Camera shutter: Degrees, Fractions and Mains Frequency

[alisterchapman]

So you have your nice camcorder, EX1, EX3, AF100, F3 or whatever and it has a function called the shutter. This function may have different ways of being set, fractions of seconds or angle. What is it that the shutter does and what’s the difference between angle and fractions. Also why is it important to know what the frequency of the local mains electricity?


180 degree film shutter
Lets start by looking at the difference between shutter speed expressed in fractions of a second and shutter angle. Shutter angle comes from film camera days when the film cameras shutter was a simple spinning disc with one half of the disc cut away to allow the light to pass from the lens to the film. The other half of the disc would rotate around, blanking off the film so it could be advanced to the next frame. If you consider that a full circle is 360 degrees, then half of a full circle is 180 degrees. So for each frame cycle with a 180 degree shutter, light is allowed to pass from the lens to the film for half of the frame rate (180 being half of 360).

Taking a frame rate of 25 frames per second, each frame lasts 1/25th of a second. Half of that is 1/50th, so with a 180 degree shutter the exposure at 25P is 1/50th of a second. There is no difference in the way the shutter works, it is just a different way of expressing the shutter timing.

If we take that and consider at a different angle, this time 90 degrees we can see that this is now one quarter of a full circle (90 is one quarter of 360 degrees). So at 25 frames per second the exposure is one quarter of 1/25th which is 1/100th and so on.

So why use angle instead of a fraction of a second? Well here’s the thing. If you set you shutter speed to 1/50th, then no matter what your frame rate the shutter speed will be 1/50th. The Sony EX and XDCAM cameras can shoot at various frame rates (as can many other cameras). It is traditional when shooting progressive, trying to create a filmic look to mimic the way a film camera behaves, so for this you would use a shutter that is open for half of the frame rate, ie. 180 degrees. When you set the shutter speed using an angle, when you change the frame rate the shutter speed will also change. Set to 180 degrees it will always be half of the frame rate. So going from 25P to 30P will change the shutter speed from 1/50th to 1/60th. Which neatly brings me on to the next bit….

Why it’s important to know the local mains frequency.

We normally take it for-granted in our home countries, shooting at our home frame rates that the pictures will be OK. But if you travel to a country where the mains frequency no longer matches the cameras base frequency then you may experience problems with flickering or strobing pictures when shooting under artificial lighting. Sometimes you will see light and dark bands slowly rolling up and down the picture. This happens because if you take your camera, lets say set to PAL (50i/25P) to the USA, the US mains frequency of 60hz will drift in and out of sync with the camera from one frame to the next. As many artificial lights brighten and dim in sync with the mains electricity you can appreciate that for one frame the lights may have one brightness and then the next frame the brightness may be different. You will possibly experience problems when the mains frequency cannot be evenly divided by the cameras shutter speed. For example shooting 30P will give problems when the mains is 50Hz as 30 will not divide evenly into 50.

So how do you counter this? Well you need to change your shutter speed to an even fraction or even multiplier of the mains frequency. So shooting 30P in a 50hz country you can use: 1/50th, 1/100th, 1/200th etc (mains frequency, frequency multiplied by 2, multiplied by 3 etc). Note that when shooting 60i you can’t normally have a 1/50th shutter so your limited to 1/100th or higher. When shooting 25P or (50i) in a 60Hz country you should use 1/60th, 1/120th, 1/240th etc. For 24P (23.98) you will often have to use the shutter when using consumer or industrial lighting using the same shutter speeds as give above, dependant on the local mains frequency.