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Exposure compensation for high speed cinematography


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#1 Ashley Barron

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Posted 19 May 2009 - 12:53 AM

Hi there,

I wasn't sure in which forum this topic would be most appropriate, so here I am.

I'm getting myself confused by trying to figure out what exposure compensation is required to shoot in high speed frame rates.

Do you need less light or more light for this? Therefore, would you open up or close down?

And while I'm at it, when someone says "add more stop" that means open up or close down?

Any advise would be great.

Cheers,
Ash.
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#2 David Mullen ASC

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Posted 19 May 2009 - 01:50 AM

The faster the film runs (or the sensor), the shorter the time each individual frame can get exposed -- so you're getting less per-frame exposure as you run the camera faster.

For example, if you have a 180 degree shutter, so it's closed 50% of the time, then at 24 fps, the exposure time is 1/48th of a second per frame. So at 48 fps, it would be 1/96th of a second per frame. So basically by doubling the frame rate, you've cut the exposure time per frame by half.

Opening the lens aperture allows more light to hit the sensor / film. So to compensate for losing half your exposure from doubling the camera speed, you'd open up the lens aperture by one-stop (like from f/4 to f/2.8 for example). Or you'd double the amount of light instead, like from 100 fc to 200 fc. Or you'd switch to a film stock that was twice as fast, i.e. had double the ASA value, like from 250 ASA to 500 ASA.

"Add more stop" tends to be a lighting comment, i.e. increase the light level to allow stopping down the lens.

"Stopping down" means closing the lens aperture (like from f/4 to f/5.6 for example.)

"Opening up" means opening the lens aperture (like from f/5.6 to f/4 for example.)
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#3 Ashley Barron

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Posted 19 May 2009 - 02:05 AM

Thank you for that David, I'm clearer on it now!
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#4 Ashley Barron

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Posted 19 May 2009 - 02:11 AM

The faster the film runs (or the sensor), the shorter the time each individual frame can get exposed -- so you're getting less per-frame exposure as you run the camera faster.

For example, if you have a 180 degree shutter, so it's closed 50% of the time, then at 24 fps, the exposure time is 1/48th of a second per frame. So at 48 fps, it would be 1/96th of a second per frame. So basically by doubling the frame rate, you've cut the exposure time per frame by half.


One other thing..

What if you alter the shutter angle aswell? How would that affect all those variables?
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#5 David Mullen ASC

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Posted 19 May 2009 - 10:36 AM

One other thing..

What if you alter the shutter angle aswell? How would that affect all those variables?


Yes, you can alter the shutter angle, which will affect shutter speed (and motion blur). Though in the case of running a camera faster, you can't really open up the shutter angle enough beyond 180 degrees to compensate, not in a film camera. For example, if you doubled the frame rate from 24 fps to 48 fps and lost half your exposure time per frame, you would have to open up the shutter angle from 180 degrees to 360 degrees to compensate if you didn't want to open up the lens aperture instead. But most film cameras can't open larger than 180 degrees, a few to 200 degrees. A film camera needs the closed shutter time to advance the film to the next frame in the gate.

Now it's easier to lose exposure by closing down the shutter angle, so in theory, you could shoot at 24 fps with a 90 degree shutter angle and then do a speed ramp to 48 fps and open up the shutter to 180 degrees to compensate, thus maintaining the same exposure through the ramp.

And in digital camera, you do have the ability to use longer exposure times than the equivalent 180 degree shutter allows -- in most cameras, you can in fact turn off the shutter completely for an effective 360 degree shutter angle.

But since shutter speed affects motion blur, the exposure times that are longer than what a traditional film camera allows can create a look that is a bit smeary and video-ish, a non-standard look.

With most video cameras that do higher frame rates, you could go from 24P to 48P and keep the exposure time per frame to 1/48th of a second, which at 48P would mean no shutter at all, a 360 degree shutter. But the look would be a bit more smeary in motion than what people are used to. But if you have no other choice, it's an option.
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