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Projection math


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#1 David Regan

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Posted 05 March 2008 - 07:54 PM

Just a point of curiosity I had, sort of off topic, and wondered if anyone could clear up for me. I understand in the camera movemnent, with a say, 180 degree shutter, the film is exposed for 1/48th of a second. How does this correlate to film projection, Is it essentially a camera with a light behind it? In other words does each frame get projected for 1/48th of a second, then pulled down as the shutter blocks the light, or is each frame projected longer, with shorter intervals between frames?

Just curious, not very familier with standard film projection.

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#2 David Mullen ASC

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Posted 05 March 2008 - 08:20 PM

Most projectors have a twin-bladed shutter so that each frame gets flashed twice in the 1/24th time interval at 24 fps. I don't know if it's exactly half of the total time taken up with the shutter closed or open though -- i.e. if the shutter blade was only 90 degrees instead of 180 but moved twice in front of the gate before the pulldown by using a "butterfly" shape, or if it's a 180 degree shutter spinning twice as fast compared to the frame rate, etc.

Flashing each frame twice reduces visible flickering on the screen. Silent movie projectors had a triple-bladed shutter since the frame rate for projection was more like 16 fps to 20 fps.
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#3 John Sprung

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Posted 05 March 2008 - 08:52 PM

Just a point of curiosity I had, sort of off topic, and wondered if anyone could clear up for me. I understand in the camera movemnent, with a say, 180 degree shutter, the film is exposed for 1/48th of a second. How does this correlate to film projection, Is it essentially a camera with a light behind it? In other words does each frame get projected for 1/48th of a second, then pulled down as the shutter blocks the light, or is each frame projected longer, with shorter intervals between frames?

Just curious, not very familier with standard film projection.

Thanks

Professional projectors pull the film down using a Geneva intermittent mechanism driving a special light weight sprocket. (It needs to be light because of the inertia, starting and stopping 24 times a second.) The vast majority use a four position Geneva, so they spend 90 degrees pulling down, and 270 with the film stationary in the gate. The shutters are typically two bladed, though the AA-2 Norelcos use a single blade with a counterweight, rotating twice as fast. (That thing would saw your fingers off if they didn't have a safety interlock.) So, you get two 90 degree presentations of each frame and two 90 degree periods of total darkness every 1/24 of a second.

There are some three blade shutters, Century was pretty much the only one to make them in 35mm. If you can control ambient light, 48 Hz flicker fusion works fine. But if you have to fight ambient and run brighter, you need a higher flicker rate. That's tough, because you need to get more light thru less shutter opening. A big pulldown blade plus two smaller interrupter blades has been tried, but you can't cheat much or you lose the flicker fusion advantage. You could go to, say, a six point Geneva, but I'm not sure if anybody ever did. With a standard three blade, you have only 90 degrees of light (three flashes of 30 degrees) and 270 degrees of dark.

There was a room put together at a studio many years ago where they found that one projector had to pull a lot more current to get the same screen brightness. After some head scratching, they figured out that they had one two blade and one three blade machine. Just watching movies, you can't tell two from three blade projection if the machines are matched for light level.

I'm not sure how much three blade was used in the silent days. I have a silent Simplex projector that has a two blade shutter. People just lived with the flicker back then, or the projectionist cranked faster. That would have reduced flicker, and got them more shows per night. When sound came in, it became necessary to standardize the projection rate. Western Electric did a survey of theaters, and found that the majority were running silent films at 24 fps. That's where the standard came from.



-- J.S.
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#4 David Regan

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Posted 05 March 2008 - 10:13 PM

Professional projectors pull the film down using a Geneva intermittent mechanism driving a special light weight sprocket. (It needs to be light because of the inertia, starting and stopping 24 times a second.) The vast majority use a four position Geneva, so they spend 90 degrees pulling down, and 270 with the film stationary in the gate. The shutters are typically two bladed, though the AA-2 Norelcos use a single blade with a counterweight, rotating twice as fast. (That thing would saw your fingers off if they didn't have a safety interlock.) So, you get two 90 degree presentations of each frame and two 90 degree periods of total darkness every 1/24 of a second.

-- J.S.


Ok so each frame is projected twice essentially? Basically if I understand, each 1/24th of a second is divided into 4ths, frameA/black/frameA/black and then frameB/black/frameB/black? Is that a correct understanding?

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#5 Dominic Case

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Posted 07 March 2008 - 01:59 AM

You've got it.

The butterfly blade spins, so in each 1/24th of a second you get 4 periods of 1/96th sec, going light, dark, light, dark. During the second of those dark bits, the film is pulled down to the next frame.

So as you say, it is f1/dark/f1/dark+pulldown/f2/dark/f2/dark+pulldown etc.

The visual system manages to live with 1/96th second dark periods, and merges the gap between each flash and the next (persistence of vision, though there is more to that than meets the eye ;) ). If it was just one flash per frame, with a 1/48th second dark period, we'd be very aware of flicker.

3D digital cinema presents another layer of complexity. The projector has to show alternate left eye and right eye images, and the glasses you wear black out the image intended for the other eye. So they generally operate at 144 flashes per second - or six per 1/24th second (a single camera frame). The projector shows alternate left and right images, but the left eye only sees the left image, so it's light/dark/dark/dark/light/dark/dark/dark/light/dark/dark/dark. Fortunately as there is no physical film pull-down, the dark intervals can be much shorter than the light ones (like a bigger shutter angle) - so even dark/dark/dark is short enough for persistence of vision to get over it.
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