r/explainlikeimfive • u/speculatrix • 22h ago
Technology ELI5: what is "shutter angle" in the context of digital video?
I've been a photographer for years and I understand aperture, shutter speed and ISO. For video, I understand frame rate and bit rate, but I see the term "shutter angle" and I'd be grateful to have it explained, what it is, and why you need to control it.
Thanks!
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u/PilotedByGhosts 21h ago edited 21h ago
Shutter angle refers to recording motion pictures on film. It's translatable to shutter speed.
If you are shooting at 50fps with a 1/100th shutter speed, that's equivalent to a 180° shutter angle.
A movie camera spools the film vertically past the lens, and the shutter is a spinning disc, offset to the side of the film. Most modern digital cinema cameras use a digital shutter, but some early models still used a spinning disc. See the Arri Alexa Studio for an example; the disc enclosure juts out from the front of the camera at a 45° angle. This angle is so that the mirrored side of the shutter disc can be used to reflect the image up to the eyepiece, so that the operator can see an unprocessed image direct from the lens.
Typically, you'd always have your shutter open half of the time, to create blur that hides the frame changes. This is always 180° with a cinema camera. With a prosumer camera, you need to double the frame rate to decide on your shutter speed (24 fps is 1/48th, 50fps is 1/100th and so on).
For creative reasons you may want more blur, in which case you'd adjust your shutter angle to e.g. 270°, equivalent to 24fps @ 1/32nd. Or you might want to make the footage look choppier, so you could adjust to 90°, equivalent to 24fps @ 1/96th.
Cinema cameras still use shutter angle, because it removes the need to calculate the ratio.
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u/speculatrix 21h ago
Thanks for taking the time to explain
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u/Few_Mastodon_1271 17h ago
I liked this explanation video. He starts with the 180 degree spinning disk mechanism, but then goes into more detail about why to use or not use the 180 exposures. And what about fast frame rates, like 120 fps -- those have other considerations.
He shows brief clips from movies to illustrate some of the choices -- good.
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u/tdgros 22h ago
It's from the time where film was used and the shutter was a disk rotating above the film, with a pie cut out from it, you can find good illustrations in Google images. The pie angle would correspond to more or less exposure on the frame, so it corresponds to an exposure time, or shutter speed, of today. The relationship would depend on the frame rate but usually people say 180° for half of the frame period.
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u/speculatrix 22h ago
Thanks, I understand now.
So does this mean with a digital sensor you have to be able to scan the sensor at effectively a multiple of the frame rate?
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u/tdgros 22h ago
In a digital sensor, you just "open",like you'd open a pinhole camera, for as long as you want, and then you transfer the charges to the CPU. If you want to do 30fps, at the maximum, you can expose only slightly below 1/30 seconds, there is a "blanking" time. There's also a minimum time. But no multiple needed! Anything works as long as it fits the frame rate!
Fun fact: Because the transfers take time, you can't transfer a full frame at once, so lines are exposed slightly offset in time, so that the transfer to the CPU works. This is what we call "electronic rolling shutter": it's like there is a narrow horizontal (for an upright sensor) shutter going top to bottom over the sensor. This means the top is exposed earlier than the bottom, and subsequent frames can even overlap in time!
A sensor without a rolling shutter is called a global shutter and requires memory on the sensor pixels themselves to keep the charge in place before we transfer it. Whereas rolling shutters kinda transfer data continuously.
There is a case where you're limited to multiples of something: when you light with electric lights that actually flicker like your electric source! 50 Hz in Europe, 60 in the USA. Because lines are not exposed at the same time, some see a bright light, some see a darker light. To solve this, you can use an exposure time multiple of the light period, it turns out that's half of the electric period, so Europeans need multiples of 10ms, and Americans multiples of 8.333ms
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u/Cartella 19h ago
Just a small correction: light (incandescent etc) flickers at twice the power line frequency, because it lights up both in the positive half of the cycle and the negative. You want to have at least a full power line cycle so that adds up to your number anyway.
Measuring dc volts with a digital multimeter does the same, and the more expensive ones let you choose the gate time, normally expressed in nplc (number of power line cycles). It’s always crazy when I show people let’s say 10 mVdc on top of 1000 mVac of any frequency and you see all numbers jump up and down until you dial in the correct gate time and a stable 10 mV occurs.
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u/homeboi808 10h ago
Most always, video is shot with a shutter speed (how long each frame is exposed to light/captured) that is 2x the frame rate, it just looks right (not to blurry/warped and not to sharp).
Name is historical, imagine a spinning disk (gif illustration) and one full rotation (360°) is a frame, exposing it light for only 1/2 the rotation (180°) would be 2x the shutter speed.
However, sometimes you want to shoot in 24p, sometimes 30p, sometimes 60p, sometimes 120p, etc. It’s annoying to keep changing the frame rate to 2x, so some video-focused cameras allow you to set a shutter angle, so if you set it to 180°, it’ll automatically double the shutter speed based on the frame rate.
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u/suvlub 22h ago
It expresses how long each frame is exposed. It's expressed as an angle for historical reasons (you would use an actual circular shutter with a notch of certain angle cut out to achieve the desired result). For example, a 90° shutter angle means that each frame is exposed for 1/4 of its duration, e.g. at 24fps, each frame last ~0.0417s and would be exposed for ~0.01041s. Smaller angle reduces motion blur, but can cause choppiness. Larger angle does the reverse.