Digital Motion Characteristics Question

[Matthew Marenyi]

Why does some digital footage that is shot 24p still have motion characteristics of video?

An incredibly simple question. But honest to god i've read tons of forums and talked to a several professionals, none of whom have been able to furnish a satisfactory answer. Its starting to drive me crazy. I know the answer has to be incredibly obvious.

I'm talking about the unnatural smoothness, that almost looks like a higher framerate. Is it the shutter speed? if so which shutter speed corresponds with the most film like characteristics? 1/40? 1/60? 1/80?

Two very well known examples of what i'm thinking of are Public Enemies and The Social Network. P.E. there is a clear video smoothness to most of the images, however the social network seems the have very filmic motion blur.

Please someone end my misery and enlighten me.

 

[Gabe Conroy]

It is the motion blur...film cameras can't leave the shutter open for the entire duration of a frame, but in a digital camera with an electronic shutter it can. So if shooting at 24p, having a shutter speed of 1/24 will give motion blur all the way from the previous frame to the next one since the shutter's essentially always open.

In film shutter terms a normal shutter speed is 180, which corresponds to the shutter being open for half of the time, giving motion blur that only extends half-way from one frame to the next. So for 24p, that'd be a shutter speed of 1/48. For 30p it'd be 1/60.

EDIT: I personally like doing even faster shutter speeds sometimes because it gives very distinct and sharp definition to movement, though it also feels jerkier to some extent. Movies like Gladiator do this a lot.

 

[Jesper Sichlau]

EDIT: I personally like doing even faster shutter speeds sometimes because it gives very distinct and sharp definition to movement, though it also feels jerkier to some extent. Movies like Gladiator do this a lot.

I believe Saving Private Ryan is the classic example of that.

 

[D Fuller]

In film shutter terms a normal shutter speed is 180, which corresponds to the shutter being open for half of the time, giving motion blur that only extends half-way from one frame to the next. So for 24p, that'd be a shutter speed of 1/48. For 30p it'd be 1/60.

Just a small explanation here: in film terms, shutter is described as shutter angle, rather than speed. For those who've never worked with a film camera, this is because a film camera's shutter usually rotates, and a 180 degree shutter means that the film is exposed for half of the revolution time. So the shutter speed for a 180 degree shutter is always exactly 1/2 of the frame rate.

 

[David Mullen ASC]

24 fps is a very low sampling rate for motion so strobing is a characteristic.

Some people shoot 24P video with longer shutter times than 1/48th, such as 1/32nd (270 degrees) or 1/24th (360 degree, i.e. no shutter) which increases blur per frame more than what a film camera can achieve with a mechanical 180 degree shutter. It also increases exposure, which is why some people use it, in low-light situations. Other people use it because they like the smearier motion. "Public Enemies" used a 360 shutter for the machine gun fire night scenes so that the flashes would expose very brightly and look very large, and actually light the actors' faces.

Another problem is how some people view 24P material, like live on an EVF or a monitor on set, where it can look a bit laggy, flickery, and/or strobey due to various in-camera and monitor/EVF processing and refresh issues. You can't really look at film "live" other than through the viewfinder or with a video tap.

 

[Matthew Marenyi]

The night gun scenes are a good example of where I'm a bit confused. The 1/24 shutter should add even more blur ass you suggested. however that scene still has a very video look. I'm particularly thinking of the scenes with Dillinger firing out of the cabin in that small bedroom. Was that scene shot 24fps or something higher with the low shutter rate?

 

[Matt W.]

Never seen that movie, but a 1/24th second shutter SHOULD contribute to a "video look."

Video (in the US at least) is shot at 60fps with a 1/60th second shutter speed (effectively). That means the camera captures 60 frames per second, each of which represents 1/60th of a second of recorded material.

Film is usually shot at 24fps with a 1/48th of a second shutter speed. This means the camera captures 24 frames per second, each of which represents 1/48th of a second of material. (The other half the time the mirror is diverting the image to the optical finder, which means when you shoot film, what you see through the finder is everything that DOESN'T get recorded to film.) Half of what you see in real life, half of what really happens, doesn't make it onto the film. This, compounded by the low frame rate, makes the image a bit choppy.

In other words, video records every 1/60th of a second for a total of 60 "full" frames per second. Film records every OTHER 1/48th of a second for a total of 24 "half" frames per second.

In Public Enemies, they shot 24fps with a 1/24 second shutter. So they recorded 24 frames per second, each frame representing 1/24th of a second. So, like film, it's shot at 24fps. But, like video, the entire motion is represented in each frame. Half of it isn't missing. So it's a lot smoother with much more motion blur.

The main reason someone would do this during a machine gun shoot-out is not for exposure-related reasons (muzzle flashes are so quick, quicker than 1/48th of a second, that shutter speed would have little to no effect on their brightness, so they would actually seem dimmer relative to the background if shot at 1/24th second shutter speed). You would shoot at this shutter speed because otherwise the camera would not pick up slightly less than half of the muzzle flashes.

I've also noticed a lot of "digital motion," as some like to call it in shots that have gone through noise reduction, since motion gets fuzzed over by the motion vector estimation.