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Question on Shutter Angle


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#1 David Calson

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Posted 15 April 2008 - 04:18 PM

So I read in a book that if you go from 180 to 90, you need to open up 1 stop. From a very new filmmaker perspective, this doesn't sound right. If your shutter angle is smaller, shouldn't that mean more light is hitting the film?

Also, can you tell me about f/stop loss at pertains to shutter speed, is there a formula?
(ex. 1/50 -> 1/100= 1 stop loss?)
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#2 David Mullen ASC

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Posted 15 April 2008 - 04:45 PM

So I read in a book that if you go from 180 to 90, you need to open up 1 stop. From a very new filmmaker perspective, this doesn't sound right. If your shutter angle is smaller, shouldn't that mean more light is hitting the film?

Also, can you tell me about f/stop loss at pertains to shutter speed, is there a formula?
(ex. 1/50 -> 1/100= 1 stop loss?)


It's 90 degrees open, so it is 270 degrees closed from a 360 degree circle.

So a 180 degree opening is twice as large as a 90 degree opening.

Think of a 90 degree shutter as a pie slice that is 1/4 of the total circle whereas a 180 degree shutter is half the pie.
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#3 Andrew Koch

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Posted 15 April 2008 - 04:58 PM

So I read in a book that if you go from 180 to 90, you need to open up 1 stop. From a very new filmmaker perspective, this doesn't sound right. If your shutter angle is smaller, shouldn't that mean more light is hitting the film?


It is correct. You need to open up a stop going from 180 to 90 and open up another when going from 90 to 45 and so on. A smaller shutter angle means LESS light is hitting the film. The shutter angle is referring to the angle of the opening in the shutter, not the dark surface of the shutter itself. A smaller angle means a smaller opening which means that a 90 degree shutter will expose the film half as long.

The formula that you are looking for is

Exposure = (Frames Per Second x 360 degrees)/Shutter Angle

The standard example:

Exposure = (24fps x 360 degrees)/ 180 degrees
Exposure = 48 which means the film is exposed for 1/48 of a second, this is your exposure time.

With a ninety degree shutter the result is 96. So your exposure time would be 1/96 of a second. That's half the exposure time as you can see.

When you open up the aperture 1 stop, you are doubling the amount of light you let in. When you close down a stop, you are letting in half the amount of light.

So if your exposure time gets cut in half by shooting at 48fps or using a 90 degree shutter, then you compensate by opening up a stop. If your exposure time is doubled by shooting at 12fps or if you have a Genesis which can shoot with a 360 degree shutter, you would compensate by closing down a stop.

If you remember to think in terms of halving and doubling as it relates to stops, it will be easier to understand it. The same rule applies to film speed as well as footcandles. (Ex: 500T is 1 stop faster than 250T). (50 footcandles is one stop brighter than 25 footcandles)
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#4 David Calson

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Posted 15 April 2008 - 09:09 PM

I'm a little confused about the formula.

So, fps x 360/shutter angle = shutter speed. It's the '='s part that has me caught up.


Do you mean equals as in, if your frames are 'x' and your angle is 'y' then the shutter speed will on it's own be formed based on those two variables.

Or is the equation a suggestion of what you SHOULD adjust shutter speed so that it achieves proper exposure, if that makes any sense.

Like is there a shutter speed knob on a film camera that you can adjust, or do you change shutter speed soley by adjusting shutter angle and/or frame rate.


Thank you for clearing up what shutter angle is!

Edited by Blade Borge, 15 April 2008 - 09:14 PM.

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#5 Michael Nash

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Posted 15 April 2008 - 11:39 PM

I'm a little confused about the formula.


It's a straight math formula; "equals" means "equals." Shutterspeed is determined by both the frame rate AND the shutter angle.

http://en.wikipedia....i/Shutter_angle . The little animation should help illustrate it.

If you're confused by the arithmetic, just think about the geometry. The smaller the shutter angle, the shorter the duration that light can come through the opening during each frame. But the slower the frame rate, the longer that opening is held over the film.
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#6 Damien Bhatti

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Posted 16 April 2008 - 08:02 AM

It's a straight math formula; "equals" means "equals." Shutterspeed is determined by both the frame rate AND the shutter angle.

http://en.wikipedia....i/Shutter_angle . The little animation should help illustrate it.


I am just curious as to the way in which the shutter moves across to expose the film. I drew a little diagram here, say for a 180 degree shutter. Now when it is exposing the film its actually coming down diagonally to the frame - making an ever decreasing angle. Is this an important measurement for the shutter? Because when we talk of shutter angles we are referring to the proportion of the disc i thought?/ ie half a pie 1/4 of a pie etc...?

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#7 David Calson

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Posted 16 April 2008 - 01:04 PM

It's a straight math formula; "equals" means "equals." Shutterspeed is determined by both the frame rate AND the shutter angle.

http://en.wikipedia....i/Shutter_angle . The little animation should help illustrate it.

