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48 or 60 fps for only a slight slow-mo effect

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Matheus Siqueira
48 or 60 fps for only a slight slow-mo effect
on Oct 18, 2015 at 5:33:56 pm

I'm planning on filming a video clip and would like to achieve the slow-motion as in this video: https://vimeo.com/34009508

My final output is 24fps. I'll be shooting with a Sony FS7. I would need to record at 48 fps or 60 fps and conform to 24 fps on post-production?

Thanks!


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Todd Terry
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 18, 2015 at 5:44:44 pm

Well firstly, I wouldn't call that "slight" slo-mo by a long stretch. I'd say it's pretty significant slo-mo. To me "slight" is shooting at maybe 30fps or 36fps... which gives you just that barely-slow-motion look that is prevalent today.

The obvious (and slightly snarky, I realize) answer is to just run some tests. Shoot footage at various frame rates and see what results you like. Also, it's easy enough to rip that Vimeo video, throw it on a timeline, and adjust the speed until the action looks "normal." If that video is the look you are going for, then that will tell you at approximately what fps the original footage was shot. Some of it didn't look all that slow, but then again some flying birds looked pretty darn slow so that leads me to believe the fps was fairly high.

So, just test it. Oh, and remember that when shooting at higher-than-normal fps you should shoot with an appropriate shutter speed for your new frame rate (the formula that you'd likely want to use is "one over twice the frame rate"... ergo, if you shoot at 48fps you'd want a shutter speed of about 1/96th... at 60fps you want 1/120th, etc.).

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rick Wise
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 18, 2015 at 6:07:35 pm

As always, Todd's advice and observations are terrific! I do have a tiny quibble with one issue: shutter speed. I suspect one does NOT need to increase the shutter speed when shooting at a higher frame rate. When we shoot at, say, 48 fps instead of 24, the amount of time each frame is exposed is 1/2 of the 24-rate. That means the image will be far more crisp. To increase the shutter speed from 48 to 96 again crisps the image even further. But too much crispness can be a problem, creating a staccato effect. While it is possible the filmmaker wants such an effect, for "normal" viewing I suspect 1/96th will be excessive. It also cuts in half the amount of light reaching the film/sensor, which is the equivalent of one f/stop. Since you have already lost one f/stop of effective sensitivity by speeding up the camera from 24 fps to 48 fps, you have by doubling the shutter speed now lost the equivalent of two f/stops of sensitivity. Could be that's no problem, if there's plenty of light and your ISO is high enough. Or the two-stop loss could in some situations make the shot impossible.

Here too, tests would be good. I have not tried shooting at 48 fps and a shutter speed of 1/48th and then again at 1/96th. Just operating from logic.... I could be wrong.

Rick Wise
Cinematographer
MFA/BFA Lighting and Camera Instructor Academy of Art University
San Francisco Bay Area
http://www.RickWiseDP.com


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Todd Terry
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 18, 2015 at 6:18:42 pm
Last Edited By Todd Terry on Oct 18, 2015 at 6:20:15 pm

I hate to argue with a sage like Rick (because I almost never can successfully), but in this case I will...

If you overcrank yet still shoot with a "normal" frame rate (say, 1/48th) you will get blurry smeary slow-mo. The brain needs just a certain amount of motion blur to interpret images as smooth, any more or any less causes problems.

You will not get the staccato effect by shooting at a higher (appropriate) shutter speed for slow mo. Think of it this way, if you were shooting in real time a subject that was actually moving slowly, you would get much less motion blur in an individual frame than if your subject was moving quickly. That's what shooting at a higher shutter speed for overcranking basically simulates.

Or think about it this way... us old timers like me and Rick used to shoot film. If you were shooting 24fps in a camera with a non-variable 180° shutter, your shutter speed would be 1/48th. BUT, if you overcranked to 60fps, your shutter speed would automatically be 1/120th... there's no changing that even if you wanted to (with a fixed-shutter camera... and even with a variable-shutter film camera you can only change that a little). And yet the footage looks "right."

