[Nick Mars] “My question is, once I have rendered them, if I then slow them by 25% (30×4 = 120) will Final Cut use all 120 of those frames now that the clip is slowed down, or simply stretch out the rendered 30fps version?”
You do not slow down that by stuff changing the speed in FC.
If you put the 120fps on a 30fps sequence, first of all, 3 of every 4 frames will be dropped.
Then if you slow it down, new frames will be created.
That’s a crap of process that will damage the picture and will make a bad slow mo.
You need to CONFORM the footage in CinemaTools to 30fps.
That will make a 30fps movie with the image slow down to a 25% speed, keeping the picture untouched.
Work always with duplicated clips.
Once you have conformed a clip to a new time-base, you can’t re-conform back to the original time-base.
rafael