Dr. Shutter angle

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#1 Mark McCann

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Posted 03 February 2011 - 05:38 PM

Hey all,

I've a question. Ive been hearing the term shutter angle thrown about and it confuses me. Why is it film cameras use shutter angle when alot of digital camera [cept the alexa if im not mistaken] use shutter speed? whats the main difference between the two? why does one use a shutter angle and the other simply lets you select the speed?

Thanks in advance for the help

Mark McCann
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Posted 03 February 2011 - 05:50 PM

Film cameras [generally] use a spinning mirror shutter with is generally 1/2 of a circle. If a full circle is 360 degrees, then 1/2 is 180 degrees, hence the use of angles. There is a formula, if you care to google (hint, try wikipedia) it, to convert shutter angle to shutter speed; but they can be used interchangeably. It's just semantics. For example a 180 degree shutter @ 24fps is 1/48th of a second shutter speed.
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#3 David Mullen ASC

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Posted 03 February 2011 - 05:55 PM

Hey all,

I've a question. Ive been hearing the term shutter angle thrown about and it confuses me. Why is it film cameras use shutter angle when alot of digital camera [cept the alexa if im not mistaken] use shutter speed? whats the main difference between the two? why does one use a shutter angle and the other simply lets you select the speed?

Thanks in advance for the help

Mark McCann

In a film camera, the shutter is a mechanical object, an intermittently spinning disk with a pie slice cut out of it to let light in. So part of the time, the shutter is open (the pie slice hole is over the film gate) and the rest of the time, the shutter is closed (the solid portion is covering the gate.)

The size of the opening is expressed in how many degrees it is out of 360 (a full circle) -- the most common is 180 degrees, a half-circle. This is referred to as the shutter ANGLE. Since with 180 degrees, the shutter is open 50% of the time, at 24 fps, the shutter SPEED would be 1/48th of a second.

Most digital cameras (except the D21) have an electronic shutter, not a mechanical one. But some allow shutter speed to be input as a shutter angle figure, for the film people who have trouble grasping the concept of shutter speed I guess. Actually, shutter angle does have one advantage, together with the frame rate, it affects the shutter speed. So if you change the frame rate and keep a 180 degree shutter angle setting, the camera will automatically calculate the change in shutter speed necessary.

Here is a good animation of a film shutter:
http://en.wikipedia....i/Shutter_angle
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#4 Mark McCann

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Posted 03 February 2011 - 07:03 PM

Hey, thanks to both of you, i think i understand now. I never knew the shutter was a spinning disk hence the confusion of where the shutter angle term came from.

This has been cleared right up for me.

Thanks
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#5 John Sprung

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Posted 03 February 2011 - 08:00 PM

If you shoot at different frame rates with the same shutter angle, you always get the same relative amount of motion blur. If you keep the same shutter speed, you'd get the same exposure, but differing motion blur. So, 45 degrees always has the same look, as does 180 or 360, etc. When you've learned the looks, you always know what you're going to get. It also protects you from asking for more time than you have, since the limit is 360.

-- J.S.
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#6 Dom Jaeger

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Posted 03 February 2011 - 10:02 PM

On professional film cameras the shutter is also a mirror, set at an angle so that during the portion of its rotation when it covers the aperture it reflects the image into the viewfinder. In this way the operator can see exactly what the lens is seeing even as the camera is running. A very clever idea that was invented by Arri way back in 1937, still used in the very latest motion picture cameras.

To avoid a very flickery and dim viewfinder image, and also because the camera needs to mechanically advance the film to the next frame during the period when the shutter covers the aperture, most film cameras are limited to a maximum 180 degree shutter angle.

The Arri D21 is the only digital camera that utilises a mechanical shutter/mirror, which allows it to have an optical viewfinder (generally preferred by operators), and a slightly more 'film-like' look, especially for fast motion capture, compared to other digital cameras that use a rolling shutter.
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#7 Chris Millar

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Posted 03 February 2011 - 10:47 PM

On professional film cameras the shutter is also a mirror, set at an angle so that during the portion of its rotation when it covers the aperture it reflects the image into the viewfinder. In this way the operator can see exactly what the lens is seeing even as the camera is running.

And interestingly you see exactly what doesn't go to film - thats always been a curiosity for me... About the only time it affects your ability to shoot is perhaps flashes/strobes - pretty much if you see it, you didn't capture all of it on film...
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Posted 04 February 2011 - 01:03 AM

You know, maybe it was because I was very tired on this last short I did, but Chris, i tell you I had this very zen/freaky moment when I realized I'm not the first person to see the film we're shooting when I'm oping a camera, and I have to wait just like everyone else. Damn
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#9 John Sprung

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Posted 04 February 2011 - 01:32 AM

The Arri D21 is the only digital camera that utilises a mechanical shutter/mirror,

IIRC, the Viper had a mechanical shutter, but not a mirror shutter viewfinder. Dalsa had an optical finder, too, IIRC.

-- J.S.
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