I've been submitting stock videos for a long time now, but when it comes to slow motion clips -- I admit it, I'm never really sure what I'm doing. 

So, my source clip was shot at 59.94 fps. I put the clip into an After Effects project, which sets the frame rate to match the source.

I then do a Time Stretch of 200%, which doubles the clip's time length.

Finally, I render the clip at 29.97 fps, and the final duration matches what I expect. (That is, 10 seconds of the clip at 59.94 takes up 20 seconds on the timeline, and creates a 20-second clip at 29.97.)

So does anyone know why what I'm doing is wrong, and what I should be doing?

Thank you very much! I'll give that a try (though it seems too easy; should be more difficult than that  ).

Perhaps so; I'm going to re-submit these new renders and find out.

The rejection reason given:
Frame Rate / Shutter Speed -- Clip exhibits issues related to frame rate or shutter speed.

Maybe it had something to do with shutter speed. I think I was using 180-degree, so the shutter speed would have been 1/125. Maybe they don't like 1/125 shutter speed rendered at 29.97? Perhaps I should always shoot at a fixed shutter speed of 1/60 when I intend to render at 29.97. I've never really understood that. 

Hmmm. I don't know if I'll ever get a handle on this stuff.

In AE, there's something called 'frame blending' that I can enable. But I have no idea if I actually need to (or should) use it in this case.

Adobe says:
"When you time-stretch or time-remap a layer to a slower frame rate or to a rate lower than the frame rate of its composition, movement can appear jerky. This jerky appearance results because the layer now has fewer frames per second than the composition. Likewise, the same jerky appearance can occur when you time-stretch or time-remap a layer to a frame rate that is faster than the frame rate of its composition. To create smoother motion when you slow down or speed up a layer, use frame blending. Don�t apply frame blending unless the video of a layer has been re-timed�that is, the video is playing at a different frame rate than the frame rate of the source video."

So, it sounds like I should have used frame blending when I did the time stretch (well, maybe). But now that I'm using increasingdifficulty's suggestion of 'interpret footage', it seems like I should not. I think. Maybe. 

Yeah, I think you're right, and increasingdifficulty's method seems to do the trick.

I've found that the rendered clips are roughly 1/2 the size with his method, and match the approximate sizes of non-slow-motion clips. I hadn't noticed before, but these were much too large. I don't know what the heck I did, but it sure wasn't right.

Maybe it had something to do with shutter speed. I think I was using 180-degree, so the shutter speed would have been 1/125. Maybe they don't like 1/125 shutter speed rendered at 29.97? Perhaps I should always shoot at a fixed shutter speed of 1/60 when I intend to render at 29.97. I've never really understood that. 

No, you did it right.

When you render at 29.97, the action plays at half the speed so the motion blur would match a 1/60 shutter if the action actually happened at that speed in real life.

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It's the same the other way around, with timelapses.

A 180-degree shutter on a 5-second interval timelapse means a 2.5-second shutter speed.

When rendered at 24/30p the motion blur matches the action as if it would've happened at that speed in real life.

Just a fast suggestion to see if it helps but it has to do with Premiere.
Right click to the clip, choose speed, set it to say 50%
but at time interpolation choose optical flow.
Render it for viewing purposes only and it will probably be smoother.

Quote from: bestravelvideo on January 27, 2018, 14:14

Just a fast suggestion to see if it helps but it has to do with Premiere.
Right click to the clip, choose speed, set it to say 50%
but at time interpolation choose optical flow.
Render it for viewing purposes only and it will probably be smoother.

This has nothing to do with what he's asking about. Optical flow is for FAKE slow motion. This is real slow motion where the frames exist.

Why not read the question and avoid further confusion?

Mea culpa!

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Maybe it had something to do with shutter speed. I think I was using 180-degree, so the shutter speed would have been 1/125.

When cine film cameras run at high speeds, they utilise faster shutter speeds compared to filming at standard speeds (24fps.) So yea the 180 shutter principle still stands regardless. Sounds like you did everything right.

By the way, I used to shoot a lot of super 8 movie film. And sometimes I would run the film at 54fps and project it at 24fps for smooth looking slow motion. Watching the footage was like poetry in motion. I wish digital was that easy.....I once shot some 50p footage with my Panasonic G6 with the intention of producing slow-mo footage. I output it as 25p with Sony Movie Studio but the footage came out at normal speed (no slow motion.)

You need to find an option called "interpret footage" or something similar in your software.

I played around with the software for a bit but couldn't find what I was looking for. I wonder if other software programs make this kind of thing easier. I do have Adobe Premier Elements which I haven't used yet. And I'm considering downloading Open Shot and trying that as well.

All you're telling the computer to do is change the playback speed, just like with film. If you have filmed 50 frames per second, it will play back at half the original speed played back at 25 fps. Instant slow motion. 

Theoretically, yes. But the tricky thing for me is working out how to do this with software. Like I said, it's so much easier with film (at least for me.) Dead easy. Run that 54fps footage through a projector and it's instant slow motion!

Ah excellent. Thankyou for the info. I'll have to start familiarising myself with Premier Elements.