What shutter speed to use if shooting 50p or 60p for 50i/60i conversion.

An interesting question got raised on Facebook today.

What shutter speed should I use if I am shooting at 50p so that my client can later convert the 50p to 50i? Of course this would also apply to shooting at 60p for 60i conversion.

Lets first of all make sure that we all understand that what’s being asked for here is to shoot at 50(60) progressive frames per second so that the footage can later be converted to 25(30) frames per second interlace – which has 50(60) fields.

If we just consider normal 50p or 60p shooing the the shutter speed that you would chooses on many factors including what you are shooting and how much light you have and personal preference.

1/48 or 1/50th of a second is normally considered the slowest shutter speed at which motion blur in a typical frame no longer significantly softens the image. This is why old point and shoot film cameras almost always had a 1/50th shutter, it was the slowest you could get away with.

Shooting with a shutter speed that is half the duration of the cameras frame rate is also know as using a 180 degree shutter, a very necessary practice with a film movie camera due to the way the mechanical shutter must be closed while the film is physically advanced to the next frame. But it isn’t essential that you have the closed shutter period with an electronic camera as there is no film to move, so you don’t have to use a 180 degree shutter if you don’t want to.

There is no reason why you can’t use a 1/50th or 1/60th shutter when shooting at 50fps or 60fps, especially if you don’t have a lot of light to work with. 1/50(1/60) at 50fps(60fps) will give you the smoothest motion as there are no breaks in the motion between each frame. But many people like to sharpen up the image still further by using 1/100th(1/120th) to reduce motion blur.  Or they prefer the slightly steppy cadence this brings as it introduces a small jump in motion between each frame. Of course 1/100th needs twice as much light. So there is no hard and fast rule and some shots will work better at 1/50th while others may work better at 1/100th.

However if you are shooting at 50fps or 60fps so that it can be converted to 50i or 60i, with each frame becoming a field, then the “normal” shutter speed you should use will be 1/50th or 1/60th to mimic a 25fps-50i camera or 30fps-60i camera which would typically have it’s shutter running at 1/50 or 1/60th. 1/100th(120th) at 50i(60i) can look a little over sharp due to an increase in aliasing due to the way a interlace video field only has half the resolution of the full frame. Particularly with 50p converted to 50i as there is no in-camera anti-aliasing and each frame will simply have it’s resolution divided by 2 to produce the equivalent of a single field. When you shoot with a “real” 50i camera line pairs on the sensor are combined and read out together as a  single field line and this slightly softens and anti-aliases each of fields. 50i has lower vertical resolution than 25p. But with simple software conversions from 50p to 50i this anti-aliasing does not occur. If you combine that with a faster than typical shutter speed the interlaced image can start to look over sharp and may have jaggies or color moire not present in the original 50/60p footage.

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12 thoughts on “What shutter speed to use if shooting 50p or 60p for 50i/60i conversion.”

  1. Great information – thanks. What is the shutter speed when a Sony camera is left in the off position. I believe the Sony manual says that this can be a variable speed set automatically by the camera. But what does that mean or how is it determined? Or is it even preferable?

    1. The notion of “shutter off” is a bit strange as a shutter must always be present to isolate one captured frame from the next, so in reality the shutter is never off. What “Shutter Off” means is that the shutter is no longer variable by the user but instead locked to the cameras sensor refresh rate. So when shooting 25p it will be 1/25, when shooting 30p it will be 1/30th, when shooting at 50fps it will be /50th etc. When shooting 25fps-50i as the sensor is refreshed twice for each frame the shutter speed will be 1/50th etc.

  2. Great stuff but I am still confused. I want to film scale models working and moving past the camera at between 20 and 40 cm. Shooting at 25fps what would be my best shutter speed. I have tried 50 to 100 so far.
    Thanks

    1. Try a range and see what works for you. There is no right or wrong way, other than the right way being the one that gives you the result you want. Be very careful if using a computer monitor though as they run at 60hz and that makes 25p jitter regardless of shutter speed.

  3. So all computer screens run at 60Hz regardless of the actual mains supply Hz.. (eg a computer plugged into mains in the UK 50 Hz).. ?

    1. They almost always run at 60Hz unless you change the desktop refresh rate. Computers don’t care about mains frequencies, instead they run at 60Hz to reduce eyestrain.

  4. Hi. I don’t know if it’s the right place to ask, but I’ll try..
    Is it deferent to shoot at 50P or 50FPS?

    1. 50p and 50fps are in most cases the same thing. Where you can get confusion is 25p and 25fps. 25p is always 25 progressive frames per second. But 25fps means 25 frames per second, but those frames could be either 25 progressive frames or 25 interlace frames with 50 fields (50i).

      1. Thank you for your replay.
        So, in this case, if I want the ability to make 50% slow motion (in a 25fps timeline) and to record sound, I’ll better record at 50P and not 50fps?

        1. Ah, now I understand your question. If you want sound then yes, you would have to record at 50p and slow it down in post.

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