Shutter angles...

[Jonathan Spear]

Hey,

 

What are the effects of using various shutter angles?

 

(Forgive the ignorance)

 

 

 

Thanks,

 

Jon

[David Mullen ASC]

Shutter angle combined with frame rate determine shutter speed (exposure time) which determines the amount of blur per frame for movement.

 

24 fps is not a particularly high frame rate, barely enough to create the sense of continuous motion, and motion blur helps smooth out our perception of the separate frames -- with no motion blur, such as with traditional stop-motion animation, motion looks jerky, strobey, too crisp. With too much motion blur, it looks smeary, although at 24 fps, you can't really get much longer exposure times than the standard 180 degree shutter angle -- a Panaflex allows 200 degrees, but that's hardly a difference.

 

With small shutter angles, like 90 degrees or 45 degrees, you get shorter and shorter expsure times, making motion crispy, jerkier, more strobey.

 

The standard shutter angle is 180 degrees, with means it's closed 50% of the time (360 being a full-circle so 180 degrees being a half-circle spinning in front of the gate.) So at 24 fps, the exposure time is 1/48th of a second per frame. Close it down to 90 degrees -- by half again -- and it becomes 1/96th of a second. Close it down to 45 degrees and it becomes 1/192nd. Less exposure time, less blur per frame.

[Laurent Andrieux]

We're talking about film cameras, of course.

 

The shutter keeps the light from coming in the camera during the period of time when the film is pulled down for the next view.

 

The cycle is devided in two : half of the period the image is exposed, half of it the shutter is closed and the image pulled down.

 

If you have a cycle of 24 frams per second, the exposing time is then of 1/48s and the shuttered time also.

 

Then the shutter angle is of 180°

 

When one talks about shutter angle, one actually talks of the "exposing" angle.

 

For instance a shutter set at an angle of 172°8 will make a 1/50s of exposure time instead of 1/48s, 144° will make 1/60s...

 

in general, the exposing time is A/CX360 where A is the exposing angle of the shutter and C the frame rate (24, 25...fps)

 

setting it at 90° will expose at 1/96s at 24 fps for instance, that allows you to have a better resolution for shooting moving objects and will occure a loss of light equivalent to 1 f stop.

 

Hope it might help

 

Regards

[Guest fstop]

Ah, this old chestnut...

 

Methinks this belongs on the upcoming FAQ.

[Guest Frank Gossimier]

So David...can you have the shutter straight up and down? Would that be 90 degrees?

 

I've only ever shot 180.

 

I know some people use a different shutter angle for blue screen work, but I've always used 180 for blue screen and the compositing guys have never had a problem.

 

Frank

[David Mullen ASC]

If a 180 degree shutter is a half-circle, then a 90 degree shutter is a circle with one-quarter pie slice cut out of it, spinning in front of the gate.

 

I guess some compositors don't like dealing with the edge transparency that results as an object moves & blurs across the bluescreen but I don't think a sharper trailing edge from a closed-down shutter, and the attendant jerkiness to the motion, is necessarily a good idea.

[John Sprung]

Some digital cameras can give you an effective shutter angle upwards of 350 degrees. A frame transfer CCD has two complete frames worth of image area, one is the light sensitive taking area, the other is masked with metal, and is just used for storage and readout. The image can be shifted very rapidly from the taking area to the masked area, fast enough that you don't even need a mechanical shutter to protect it during the transfer.

 

Film cameras are limited by the mechanics of pulldown. Typically they spend nearly half the cycle time pulling down, so 180 - 200 degrees is the max for such movements. Before there was video tape, TV was recorded on film using a "hot kinescope" camera that used a special movement to get a shutter angle of 288 degrees.

 

Motion blur is essential to the illusion of motion. Without enough of it, you see a very rapid slide show of very similar slides instead of a movie.

 

 

 

-- J.S.

[Guest Frank Gossimier]

Ok David now I think I am confused?

 

So does the shutter angle actually change inside the camera then?

