FCPX choppy slow motion (not the usual problem)

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FCPX choppy slow motion (not the usual problem)

Hi guys,

I am having serious issues with my Final Cut Pro X and the way it handles slow motion footage. When I slow down 50p footage in a 24p timeline (slowed to 48%) I get this choppy effect each few seconds during the clip. At first I thought it was just the laggy playback in FCPX, but then I exported the clip and exactly the same happened.

I did some more testing and found out that each 25 frames one frame is played back twice. Let's say I play the video - in a 24p timeline - and at 04:05 that frame is played twice (at 04:05 and at 04:06) exactly 25 frames later at 05:06 that frames is played twice (at 05:06 and at 05:07).

This is definitely a problem that occurs in FCPX, because I slowed the exact same clips down to 50% in iMovie and the playback was smooth.

If I change the video quality in the retime menu to frame blending or optical flow this issue is resolved but I do not like the look of these two options. It should work in the normal setting for video quality because I have sufficient frames. The same thing happens when I slow down to 60% or anything else slower than 100%.

Do any of you know what I am doing wrong or if there are some settings that I should change?

Posted on Aug 18, 2020 8:32 AM

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Answer 1

Aug 18, 2020 9:20 AM in response to Barni527

Did you specifically set the retiming to 48%? If so, try choosing "Automatic Speed" instead.

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Answer 2

Aug 19, 2020 4:33 AM in response to Barni527

> each 25 frames one frame is played back twice

In my experience FCP ignores 24 <-> 25 frame rate conversion i.e. 96% or 104% speed is applied behind the scenes but not shown to the user (so AFAIR the duration changes accordingly). So should you just apply a 50% slow-motion so in 50 -> 24fps unique frames slow-motion no frames are duplicated?

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Answer 3

Tom Wolsky

Level 10

152,780 points

Aug 18, 2020 10:00 AM in response to Barni527

Now it’s 1/100 at 24fps. The sharp image, fewer times per second, creates a more noticeable change between each frame. 24 would need to have motion blur added to make the movement less pronounced. Optical flow tries to do something similar. A third party effect with controls might look better.

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Answer 4

Barni527 Author

Level 1

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Aug 18, 2020 12:55 PM in response to Tom Wolsky

What should I do then with my frame rate when shooting panning shots and what should my shutter speed be for the intended delivery rate?

Does optical flow change the way motion is rendered in my video or does it just make everything "flow" better? I have noticed that frame blending gives this slight halo effect on some clips.

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Answer 5

Barni527 Author

Level 1

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Aug 18, 2020 9:38 AM in response to Luis Sequeira1

I used the "Automatic Speed" setting and that retimed it to 48%.

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Answer 6

Tom Wolsky

Level 10

152,780 points

Aug 18, 2020 9:48 AM in response to Barni527

What's the shutter speed?

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Answer 7

Barni527 Author

Level 1

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Aug 18, 2020 9:55 AM in response to Tom Wolsky

The shutter was 1/100. The choppy effect only occurs when I slow the footage.

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Answer 8

Tom Wolsky

Level 10

152,780 points

Aug 18, 2020 12:51 PM in response to Barni527

Planning shots are always poor at 24fps, which is why they’re usually avoided at the film frame rate.

The shutter speed is correct for your media, but not for your intended delivery rate.

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Answer 9

Barni527 Author

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Aug 18, 2020 1:14 PM in response to Tom Wolsky

What if I want to get B-roll shots of a location? Then I would normally shoot a slow panning shot at 24mm at 50fps and slow it down in post? Is this wrong?

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Answer 10

Tom Wolsky

Level 10

152,780 points

Aug 18, 2020 1:36 PM in response to Barni527

There is no right or wrong, except that if you want 24fps it’s generally thought best to avoid shooting slow planning shots, or any panning shots. You better off shooting a series a individual shots.

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Answer 11

Barni527 Author

Level 1

12 points

Aug 18, 2020 12:46 PM in response to Tom Wolsky

I shot in 50fps with the shutter at 1/100. So that abides to the 180 degree shutter rule... Optical flow helps with the problem, but I understand that optical flow (and frame blending) are mostly for slowing footage more than your frames per second allow. For instance slowing 24fps footage to 50%. I shot in 50fps and this effect happens even when I slowed to 60% where I should easily slow to 48% in normal video quality.

The funny thing is it is not like the normal choppy effect you get when slowing 24fps footage to let's say 50%. It only happens once every second. It's like a glitch. Did I mention it is way more pronounced with panning shots?

Creating proxy media also helps in some cases.

Thanks for the tips so far guys!

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Answer 12

Tom Wolsky

Level 10

152,780 points

Aug 18, 2020 1:03 PM in response to Barni527

Only use planning shots that follow something as it moves, or moves quickly from one place to another. Avoid planning at 24fps. If you want to use the film frame rate then the shutter speed should be appropriate so you get the appropriate motion blur. High frame rates are better for motion, but it really depends on what you need to deliver.

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FCPX choppy slow motion (not the usual problem)

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