"Time Stretch" vs "Conform to Frame Rate"

[oklava]

Hi,
I have a footage which is in 100 frame rate. I work in 25fps.
My question is;

are the ways of making that footage slow-motion

- in the "interpret footage->main-> conform to frame rate= 25"  or

-in timeline "time stretch->Stretch factor:%400"  (while frame rate="100")

exact same things?

I did it by stretching time, but I suspect this issue because there was some black frames and I thought this could be the artificial way to making slow-mo. But after I close and open after effects again, those black frames were gone. Probably it's irrelevant. But the question remained

[Rick Gerard · Accepted solution]

The easiest way and the only guaranteed way to use every frame captured with any camera is to match the frame rate of the composition to the frame rate of the footage. If you have true 100fps footage, your standard composition is 25 fps, and you want to use every frame then just change the interpretation to match the comp. If you do that and you want to make further time changes you will get better results using Time Remapping (best standard option) with the most effective frame blending you can choose for the type of footage you are using. Time Stretch is also OK but I prefer time remapping for a ton of reasons that I won't go into right now.

Your black frames may have been caused by a cache issue. When you are changing playback speed, and generally, it is always a good idea to purge memory and disk cache before you render using the output module.

[oklava]

Rick Gerard Thank you for your answer and valuable information,
I will take your advice for further projects, but I did the whole project by time stretching. So as I understand from your explanation, It's better to keep this as it is in order to not lose time, right?

[Rick Gerard]

It's not loosing time that is the problem, It is that you can't count on the math. If your 100 fps footage was actually 104 fps for most of the shot and you want it to be played back at 25 fps so you change 1 second to 4 seconds you are going to end up with interpolated and blended frames. It's just so much easier to conform the footage to the comp frame rate and then do your speed changes. I often will take footage that is shot at 120 or even 500 fps and play it in real time for a few seconds and then make the shot slow motion. The footage is a lot easier to handle if you interpret it at the comps frame rate so that it is now playing at the optimal slow motion speed and then you start speeding up the action by turning 10 seconds into 2 than it is to try and figure out how to turn 2 seconds into 10 and guarantee that you are showing the optimal slow motion footage.