Rick's argument makes a bit of sense in theory, but in practicality you need the "right" shutter speed for the corresponding frame rate.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rick Wise
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 18, 2015 at 6:25:39 pm

I look forward to someone shooting a test that proves me right.....

Rick Wise
Cinematographer
MFA/BFA Lighting and Camera Instructor Academy of Art University
San Francisco Bay Area
http://www.RickWiseDP.com


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Matheus Siqueira
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 18, 2015 at 6:37:28 pm

If I manage some spare time with the camera I'll do some shots at 48 fps with both shutter speeds and upload it to share here. Once again thank you very much Todd and Rick!


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Matheus Siqueira
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 18, 2015 at 6:10:08 pm

Thanks for the amazing answer Todd! The idea of ripping the vimeo video and checking it out on a timeline was fantastic, I´ll do this since unfortunately the Sony FS7 I'll be using is rented for the exact amount of shooting days so I won't have a lot of time to make tests and see what works best.


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Fernando Mol
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 19, 2015 at 2:44:36 pm

Look like a 48 to me, but I agree with the idea of doing a test. If your camera has an "auto" option for the shutter speed it will fix it to half the frame rate speed, as mentioned by others.

For 24, 48. To shoot at 90, you'll have a speed of a 180.

But, if the camera has cine options, it will not show a shutter speed, but a shutter angle that should be of 180 degrees and you don't have to adjust.

I hope this helps.

https://vimeo.com/fernandomol


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Todd Terry
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 19, 2015 at 4:00:19 pm

I have to say I'm a little surprised at the answer....

I had a minute to kill so I ripped that video and threw it on a timeline. After playing with a couple of speeds I found that the most of the slow-mo footage looked to be perfectly "normal" speed if played back at 150%.

So... that tells us it was shot at 36fps... or thereabouts.

I'm pretty surprised, to my eyeball I would have guessed the frame rate to be much higher than that... so I have to reluctantly recant my original guess that it was much slower slow-mo than I first thought.

Oh well, live and learn.....

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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Rich Rubasch
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 22, 2015 at 7:14:35 pm

Here is our (very cool) workflow. Shoot shots we want to slo mo at 120, 90 or 60 fps, depending on the camera. Shoot the rest at 23.976.

Convert to ProRes and run the slo mo high frame rate clips thru Cinema Tools and conform to 23.976. Now you can play them at full slo mo or use the speed tools to speed it up....works at slo mo and works sped up with total flexibility.

In premier you can just bring in the raw and change the inspector frame rate of the clip from the original frame rate to 23.976 and it works the same way.

Very cool.

Rich Rubasch
Tilt Media Inc.
Video Production, Post, Studio Sound Stage
Founder/President/Editor/Designer/Animator
http://www.tiltmedia.com


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Todd Terry
Re: 48 or 60 fps for only a slight slow-mo effect
on Oct 22, 2015 at 7:24:57 pm

[Rich Rubasch] "use the speed tools to speed it up....works at slo mo and works sped up with total flexibility."

Yes, of course you can always speed up slow-mo footage to normal speed, and I've had people ask me "Why shouldn't I just overcrank everything, so that if I want to slow-mo something later I have that option." Well you certainly can do that, but when you "normalize" the speed you risk very choppy staccato video if you have much action in your frame, obviously because as the frame rate increases the shutter speed goes up, too. It's exactly the same combo if you are shooting 24fps but with a narrow shutter. Great for "Saving Private Ryan," but for other things... not so much.

So Rich is absolutely right in that he only shoots 23.976 for normal speed shots. Overcranking should really be reserved for when you pretty much know the shot will be slow-mo'd. You can use overcranked footage at normal speed, but I'd definitely save that for the emergency no-other-options situations and be prepared for that narrow-shutter action-movie look.

T2

__________________________________
Todd Terry
Creative Director
Fantastic Plastic Entertainment, Inc.
fantasticplastic.com



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