half of a second long and theme-styled transitions (which are available only if you have set a theme for your project) are two seconds long. However, you can set the duration for each transition in your project individually, or you can change the default duration of transitions in your project so that when you add them, they’re already the desired length.
You can also set how much transitions overlap with the clips they’re adjacent to. This setting affects your project’s duration.
To change the duration of individual transitions in a project:
In the Project browser, double-click the transition icon. (There are many styles of transition icons; several are shown below.)
In the inspector that opens, type the number of seconds you want the transition to last.
If iMovie is set to display timecode values for project clips, type the duration using the following convention: HH:MM:SS:Frames (hours, minutes, seconds, frames).
If you want this duration to apply to all transitions you’ve already added to the project and all those you add later, select “Applies to all transitions.”
To change the default duration of transitions in a project:
With your project open, choose File > Project Properties.
The File menu appears in a light gray bar across the top of your computer screen.
Move the Transition Duration slider to set the length, in seconds, of non-themed transitions.
Move the Theme Transition Duration slider to set the length, in seconds, of theme-styled transitions.
If this slider is white and can’t be moved, it means your project has no theme.
Select one of the following:
“Applies to all transitions,” if you want the settings to apply to all transitions, including all transitions you’ve already added and all those you add from now on.
“Applies when added to project,” if you want the settings to apply to just the transitions you add from now on.
To set how much transitions overlap with adjacent clips:
Whether transitions are added to your project automatically or manually, you can set how much they overlap with the clips they’re adjacent to. This setting, which applies to all transitions in your project, affects the duration of your project.
Double-click any transition icon in your project. In the inspector that opens, choose an option from the Overlap pop-up menu:
All - Maintain Clip Range: The two clips are made to overlap by the length of the transition, and the transition is placed over the overlapping region. In this case, no additional content from clip ends is added. The total duration of your project decreases by the length of the transition.
Half - Maintain Project Duration: The transition is placed so that it spans the clips; one half of the transition overlaps each clip. In this case, additional content from the end of each clip is added to fill out the transition. The total duration of your project remains the same.