 

For instance if I take the lens off my camera right now I see the shutter laying back on an angle toward the gate. Like this in side view "/"

 

Can that angle be changed to look more like this (side view) "I". Straight up and down?

 

Or when you say you're changing the shutter angle do you mean changing the mirror/disc so that the opening is different? Like 1/4 of the circle cut out instead of half?

 

In that case by changing the shutter angle you mean taking out the rotating disc cut in half and replacing it with a disc with a 1/4 cut out of it.

 

Frank

[Guest Sean McVeigh]

Ok David now I think I am confused?

 

So does the shutter angle actually change inside the camera then?

 

For instance if I take the lens off my camera right now I see the shutter laying back on an angle toward the gate.  Like this in side view "/"

 

Can that angle be changed to look more like this (side view) "I".  Straight up and down?

 

Or when you say you're changing the shutter angle do you mean changing the mirror/disc so that the opening is different? Like 1/4 of the circle cut out instead of half?

 

In that case by changing the shutter angle you mean taking out the rotating disc cut in half and replacing it with a disc with a 1/4 cut out of it.

 

Frank

<{POST_SNAPBACK}>

 

It's like pac-man opening his mouth wider. The angle on the disc increases or decreases. Adjusting the mounting angle of the shutter would do nothing but muck up your viewfinder and fog your film :)

[Greg Gross]

A change in camera speed causes a change in shutter speed. Picture a ro-

tating disk that has a 180 degree opening or cut out. As the disk rotates it

covers the aperture,at the same time the film is advancing to position at the

gate. As the disk rotates more,the opening(cut out) lets the frame be exposed.

Then the cut out is covered again while the pull down claw is in process of pos-

itioning next frame for exposure. The shutter is always rotating so that half of

the time the film is exposed,the other half it is covered. If the camera is runn-

ing at 24fps actual time of exposure for each frame is 1/48 of a sec.. Now lets

slow camera down to 12fps(slow down movement to 1/2) exposure increases

to 1/24 sec. for each frame. If we speed up the movement to twice the normal

at 48fps,the exposure time of each frame is reduced to 1/96 of a sec.. If you

understand this process of exposure, you can adjust the fstop for the change

in exposure when shooting slow or fast motion. A change in the movement of

film speed can be useful shooting in low light. May not be practical though with

moving cars,people in background. If you reduce speed to 12fps you double

the exposure for each frame. If you vary the angle of the cut out you can regu-

late exposure. You need variable shutter. A 90 degree cut out transmits 1/2 as

much light as a 180 degree cut out. Stroboscopic effects can be created by vary-

ing the shutter. A narrow shutter can cause skipping, where say the railings of

a fence appear to be vibrating. Greg Gross

[David Mullen ASC]

The shutter is a rotating mirrored disk that can be closed down from a half circle to smaller openings like a quarter or an eighth of the circle -- assuming the camera even has a variable shutter. Some cameras don't.

 

There is no up or down -- it spins. If it's a variable shutter, you can see the degrees marked inside the shutter as you close it down.

[Guest Frank Gossimier]

Ok all clear now. You can tell how often I shoot with any thing other than 180.

 

Frank

[Mike Donis]

A film teacher of mine told me that the majority of film cameras have the 172.8 degree shutter, with 1/50th a second, not the 180 degree as I said (he seems to like making students feel they know less than him). But from what's being said here it seems that 180 is in fact more common.

 

I know it's not really important, but I was just wondering...

[David Mullen ASC]

Maybe in 50 hz countries, a 172.8 shutter is more common, but here in the U.S. it's a 180 degree shutter (although technically 144 degrees is optimal for 60 hz lighting.)

[Greg Gross]

ARRIFLEX 235, shutter is manually adjusted to:

 

45,60,75,90,105,120,135,144,150,172.8,180

 

ARRIFLEX 435 XTREME:

 

11.2 to 180

 

Just some examples from ARRI News.

 

 

Greg Gross

[Mike Donis]

Thanks!

 

I had always read that 180 was the standard here - film tech teachers.... :P