Thread: 30fps and PAL
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Old 19th January 2014
davidfarquhar davidfarquhar is offline
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Re: 30fps and PAL

My understanding is that the flicker in artificial light is not related to the fps of the video being taken. It is actually related to the shutter speed used in the footage.

UK electricity runs at 50 Hz, so every 1/50 of a second the lights go through an entire cycle. During the cycle the colour and intensity of the light varies slightly as the electricity cycles (thats about as technical as I get on that part of it).

If you are shooting at 1/60 shutter speed then every frame on your video will capture 5/6 of a cycle, and given that each frame starts 1/30 second after the last, you cannot ensure that the same 5/6 of a cycle is captured on every frame. Hence the colour and intensity of the light will vary slightly from frame to frame. This gives rise to the flicker as you see different intensities on each frame

The solution to this is to shoot at 1/50 second shutter speed. Every frame will capture a full cycle of the electricity, regardless of when you start it. Thus each frame will capture the same average light colour and intensity, and the resulting video will have no flicker.

I believe (from distant memory) that the E-M5 will default to the conventional norm of "shutter speed half your frame rate" in automatic mode. This norm is there to give your footage a "normal" feel to it. If you shoot 30fps at a shutter speed of 1/120, then the footage will appear very jerky. Shooting it at 1/30 will appear sluggish. So over time the normal convention has arisen.

However if you shoot in manual mode you get to control both shutter speed and aperture, so you can manually set shutter speed to 1/50 (not quite half frame rate but so close as to not be noticeable), and then use aperture and ISO to adjust to give the correct exposure. I always shoot movies in manual for this reason, with shutter speed 1/50 indoors to avoid flicker.

I hope this long explanation helps - try it out on manual mode its really easy to see the effect even in the viewfinder. Please note I'm not a professional videographer so if any of my above is slightly wrong I apologise - I do know however that 1/50 will sort out your flicker

David
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