Slow motion is a common technique for creating emotional impact in a video piece. Although there is a Retiming tool in Final Cut Pro X to do this, it's a technique that is forced to create fake frames from existing frames and may not yield the best quality results. A better way to do this is by using the frame rate setting in your camera and then conforming in a Timeline of a slower frame rate. Let's see how this works.
Set your camera to shoot 60 frames per second. Also be sure you are shooting progressive shots. Interlace formats do not lend themselves to quality speed effects. Faster shutters speeds will give you more clarity in your resulting slow motion shots. Slower shutters speeds will introduce more motion blur to the original shots. Neither is right or wrong, it simply depends on what stylized look and emotional content you want your final product to have.
Import your shots into an Event in Final Cut Pro X. Create a new Compound Clip or Project to begin editing in. When prompted for the settings to this new Timeline, be sure to not use the automatic settings. Select the custom settings and verify this new timeline will be set to 24 progressive frames per second, or '24p'.
Select your original 60 progressive frames per second, or 60p shot in the Event Browser. Drop it into the 24p Timeline you just created. It will play at normal speed. It may look slightly choppy, depending on the amount and type of movement in the clip. This is because FCP is forcing those 60 frame per second to fit inside of a 24 frames of play per second. The result is that certain frames are dropped and ignored. But that's fine, we're not finished yet.
Highlight the 60p clip in the Timeline and go to the Modify menu, to the Retime sub-menu and select Conform Speed.
For those of you who remember the Cinema Tools application that was included with the legacy Final Cut Studio suite, this is in fact Cinema Tools carried over into Final Cut Pro X. With this technique you will end up with a clip that plays back at about 2.5 times slower. The image quality will be as clear as possible. Don't be afraid to slow it down more, also if desired. Simply select the clip and in the Modify menu, back to the Retime sub-menu, to the Slow option and pick a speed to begin with. Then grab the handle at the end of the Retiming bar in the Timeline and drag it out to customize the speed.
Remember at the beginning of this article I said if you drop a 60p shot into a 24p Timeline it may look jerky at tad bit? Use this same technique, but once conformed go to the Modify menu, to the Retime sub-menu, and chose Fast with the 4x or 2x choices. Then grab the retiming handle at the end of the clip and adjust so that you're getting your normal speed. Because you've kicked in the Conform Speed function, you often can element much of the jerkiness.
Shooting with the proper camera settings, creating a Timeline with the proper manual settings, and using the Conform function in Final Cut Pro X, you can achieve some amazing slow motion results.