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Shooting on film: High/low speed motion blur & shutter angle

film high speed motion blur 180 degrees shutter angle

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#1 PJ Palomaki

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Posted 08 August 2015 - 04:19 AM

Hi,

 

I recently had an opportunity to shoot on film for the first time in my life (crazy, I know) and a question came up which left me thinking.. Apologies as it's probably a very noob question:

 

When I shoot slow/fast motion on digital, I change the shutter speed to maintain the same motion blur as for the 'normal' speed, i.e. shooting on 25p, 1/50 'normally' and then do 50p for instance, I shoot at 1/100 to have the same amount motion blur. Assuming you want the same motion blur, how would you do this shooting on film?

 

I am right in understanding that 180 degree shutter angle will remain 180 regardless of what frame rate you shoot? I.e. shooting 25fps and 50fps means that the shutter turns twice as fast to maintain 180 degree shutter angle? I hadn't noticed but was told that Red has an option for shutter to have either a shutter speed (1/xxxx) or angle (degrees), does this mean if you want to shoot everything at 180 degrees, you don't have to keep changing the angle for different fps?

 

But surely, even if you can capture everything at 180 degrees, in playback, there will be variations of motion blur as the angle was relative to the fps? I.e. 25fps has 180 degrees and 50p has 180 degrees, but compared to each other, they don't match?

 

I would've automatically calculated the difference between the degrees when under/overcranking and changed the shutter accordingly, but I was told by two working AC's that this is incorrect...

 

If anyone can shed any light or knows of any resource on this, I'd really appreciate it!

 

Thanks,

 

PJ


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#2 Mark Dunn

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Posted 08 August 2015 - 05:07 AM

Variable shutters for film will close to a small angle, but they can't open very wide as there has to be time for film transport.

So you won't be able to maintain the same effective shutter speed over a wide range of running speeds. It's a physical limitation, not an electronic one.


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#3 David Mullen ASC

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Posted 08 August 2015 - 10:32 AM

The shutter time / speed -- which controls the amount of motion blur per frame -- is the result of the combination of frame rate and shutter angle.  So if the frame rates are different and the shutter angle is the same, then the shutter times will still be different.

 

The shutter is a spinning metal disk in front of the film with a pie slice hole cut out to let light pass through, so the shutter angle is a measurement of that gap or hole -- 180 degrees means it's a half-circle, half open, since 180 is half of 360.  So the shutter is open for half the time that the frame rate allows.  If the camera is taking a new picture 24 times a second, then the shutter is open for 1/48th of a second.

 

With a film camera, you can't open the shutter angle more than 180 degrees (except for the few that do 200 degrees max) but you can close down the shutter.

 

25 fps and 50 fps are easy to calculate the difference in exposure time at 180 degrees, it would be 1/50th and 1/100th, but there is no way to shoot at 1/50th at 50 fps because you'd need a 360 degree shutter angle, i.e. no shutter at all, but you can easily get 25 fps to be 1/100th just by switching to a 90 degree shutter angle.

 

If you want a 180 degree shutter angle, you set the camera to that -- it won't change as the frame rate changes unless you've got some fancy 35mm camera that can do speed ramps where the exposure stays the same as the speed changes by changing the shutter angle instead of the f-stop.  In that case, you could ramp from 25 fps to 50 fps and the camera would change the shutter angle from 90 degrees to 180 degrees to keep the exposure consistent.

 

The Red camera is tricky because there is some setting called "relative" which I think can change the shutter time or angle to something other than what you set, I'm not sure.  In theory, yes, you use use shutter time or angle, so that if you use angle, the time will change with the frame rate.


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#4 dan kessler

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Posted 08 August 2015 - 01:30 PM

So with regard to your motion blur question, the answer is no, the amount of motion blur will not be the same when shooting at different frame rates with a film camera.  

As you increase the frame rate, the exposure time decreases, so the amount of motion blur also decreases.


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#5 PJ Palomaki

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Posted 08 August 2015 - 03:15 PM

Thanks to all for replying and filling in, very helpful!

 

So, a follow-up question - When shooting film, do you just have to settle for very little motion blur, if you shoot at 120fps, for instance (5 times less the amount of 24p motion blur at 180 degrees)? Or is this something you wouldn't even care aesthetically as it's slow motion?

 

But when you under-crank, you should be able to match the motion blur as long as you adjust the shutter angle to match exposure time of 24/25fps, i.e. 24fps at 180 degrees = 12fps at 90 degrees? And as an artistic choice, you could shoot at, say 1fps at 180 degrees and have a lot ton of motion blur (providing you expose correctly)?


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#6 David Mullen ASC

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Posted 08 August 2015 - 05:48 PM

It actually looks odd if you open up the shutter on high speed shots, like using a digital camera running at 60P with 1/60th of a second (360 degree shutter), even though in theory that's the same motion blur PER FRAME as something shot at 30P with a 1/60th shutter time (180 degree shutter) -- keep in mind that you are watching the high frame rate shot played back at something like 24 fps, so the motion is unique, being slowed.  If it is blurrier on top of being slower, it looks sort of smeary, dreamy.

 

If anything, sports movies shooting at high frame rates often use smaller shutter angles than 180 degrees to reduce blur since one reason you are using the faster frame rate is to sample the motion more times and see it reconstructed more clearly, so having less blur per frame makes the action more clear.  I recall a Nike commercial of some runners shot at something like 250 fps with a 45 degree shutter angle so you could see every speck of dirt and sweat fly.

 

Also keep in mind that we've had a 100 years of slow-motion footage in movies shot with a 180 degree shutter so I'm a bit surprised that you are concerned about the effect being odd aesthetically -- surely you made up your mind about that a long time ago?

 

Again, for under-cranked motion, it's the same thing, if the footage is going to be projected at 24 fps and look oddly sped-up, the amount of motion blur is not what you really notice, but either way, we are used to seeing it shot with a 180 degree shutter.  Low frame rate motion already looks sort of choppy due to the low sampling rate, so if you shortened the shutter angle to keep a 1/48th shutter time, then you are going to make the sped-up motion look even choppier, more staccato... at least more motion blur takes some of the curse off of that stop-motion pixellated look.

 

Now if you are going to take that low frame rate stuff and play it back at the same rate so that the speed is normal but the look is very steppy -- what used to be done with optical printing, repeating frames until they added back up to 24 -- then with a normal 180 degree shutter, you can this look of steppy motion with a lot of blur; you see this is some Wong Kar Wei movies.  It's already a weird effect, but if you wanted to shorten the shutter time to get less blur, you could, but the motion would still be very steppy, maybe even more so without as much blur.


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#7 PJ Palomaki

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Posted 10 August 2015 - 10:39 AM

Thank you David, invaluable information! Much appreciated all the effort and patience in replying and clarifying. The point about high frame rates and sports footage makes good sense.

 

Thank you,

 

PJ


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