Technical question about the shutter

Hugo Miro · March 7, 2010

Hi people.

Under normal conditions we (in europe) record at 25 fps with the shutter at 1/50 (180º). If we change the shutter to -for example- 1/500 what we are doing is closing the shutter angle. But... What happens when we increase the shutter to 1/16? I ask this because the maximum aperture is 200º approximately... What does the video camera then? Is it Recording unless fps? If so... Is reproduction maintained at the same frame rate (if not all would go on fast motion)?

I hope I explained me well :huh:. Sorry for my poor english :unsure:

Thank you.

Mathew Rudenberg · March 7, 2010

Hi people.

Under normal conditions we (in europe) record at 25 fps with the shutter at 1/50 (180º). If we change the shutter to -for example- 1/500 what we are doing is closing the shutter angle. But... What happens when we increase the shutter to 1/16? I ask this because the maximum aperture is 200º approximately... What does the video camera then? Is it Recording unless fps? If so... Is reproduction maintained at the same frame rate (if not all would go on fast motion)?

I hope I explained me well :huh:. Sorry for my poor english :unsure:

Thank you.

The maximum your shutter can be open while maintaining your frame rate is 360 degrees. That is, 25 fps with a shutter angle of 1/25.

If you go below 1/25 at 25fps you are no longer recording 25 individual frames per second.

On some cameras you may reduce the fps and thus get fast motion, but on most common consumer cameras the camera will just duplicate frames to maintain 'normal' speed.

So, if your shutter is 1/16th of a second but your frame rate is 25fps, 16 individual frames are stretched over the 25 frames recorded. This is why when you do so the image will appear more stuttery or stepped.

David Mullen ASC · March 8, 2010

Yes, you'd be breaking the laws of space and time to have a per-frame exposure time that was longer than the time each frame was "in the gate" so to speak -- i.e. if a camera is taking 25 pictures per second, it can't expose each of those pictures for longer than 1/25th of a second. So cameras that allow even longer exposure times are employing certain tricks to do it, like reducing the frame rate and stretching it back to look normal.

John Sprung · March 8, 2010

Another trick that could be done is to record, say, 25 frames per second with a 360 degree shutter, and then combine consecutive frames in post. The first frame you see would be a combination of camera original frames 1 and 2. The next would be made from camera frames 2 and 3, and so forth. The motion would be at normal speed, but with four times the normal motion blur, 720 degrees instead of 180. Interesting for a stoned POV, where you see things clearly unless they move.

-- J.S.

Hugo Miro · March 9, 2010

Is in film around 200º the maximum aperture?

I am very grateful for the answers, thank you.

Tom Mitchell · March 9, 2010

Is in film around 200º the maximum aperture?

I am very grateful for the answers, thank you.

200º is reference to shutter angle, not aperture which is something entirely different.

The shutter is essentially a spinning disk of set to one side, where a slice has be cut out of it. That slice size can be changed, but the standard is 180º so half the disk is missing. The effect when you increase the angle, is that you let more light in but also increase the time the fame is being exposed increasing the motion blur that is incurred. and you get sharp crisp images in small shutter angles like 45º.

There is a section of the shutter that has a mirror on it, to bounce incoming light to the eye peace, so you are limited to the maximal shutter angle by how you fold up the sections of the shutter but also the size of the mirror. as far as I am aware this can vary from camera to camera.

when comparing shutter fractions with angle you have to remember a fraction is a measurement of time, and and angle is a fraction of fraction.

Lets take UK and US standards of 25FPS and 30FPS at take 180º as the shutter angle. 180 basically meaning 1/2.

@25FPS 180º=1/50th thats 1/25 divided by two. and 30FPS 180º=1/60th.

And as for aperture, that is the hole the light comes through at the front to the camera, that can increase or decrease. it is measured in F stops or in film T stops though the numbers are the same.

Increasing the aperture size will let more light in and decrease the depth of field (the Z distance that will be in focus) large apertures are represented with smaller T/F stops I.E T1.3 and small apertures (lke a pin hole camera) will have large numbers T16.

(T stops measure actual light entering the camera post loss from diffraction and diffusion of the glass elements in the lens. and F stop is the amount of light that should be going through the lens and doesn't account for any loss.) T stops are used in film so we can mach lenses of different sizes and manufacture and get the same exposure.)

There you go a bit of 101 film making for you hope that shed some 'light' on the situation

Hugo Miro · March 9, 2010

200º is reference to shutter angle, not aperture which is something entirely different.

The shutter is essentially a spinning disk of set to one side, where a slice has be cut out of it. That slice size can be changed, but the standard is 180º so half the disk is missing. The effect when you increase the angle, is that you let more light in but also increase the time the fame is being exposed increasing the motion blur that is incurred. and you get sharp crisp images in small shutter angles like 45º.

There is a section of the shutter that has a mirror on it, to bounce incoming light to the eye peace, so you are limited to the maximal shutter angle by how you fold up the sections of the shutter but also the size of the mirror. as far as I am aware this can vary from camera to camera.

when comparing shutter fractions with angle you have to remember a fraction is a measurement of time, and and angle is a fraction of fraction.

Lets take UK and US standards of 25FPS and 30FPS at take 180º as the shutter angle. 180 basically meaning 1/2.

@25FPS 180º=1/50th thats 1/25 divided by two. and 30FPS 180º=1/60th.

And as for aperture, that is the hole the light comes through at the front to the camera, that can increase or decrease. it is measured in F stops or in film T stops though the numbers are the same.

Increasing the aperture size will let more light in and decrease the depth of field (the Z distance that will be in focus) large apertures are represented with smaller T/F stops I.E T1.3 and small apertures (lke a pin hole camera) will have large numbers T16.

(T stops measure actual light entering the camera post loss from diffraction and diffusion of the glass elements in the lens. and F stop is the amount of light that should be going through the lens and doesn't account for any loss.) T stops are used in film so we can mach lenses of different sizes and manufacture and get the same exposure.)

There you go a bit of 101 film making for you hope that shed some 'light' on the situation

Sorry, I meant to say "angle" instead of "aperture", as in my first post. Lapsus linguae, but thank you.

Tom Mitchell · March 9, 2010

Sorry, I meant to say "angle" instead of "aperture", as in my first post. Lapsus linguae, but thank you.

I did think it was just a typo/miss wording, but i just went in to detail more so for outer people reading that might not know.

I also forgot to add the other limiting factor in shutter angle, is that you got to move the next frame in to the gate under cover of darkness, or you get unwanted vertical blur. So it also a physical issue how how quick you can do that and not break the film (even at higher frame rates).

How ever, at this point is where i start to know less about how this works. in high speed film cameras they have a system with no shutter but a spinning prism that matches the speed that the film is continuously moving through that camera. I think that gives you a 360 shutter or close to. I'm sure other peeps can expand on that.

John Sprung · March 9, 2010

Is in film around 200º the maximum aperture?

Most film cameras are under 200º maximum. Typical maxima are around 175 - 185. It depends on the pulldown mechanism. The Eyemo, for instance, is 165, and the early Arriflexes were 120. When they went from eccentric screw to cardioid cam, the Arri's were able to increase their shutter angle from 120 to 180.

One very special case is the old Acme kinescope camera, used for recording 24 fps film from 60 field NTSC TV, back before there was 2" quad video tape. They were designed for a 288 degree shutter, and had a 2:1 gear reduction between the vertical and horizontal cams, so the claws would pull down, go back up and down again without engaging the film, then back up for the next pulldown. That's how West coast network delay was done in the early 1950's.

-- J.S.