Working with 120 fps Footage

[James@doorbellmedia.com]

Hi,

I shot on A7S ii at 120 fps with the intention of using the footage in super slow motion. Shot in 1080.

Brought footage into Premiere Pro CC 10.4. In the project window, selected clips and CTRL-Clicked or Right-Clicked MODIFY/INTERPRET FOOTAGE and selected "Assume This Frame Rate:". Manually entered "23.976" which is what the rest of my footage was shot at.

For most of my clips this worked fine and resulted in nice smooth slow motion. However, there were some shots that had Christmas lights and a TV. These shots, either at the native 120 fps or the interpreted 23.976 fps resulted in extreme strobing of the light sources.

I don't know if the problem is that PP doesn't really recognize 120 fps - at least there isn't a setting for it.

The only workaround I found which ended up being a bit of a compromise was to transcode the original 120 fps footage in Media Encoder to 59.94. Then I took those transcoded files, brought them into PP and interpreted them as 23.976. The result was a slow motion clip without the strobing, however it was not as slow as interpreting 120 fps as 23.976.

Tried using Apple Cinema Tools which is how I used to slow down high frame rate footage, but it didn't recognize the .mp4 files from the A7S ii.

If anyone has any other suggestions, explanations, workflows please comment.

Thanks,

James

[SAFEHARBOR11]

Hi James,

I think you are trying too hard - you don't need to interpret the footage first, in fact that may be what's causing the issues!

I often edit 60p clips in a 30p sequence. All I do is drop them into the sequence - Premiere is intelligent enough to handle things properly, clips play at proper speed and look fine. Should be no different dropping 120fps clips into 24p timeline, Premiere will just skip some frames and play the clip proper speed at 24fps.

However, if you apply slow motion to clip, then the extra frames will be utilized for buttery-smooth slow motion effect! I think that by conforming frame rate up front, you are "losing" those extra frames perhaps. Premiere then treats those 120fps clips as if they are only 24fps, so adding slow motion duplicates frames maybe, losing benefit of the high frame rate??

I apply 50% speed to 60p in 30p sequence and it looks gorgeous.

Try it natively without conforming

Thanks

Jeff Pulera
Safe Harbor Computers

[James@doorbellmedia.com]

Thanks, Jeff. I think what we've described is two ways of achieving slow motion - interpreting high frame rate as lower or dropping high frame rate into a lower frame rate sequence and changing clip speed. You don't lose the frames when reinterpreting - the clip becomes longer.

In either situation the strobing is still there. Tried both.

It's very similar to shooting a monitor or under fluorescent lights that have refresh rates that cause flicker at certain shutter speeds and frame rates.

I was able to eliminate the flicker by converting 120 fps to 60 fps. Still able to get slow motion slowing down to 23.98 but not as much as going from 120 to 23.98.

Thanks,

James

[Joe Denner]

I know this is a really old thread, but I just want to confirm this: you are saying there is no difference between A) interpreting 120fps footage as 24fps and B) putting 120fps footage into 24fps timeline and slowing to 50%?

[SAFEHARBOR11]

Well, not quite. It would be 120fps in 24fps sequence at 20%.

Example: If you have 60fps footage and Interpret as 30fps, that does in effect then double the playback duration. It plays back at 30fps rather than the original 60fps, using all the original frames. Alternately, put the 60p clip into a 30p sequence, and apply 50% slow and get the same result without interpreting.

120 divided by 24 = 5. So put the 120fps clip into 24fps sequence and set to 20% slow and it plays all frames, resulting in 5x the original duration.

You could also do 120p into 60p sequence at 50%, or 120p into 30p at 25% and use all frames for perfectly smooth slow playback.

Thanks

Jeff

[Ann Bens]

Optical Flow Time Remapping: Tips & Tricks for Best Results | Creative Cloud blog by Adobe

[James@doorbellmedia.com]

Here's a link to some of my footage, illustrating the issue I'm having:

Vimeo

[SAFEHARBOR11]

Interesting. Unlike incandescent light sources which are simply a glowing filament providing constant light (similar to a candle flame), other sources like flourescent and LED actually flick the light source on and off very rapidly. Looks like a steady light to our eyes, but cameras can pick up the flicker due to shutter opening and closing and catching the light when it is OFF/cycling. Looks to me like this is the issue - if the light source on tree was constant, there would be no dark frames in playback, because the light would always be on.

I believe the problem is inherent in the footage as shot and nothing to do with slow-motion in Premiere. There are third-party plugins available to repair. search web for "flicker fixer".

Try this - open a 60p project. Import the 120p clip, and interpret as 120fps. This should double the length when clip is put on timeline. Now play the footage frame by frame using arrow key. You should be seeing every single frame you shot. Bet you have frames with lights on, and frames with lights off. You don't notice flicker when playing the footage realtime in 24p sequence because you are not seeing all the frames, or they blend together.

Thanks

Jeff

EDIT: remember watching old Westerns and the wagon wheels seem to turn backwards? Due to how the shutter freeze-frames the spokes at various positions in time. I am now seeing this same phenomenon on the aluminum-spoke wheels of cars driving next to me on the expressway! Now, my eyes don't have shutters in them, so the only explanation is that the new type of highway lighting being used is flickering, so the wheel spokes are not constantly lit and like the old film camera shutters, light up the wheel spokes with such timing that causes them to appear to spin in reverse. Wild. With steady (incandescent) lighting, the wheel spokes should be a blur at 70mph, but I can see the spokes due to flickering lights. Like a strobe light. Any old-time mechanics here that have used a timing light? Then you know exactly what I am referring to!

[Artie Medvedev]

I have the same issue with 120 fps footage from sony a7iii. And I've shot videos under mid day sun light on the beach without any strobbing light sources.

[rodneyb56060189]

I think for some reason the little Sony ( a very nice camera ) is a little prone to being sensitive. I don't have one. I got some footage from a friend so I could test my computer using 4k S log, so he sent me stuff on a memory stick straight from his camera.

This is 2 minutes and if you look carefully you can see some lights strobing.

TOM FINISHED on Vimeo

To me this is complicated cause one would think that 30fps at 180 would be the only safe U.S. combination.

This video sample is the U.S. on 60hz grid, so 23.976 ( or 24) fps should be fine with a 180 deg shutter.  In the UK, France, you'd have to be at around 171 deg shutter I think ( or 25 fps at 180 ) cause they are on a 50hz grid.

The shutter and fps combination is really important ( boils down to fps and shutter SPEED).

Some LED's are further confusing things due to some strobing on some of those ( and of course fluorescent and sodium vapor are fun too )… but basically what comes out of the camera is the main thing to start with.

So if you are getting strobing from lights, or from sunlight, double check your shutter ANGLE ( there are charts that will give you equiv angles to 'speeds' settings...  ( e.g. 180 at 24 fps would be 1/48th of a second ).

[Tim_Ouillette_75]

I thought that you would have to have a shutter speed of 1/240 if you wanted to do this without flikering in the US.