iPhone media is VFR ... variable frame-rate. Meaning you may have the FPS set to 30fps, but ... depending on how much motion the phone 'sees' ... it may decide there's so little motion that at this moment 28.3fps is fine, but a bit later, there's a lot of motion, and it might even go above 30fps for a bit.
My Samsung phones do the same. A 30fps file will in reality vary from maybe 28.-something fps to 31.2 fps or so, continually changing through the length of the recording.
The sound, of course, is recorded solidly at a steady rate.
This is done in the camera so that it saves the amount of space and effort writing files to disc. And if you're not editing the clips, works ok for viewing also.
But when editing it ... an app like PrPro needs to be displaying X frames per second, period, and so it will have to make the in-between frames on the fly if and as it can. Which is why though it might be able to work with at least some VFR, it's normally better to convert the clips to CFR prior to importing into Premiere.
And that's where the free utility app HandBrake comes in. It can convert the clips to a solid CFR at whatever frame-rate you set. And though it's an odd process, it can batch process say an entire folder of clips.
I set that to working when I'm going to be away from the computer, or have it running on my laptop while I'm working on my desktop, so it doesn't affect anything I'm working on at the moment. Then import the converted files into PrPro.
And ... I'm wondering if PrPro is having an issue with this media, being as it's needing to create frames that aren't actually there on the fly, while you're telling it to also scale the clip up or down.
Many thanks Neil, if that information is right (I believe that of course), then that's really important to know!
So as far as I understand, "handbreak" will convert everything to a fix FPS. In which format is the output then?
So maybe my comparison with Aftereffects was then also not correctly, because I did not import the six minutes of recorded MP4 directly, instead I exported from PrPro as MXF (DNx... ) and imported into AE. This export took long and lead also to an incredible huge file size, but maybe has converted to a fix frame rate.
May I ask you, because you seem to know a lot in detail, about the recommended formats to do postproduction?
I'm working on windows (which is lacking the apple formats).
A mate once told me, I need to convert all recorded Mp4 (mostly from a Sony or Canon camera) manually to MXF, in order to have a lossless workflow and to achieve fast reading times in compositings.
Can you confirm this or is there another format better on Windows? Are there anywhere clearer instructions on which format settings to choose?
I really wonder why a google search does not give any clear advice on this topic.