Movie Camera Shutter?

[Michel Hardy-Vallée]

Ok, here's a question for the movie buffs: What is the speed of a movie camera shutter?

Let's consider that one is shooting Tri-X in sunlight on a beautiful Paris day (to make a Nouvelle Vague movie). Provided that a 35mm movie camera shoots at 24fps, does that mean that the shutter speed for each frame is 1/24th of a second? Even at f/22 that would be overexposure, and each frame would be blurred!

So does that mean that a camera's shutter would flick at, say 1/100th of a second, 24 times per second? But then why does my light meter has shutter speed of 1/8, 1/16, 1/32, 1/64 for cinema? Is it the case that motion blur, while visible on a static frame, disappears when such frames are projected?

Eeehh, I don't get it, and I'll never make it to movie school!

 

[wildbill]

The normal shutter "speed" is 1/48th. That also happens to be 180deg. shutter angle. Panavision cameras have butterfly shutter with a 200deg standard. This is variable on most modern cameras usually but not limited to 200deg down to 45deg. Just about all action movies you see will be filmed with some shots at 90 or 45deg to give a sharper image that doesn't blur together as much as the normal setting. Much of Saving Private Ryan was filmed with a 45deg shutter and they rubbed of the lens coatings to increase flare as well. You'll notice it quite a but when the dirt flies from explosions. This movie sparked an increase in variable shutter use. One cinematographer i've worked with used it on every music video and action sequence.
The arri 435 extreme can change the shutter during a shot and close or open the lens aperture to compensate.

 

[Dan Fromm]

For rotary shutters, shutter speed depends on the shutter angle and on frames/second. For reciprocating shutters, as used on, e.g., Beaulieus, it depends on slit width and frames/second.

Your light meter is calibrated for an 8mm cine camera with 180 degree shutter. "Normal" frame rate for 8/8 mm is 16 fps. For Super 8 its 18 fps.

Motion blur disappears because of persistence of vision.

You'll never make it to film school? Apply, you might find one that will have you.

 

[Michel Hardy-Vallée]

Thanks for the excellent answers guys, but I'm not familiar with the types of shutters on movie camera. How do you define a butterfly, a rotary shutter? What do you mean by "angle of the shutter" ? Know of any place on the web with a cute diagram?

I made to grad school, I was just joking about film school

 

[Dan Fromm]

A rotary shutter is a disc with a sector cut out. The sector's edges lie on radii of the disc. The shutter angle is the angle between the sector's edges. A butterfly shutter is effectively a disc with two sectors cut out; in this case, the sectors are opposite, i.e., the leading edge of one and the trailing edge of the other are on the same diameter of the disc.

With both types, the shutter's axis of rotation is the disc's center. The film is exposed while the open part of the disc sweeps over it. It is pulled down to position the next frame for exposure while the closed part of the disc sweeps over it.

 

[jimgalli]

Our cameras at work have adjustable shutter angles. 5, 10, 15, 20, etc. up to 120 degrees. The shutter is a pie shaped disc that is going around in a circle blocking light from the film. If I set it at 10 degrees, there's a 10 degree pie wedge in that circle that will let light in as it rotates past the film that's in the gate. 10 degrees out of 360 possible gives a shutter factor of 36. When the cameras are cranked up to full tilt they are capable of 360 frames per second. You multiply the shutter factor times FPS to get the effective shutter speed. In that case, 36 X 360 = 12960. My shutter speed would be 1/12960 sec. You need pretty speedy film and lenses at that point. A more common setting might be shutter angle 35. 360 / 35 = 10 which becomes my shutter factor. 10 X 360 = 1/3600th of a second. Very useable around f4 with 400 asa ektachrome and an EV of 15 or so. We use prime Pentax 6X7 90mm f2.8 lenses which are sharp as hell at f4 expecially since we're only getting half frame 35mm which is the sweetest spot in the center of the lens.

I should add that our cameras are "pin registered". That means that 360 times a SECOND new film is being pulled down into the gate where the exposure happens, and registration pins are going into the perfs to hold it still while the exposure happens. The mechanism that makes that happen is nothing short of awesome. It breaks my heart to think we're right on the threshold of digital cameras replacing these machines. 35mm film.

 

[Michel Hardy-Vallée]

Jim, that means that the shutter rotates once per frame? That way the time it exposes the film is in direct proportion to the ratio between the angle of the shutter and 360 degrees.

The cameras you describe are nothing short of awe-inspiring. If we compare to video, while still another analog process, the fact that it is entirely mechanical is nothing like the magnetic heads on a drum. Even electronics feels like cheating!

 

[jimgalli]

Jim, that means that the shutter rotates once per frame? That way the time it exposes the film is in direct proportion to the ratio between the angle of the shutter and 360 degrees.

The cameras you describe are nothing short of awe-inspiring. If we compare to video, while still another analog process, the fact that it is entirely mechanical is nothing like the magnetic heads on a drum. Even electronics feels like cheating!

They have a little vacuum pump that sucks the film flat against the platten for perfect flatness. Image quality approaches a Nikon. I have an 11X11 blow up from one of our square frame cameras just to show folks how good they are. Neat stuff. Your tax $$ at work. Unlike digital, a lot can go wrong and we can still get good images.

 

[nick mulder]

I have a Bolex RX4 16mm - Its shutter is a max 130 deg which gives it a shutter speed of ~ 1/66 at 24fps -

Most Bolex literature lists its shutter speed at 1/80 sec at 24 fps :confused:

I've finally found out why..

Its the 'reflex' beam splitter factor - a certain amount of light is sent to the viewfinder by a partially silvered prism and a shutter speed of 1/80 would give you the same amount of light as if it weren't there in the first place ...

I find that kind of logic fine for quick rules of thumb - but it doesn't help anyone learning the reality of the mechanism.