If you're confused by the arithmetic, just think about the geometry. The smaller the shutter angle, the shorter the duration that light can come through the opening during each frame. But the slower the frame rate, the longer that opening is held over the film.



I see, so to increase shutter speed, either increase frame rate, or shorten the shutter angle.

I guess one thing that threw me off was how digital cameras allow you to control shutter speed by itself.

Edited by Blade Borge, 16 April 2008 - 01:09 PM.

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#8 Michael Nash

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Posted 16 April 2008 - 02:50 PM

I guess one thing that threw me off was how digital cameras allow you to control shutter speed by itself.


In that case "shutterspeed" is just a term held over from mechanical/film photography. Digital cameras (both still and motion) electronically control the length of time that the sensor is "scanned" instead of using a physical shutter.
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#9 David Mullen ASC

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Posted 16 April 2008 - 05:49 PM

I am just curious as to the way in which the shutter moves across to expose the film. I drew a little diagram here, say for a 180 degree shutter. Now when it is exposing the film its actually coming down diagonally to the frame - making an ever decreasing angle. Is this an important measurement for the shutter? Because when we talk of shutter angles we are referring to the proportion of the disc i thought?/ ie half a pie 1/4 of a pie etc...?

Posted Image


You need to draw a bigger shutter circle or a smaller rectangular film gate -- imagine the center of the circle, the pivot point being outside the film frame on one side so that a half circle completely covers the frame.
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#10 Michael Nash

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Posted 16 April 2008 - 06:03 PM

From the Wikipedia article I linked:
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#11 Damien Bhatti

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Posted 18 April 2008 - 12:08 PM

Maybe I am barking up the wrong tree here, but what I am curious is to - the actual diagonal movement illustrated in that little animation there. it cuts down across the frame and then the claw drags the film along as the shutter hides the film.

but why isnt that movement of the shutter registered on the film?/ is going to fast - maybe its a real simple photographical question but i am curious.

in my stupid logic - the way the mirror shutter moves would leave a little trace ?/ but it does not, a miracle - maybe i am not thinking about the situation "correctly," thanks.
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#12 David Mullen ASC

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Posted 18 April 2008 - 01:19 PM

The film is not pulled-down/advanced until the shutter has completely closed over the gate -- otherwise, if it did start to advance while the shutter was still closing, you'd get the mistimed shutter effect used in "Saving Private Ryan" where bright highlights are streaking up vertically.
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#13 Michael Nash

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Posted 18 April 2008 - 05:43 PM

but why isnt that movement of the shutter registered on the film?/ is going to fast - maybe its a real simple photographical question but i am curious.


In the strictest sense it does leave a trace, but it's exposed for a much shorter duration than the time the shutter is completely open. The sum total of exposure to light tends to obscure the motion blur artifacts. Plus, the shutter is changing angle as it wipes the frame, making an irregular artifact.

Some of these new CMOS cameras do see an artifact from an "effective" moving shutter that goes vertically down the frame. Motion that goes horizontally gets exposed at the top of the frame first and the bottom of the frame last, resulting in a diagonal displacement of motion blur.

http://dvxuser.com/jason/CMOS-CCD/
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#14 Damien Bhatti

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Posted 21 April 2008 - 01:07 PM

Plus, the shutter is changing angle as it wipes the frame, making an irregular artifact.

http://dvxuser.com/jason/CMOS-CCD/


Is this "changing angle," as it sweeps across the frame, the important angle when we discuss shutter angles?/ or is it the shape of the 180 degree mirror?

- where the mirror is placed in the camera effects this diminishing angle and wouldnt that negate whether the shutter is 90 or 180 degrees? (If the mirror is rotating right near the gate the angle will be more exaggerated?)

I am just trying to work out in my minds eye the relationship between these two measurements....if you understand how i phrased it all//

Thanks :)
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#15 Michael Nash

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Posted 21 April 2008 - 03:38 PM

Is this "changing angle," as it sweeps across the frame, the important angle when we discuss shutter angles?/ or is it the shape of the 180 degree mirror?


The "changing angle" has nothing to do with it. For all practical purposes just forget about the shutter wiping the frame; with a film camera/rotating shutter the effect is negligible and it doesn't affect exposure.

All that matters for exposure is the amount of time the shutter is opened and exposing the film to light. The angle of the shutter is just a measure of the size of the opening, which determines the amount of time the film is exposed, at a given frame rate. It really doesn't matter what direction the shutter moves.
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#16 Damien Bhatti

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Posted 21 April 2008 - 04:02 PM

The "changing angle" has nothing to do with it. For all practical purposes just forget about the shutter wiping the frame; with a film camera/rotating shutter the effect is negligible and it doesn't affect exposure.

All that matters for exposure is the amount of time the shutter is opened and exposing the film to light. The angle of the shutter is just a measure of the size of the opening, which determines the amount of time the film is exposed, at a given frame rate. It really doesn't matter what direction the shutter moves.


query = cleared up, thanks

db
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