Previously, I posted about a common mistake that occurs when you use stock footage. I gave the example of inserting a slo-mo 24 fps stock clip into a montage of 30 fps footage. The sequence was natively 30 fps. (I’ve rounded 23.976 fps to 24 fps and 29.97 fps to 30 fps for clarity.)
It turns out that edit software is so obliging when it inserts clips that are the wrong frame rate into a timeline. The software automatically compensates for the difference in frame rates. In the above example, it repeats a frame every five frames. Sometimes you won’t see a problem, but other times you may notice the repetition of frames as an occasional stutter in the footage.
If the repeat happened every frame or every other frame, you might apply a speed change to the clip to overcome the problem. But a repeat every five frames? That doesn’t make the math easy.
Most editing applications have different ways of smoothing out frame rate differences. The simplest method is to blend two frames together to either add or remove a frame. A more sophisticated “Optical Flow” tries to calculate the direction that objects—represented by pixels—are traveling and then it creates “in-between” pixels based on that estimation.
Rather than using those methods to create new frames, think about using the frames you already have. Consider that, in this instance, the footage isn’t in real time. If the speed of the footage were changed slightly, would it be noticeable? Maybe not as noticeable as stuttering frames.
To achieve this change, tell the software to play back the footage so each frame of the stock footage is displayed only once. This isn’t done using the usual frame interpolation settings that you use when changing the speed of the clip.
Instead, you change what the software thinks the clip’s frame rate is. In Blackmagic Design’s DaVinci Resolve, you right click on a clip in the bin and select “Clip Attributes.” Then change the “Video Frame Rate” pull-down selection so it matches your timeline frame rate.
In Adobe’s Premiere Pro, right click on the clip and select “Modify”, then “Interpret Footage…”. The Frame Rate adjustment is at the top of the window. The default is “Use Frame Rate from File:”. To reinterpret the footage, select “Assume this frame rate:” and then enter the sequence frame rate you’re using.
Once you’ve made this modification, the clip will play back without creating new “artificial” frames. The stutter will go away, and the slight speed change should be more than acceptable.
Obviously, this is a solution for clips that aren’t running in real time, but it’s an easy fix to a common problem when you use stock footage.