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Richard El Asmar

Shutter speed and shutter angle

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The rotating metal disc that is the shutter controls exposure time by the size of the pie slice cut out of the circle, typically a half open circle (180 degrees) in combination with the frame rate which controls the total time a piece of film sits in the gate. So at 24 fps, you have a new frame every 1/24th of a second, which is exposed for half that time by the 180 degree opening (aka shutter ANGLE) in the shutter disc, so the shutter TIME is 1/48th of a second per frame.

 

It works the same in both 16mm and 35mm cameras though not all use mirrors on the shutter to send light to the viewfinder while the shutter is covering the gate. There are other types of shutters which may affect whether you can get 180 degrees.

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Thanks I will try to be more precise: if we speed up the shutter, what effect do we get? And if we change the shutter angle, what effect do we get?

 

Are they the same?

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As David said, the shutter angle defines the shutter speed for a given framing rate. Reducing one reduces the other. The effect is to increase the appearance of flicker on moving objects because they are less blurred.

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Thanks I will try to be more precise: if we speed up the shutter, what effect do we get? And if we change the shutter angle, what effect do we get?

 

Are they the same?

 

I'll try my hand to fill in the conceptual gap here :)

 

They are mechanically (and mathematically) related via frame rate.

 

Increasing frame rate makes the shutter speed take less time i.e. 'faster'

Decreasing the shutter angle makes the shutter speed faster

 

You could (if you had a shutter that allowed for it) actually increase frame rate and increase shutter angle such that the shutter speed were the same before and after the changes.

 

It is best understood when you look at an actual shutter and see how it behaves as it rotates over time - you have tried wikipedia huh?

 

http://en.wikipedia.org/wiki/Rotary_disc_shutter

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You change the exposure/shutter TIME per frame by adjusting the shutter ANGLE or the frame rate, or both together. Standard shutter angle is 180 degrees.

 

Smaller angles like 90 degrees will reduce the exposure time, reducing the amount of motion blur per frame -- which at speeds like 24 fps, which is barely fast enough to reproduce motion smoothly, will make the motion look choppier, more strobing, because a certain amount of blur is needed to compensate for the relatively low sampling rate of 24X, to smooth the perception of motion as the frames are flashed in front of the viewer.

 

As a frame of reference, some of the strobing, staccato motion in the action scenes in "Saving Private Ryan" and "Gladiator" were achieved with a 45 degree shutter angle, which reduced the shutter time per frame from 1/48th of a second at 24 fps to 1/192nd of a second, a light loss of two f-stops of exposure.

 

Because they use electronic shutters, digital cameras can employ longer shutter times than what a 180 degree mechanical shutter allows (well, a Panaflex film camera can go to 200 degrees but that's barely an increase in exposure time over 180 degrees) - and longer exposure times mean more motion blur per frame. So instead of motion looking strobing and staccato as with a short shutter angle, with a digital camera set to an effective 270 degrees or 360 degrees (i.e. shutter is open all the time), you have the effect of more smeary motion.

 

It's not a question of choosing between changing the shutter time or the shutter angle in a film camera -- the only way you can change the shutter time is by either changing the shutter angle, the frame rate or both. There is no separate time adjustment setting on a film camera, and if there were, it would only be another way of telling you that you were actually adjusting the shutter angle, frame rate, or both. Just as with a digital camera that allows you to set either shutter time or shutter angle -- in this case, it's the opposite, if the digital camera has no mechanical spinning shutter in front of the sensor, then shutter angle settings are just a way of actually changing the electronic shutter time.

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Shorter shutter speeds give the effect of sharper images with less motion blur and also with slightly longer dwell times between exposed frames, creating a more staccato effect on moving objects. It's more obvious that the moving pictures you are seeing are actually a series of still images flashing by in rapid sequence.

 

Conversely, longer shutter speeds give the effect of more motion blur and a more continuous representation of subject movement over time. You feel more of the legato effect of time's passage. Since 1/48 second is the nominal shutter speed at 24fps, slower or faster speeds create a look that is increasingly impressionistic at the extremes.

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I think most of the confusion comes from the specific terminology and also general use of english.

 

As David has pointed out 'shutter speed' actually refers to a time period, so 'faster' which means a higher number in the reciprocal actually results in a shorter, i.e. smaller time period.

 

Now when I say 'smaller' do I mean it in an absolute sense or a relative one ?? :wacko: :rolleyes: ;)

 

I understand it all makes sense to people who have figured it out, but try if you can to decouple your understanding and see it afresh - to newcomers it's a bit confusing. I still think looking at an actual shutter and reverse engineering the explanations and terminology from that is the best method to truly understand it.

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Shorter shutter speeds give the effect of sharper images with less motion blur and also with slightly longer dwell times between exposed frames, creating a more staccato effect on moving objects. It's more obvious that the moving pictures you are seeing are actually a series of still images flashing by in rapid sequence.

Conversely, longer shutter speeds give the effect of more motion blur and a more continuous representation of subject movement over time. You feel more of the legato effect of time's passage. Since 1/48 second is the nominal shutter speed at 24fps, slower or faster speeds create a look that is increasingly impressionistic at the extremes.

I love the staccato and the legato comparision. Right on!

 

G

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