You guys are asking good questions
My rule of thumb (so to speak) is to set the project FPS and produce to the highest frame rate used in your source clips. Why? Because all that detail and info is present in your 50/60p clips and you'll miss out on it if you produce to 25/30p.
By setting the frame rate to match the highest FR clip, you ensure that all transitions map correctly to each and every frame. PD will simply double every 30p frame to run at 60p, but you'd only really notice that if you put in some slow motion on the 30p clips.
On the other hand, if there really isn't much motion (of the camera, subject, or both) OR if you want the smallest output file size, then set the project FPS to the lowest frame rate of the clips you're using and produce to that rate as well.
This time, PD will skip every other 60p frame when adding transitions and producing, and everything in your produced video will be at 30p.
The same goes for unusual framerates, like 48p, where PD will add frames (by duplication or interpolation) if you're producing to 60p; and will remove frames if you're producing to 30p.
YouTube/optodata
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