The project
Choose a shot with a clear action, steady enough detail to judge motion, and additional source handles. A runner, a passing object, or a camera move can work. Keep an unretimed version for comparison.
1. Identify the emphasis
Decide which instant should feel fast, slow, or held. Mark the important action before opening the controls. A speed effect works best when its timing supports something the audience should notice.
Check the source’s frame rate and motion blur. Slowing footage does not create information that was never recorded.
2. Adjust the retime
Select the clip and open Retime (⌘R). Establish the desired speed behavior and review the resulting duration. Refine the transition into and out of the emphasized moment using the available retime controls.
Check neighboring edits after applying the change. A duration change can affect where the next event lands in the sequence.
3. Compare picture treatments
Preview frame sampling and optical-flow treatment where available. Inspect the moving subject, crossings, thin details, reflections, and newly revealed background. Optical flow estimates motion and can create visible artifacts around difficult movement.
Review the actual moving segment at normal speed, then frame-step through suspicious areas. A single appealing frame does not prove a clean interpolation.
4. Rebuild the sound if needed
Listen to linked audio after retiming. Consider whether the scene needs retained location sound, a deliberately shaped sound effect, or only the music bed. Keep synchronization intentional.
Review your result
Export the effect with original media and inspect both the speed transitions and the clip boundaries. Compare with the unretimed version. Keep the effect when it strengthens the action, not just because the controls allow it.