understanding fps when child comp's fps don't match

[TimSx]

I have a master comp running at 60fps.

Inside of it, I have a child comp, running at 30fps. Child comp contains static images and some animations, but no video at the moment

When I run the animation in the master comp, the child comp updates on every frame of the master comp even though it is set at 30 fps. Why is that?

Also... Does it make any sense to change the child comps fps to 60? The child comp fps does not even look to have an effect at all. Is render speed affected by the mismatch?

[Andrew Yoole · Accepted solution]

One of the GREAT things about After Effects is that it will sample time accurately in nested comps, regardless of the frame rate.

But, if for some reason you need it to respect the underlying comp frame rates, go to Composition Settings/Advanced in the nested comps and tick the box for "Preserve frame rate when nested or in Render Queue".

You can also apply the "Posterize Time" effect to your precomp's layer in your main comp and tell it what frame rate you want the layer to run at.

[Mylenium]

As Andrew already said - AE samples on genuine infinite precision time, not discrete frames. All that will happenb is that for two frames of your 60 FPS comp AE will sample the same frame in the 30 FPS comp 'cos naturally the frame duration is twice as long. Whether or not using different FPS makes any sense is entirely up to you and the specifics of the project.

Mylenium

[Rick Gerard]

As others have said, keyframes are based on time, not frames so the FPS of a nested comp does not matter as long as properties in that comp are animated with keyframes. You can force the frame rate to matter so you get 3 duplicated frames in a nested comp if you select preserve frame rate when nested or rendering in the comp settings. You'll find that in the Advanced tab of Composition Settings. That is the only time you will end up with duplicate or missing frames in a nested comp.

What does matter is the frame rate of any footage or image sequences you have in your comp. If you want to see all of the frames then the frame rate of the footage or image sequence needs to match the frame rate of the comp. If it doesn't you'll get blended or missing frames. The frame rate determines the speed of the playback. You have to know what that is. If you shoot at 60 and your comp is at 30 then 10 seconds of real-time will still be 10 seconds of screen time but you will be missing every other frame. If you interpret 60fps footage at 30 then 10 seconds of real-time will take 20 seconds of screen time even if the footage is put in a comp that is 60 fps.

Did you get all that? Most folks that didn't grow up shooting film don't really understand frame rates until they play with them for a while.

[TimSx]

Thank you all!  Topic is crystal clear!

Rick, understanding time codes of 29.97 fps footage was way more challenging

[Rick Gerard]

Just remember that drop frame timecode gives you accurate time to the nearest frame and non-drop frame timecode does not give you accurate running time. No frames are dropped, just frame numbers so the clock is accurate to the nearest frame.