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#1 Craig Agee

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Posted 28 February 2006 - 01:30 AM

alright...i must warn i am not a DP,i am a still photographer who is curious and interested in film.this might sound like an ignorant question but i am asking in the most humble way heh...hope this isnt lengthy.i know about exposure,i know very well how still camera's operate.in still photography the combination of aperture/shutter speed determines exposure obviously and i assume the same with cinema...of course aperture determines DOF...when shooting of course i will usually set to aperture priority and whatever shutter needed (unless a specific effect is desired) set it on a tripod...exposure could be anywhere from 5 seconds to 1/500th....here is my question...can someone explain to me how shutter speeds work in cinema camera's?...i assume shooting a poorly lit interior you wouldnt do long exposures because there is a frame rate...at 24fps what is the slowest shutter speed possible?Would appriciate anyone who took the time to explain this to me.thank you
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#2 David Mullen ASC

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Posted 28 February 2006 - 01:43 AM

The shutter speed is determined by two things: the speed of the camera (frame rate) and the size of the shutter ANGLE. You see, the shutter in a film camera is a spinning disk with a pie slice hole cut out that allows light to pass through and expose the film. For most cameras, that shutter is a half-circle, i.e. a 180 degree shutter angle. So as the shutter spins, half the time it's open and exposing the film, and the other half of the time it is closed over the film, allowing time for the movement to advance the film to the next frame for exposure.

So if the camera is running at 24 fps and the shutter angle is 180 degrees, then the exposure time is half of 1/24th of a second: 1/48th of a second.

Unfortunately you can't really increase the exposure time for low-light without slowing the frame rate down, because most film cameras don't open the angle more than 180 degrees (Panaflexes can open to 200 degrees but that's hardly an increase in exposure).

However, you can close down the shutter angle to shorten the exposure times. Closing from 180 to 90 degrees will cut the exposure by half again, from 1/48th to 1/96th of a second per frame at 24 fps. But then you start to get rather choppy-looking motion. So playing with the shutter angle to control exposure time is not generally done; usually you only mess with the shutter angle to create some odd motion effects or to sync with a cycling light source of a certain frequency.
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#3 Craig Agee

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Posted 28 February 2006 - 01:51 AM

that is very interesting,obviously ive never touched a cinema camera hehe...my brain has to see something to understand something usually,although that was explained well and i think i understand a bit better.so exposure is most easily controlled with iris/aperture right?and i assume film speed plays a major roll.thank you for responding to my questions.
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#4 David Mullen ASC

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Posted 28 February 2006 - 02:17 AM

that is very interesting,obviously ive never touched a cinema camera hehe...my brain has to see something to understand something usually,although that was explained well and i think i understand a bit better.so exposure is most easily controlled with iris/aperture right?and i assume film speed plays a major roll.thank you for responding to my questions.


Yes, exposure is controlled more often by iris (f-stop), film stock speed, filters, and light levels created rather than manipulating shutter angle.
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