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frame rate/ shutter angle for flicker free images (first time shooting in the US)


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#1 ben jones

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Posted 31 January 2011 - 01:19 PM

Hello.

Coming over from the UK to shoot a short in arizona (CANON 7D). I'm concerned about the right format to shoot with regard to the difference in power hz. Is there a way I can I still shoot in PAL and avoid the potential flicker from flouros and other lights? Or do I need to be shooting in NTSC.

The film is to be finished in the UK with all editing/post/sound work being completed back home.

Thanks in advance to all suggestions and advice.

Ben
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#2 Simon Wyss

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Posted 31 January 2011 - 02:56 PM

♪ Winchester Cathedral ♫

Holy Winter, what a mess! Format, Canon 7D, power hz, PAL, NTSC, film?

Judging by the way you’re writing, it’s a digital video. To me Canon 7D is Canon EOS 7 D with which you can switch from 24 to 25 fps, and 30. Light from 60 Hertz mains will not cause any problem when camera is set to according mode. PAL and NTSC are analogue television systems. Your camera is a digital one. Haven’t you got a manual for it?

Please don’t say film. Film has a photochemical layer and perforation holes. I am totally wacky in this, forgive me. Video is not film. Ne-ver.
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#3 ben jones

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Posted 31 January 2011 - 05:42 PM

Simon

Thankyou for you response and for your concern!

Hardly in a mess with over a week till shoot. Yes the camera is switchable between PAL and NTSC, yes I do have a manuel and yes you do run into potential problems when shooting certain frame rates with certain lights, according the the native power frequency.

Im fully aware of the differences between film and video, but you dont hear people referring to digitally originated full length motion pictures as "Feature videos"

Just a thought.
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#4 John Sprung

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Posted 31 January 2011 - 06:33 PM

Ben, very likely the best choice would be to shoot at 24 fps, and forever after just pretend that it had been 25. Your footage will run 4% fast in the UK, as does everything we make here and sell over there. No frame interpolation, no blending, no Twixtor or any of that. Just present the exact same frames you shot, only at 25 instead of 24.

There are two ways to deal with pulsating light sources, magic frame rates, or magic shutter angles. If you pick a magic frame rate, you can use whatever shutter angle you want to get the motion blur and exposure you want. If you have to use a non-magic frame rate, you're stuck to pick one of the shutter angles that are "magic" for that frame rate.

Light sources that pulsate due to AC power produce light on both the positive and negative half cycles, so they pulsate at twice the power frequency, 100 Hz in the UK, 120 Hz here in the US. Your goal is to get the same amount of light in every frame.

The first way to do that is to choose a frame rate that divides evenly into the pulse rate. That way each complete exposure and pulldown cycle of the camera corresponds to an integer number of light pulses. So, the shutter always opens at the same point in the light pulse cycle, and closes after admitting the same amount of light. That allows you to use any shutter angle you want. 24 works here, because 120 = 5 x 24. 25 works there, because 100 = 4 x 25.

For over or under cranking, the possibilities are:

120 = 1 x 120 = 2 x 60 = 3 x 40 = 4 x 30 = 5 x 24 = 6 x 20 = 7 x 17.143 = 8 x 15 = 9 x 13.333 = 10 x 12 .... = 20 x 6, etc.

If you can't use one of the magic frame rates, you're stuck with the more difficult approach. You have to find the shutter angles that, at the required frame rate, have the shutter open for an integer number of light pulses. Suppose you must use 25 fps with 120 light pulses per second. In that case, you want a shutter angle A such that

(1/25) x (A/360) = (N/120) where N is an integer. The possible N values are 1, 2 ,3, and 4, because at N = 5, the exposure becomes longer than the cycle time, so your shutter angle would have to be over 360 degrees. Working the math, we get:

A = 75 x N

So, the possible angles are 75, 150, 225, and 300. (The next one would be 375, but that's impossible.)

It should be clear that magic frame rates are the easy way, and 24 is plenty close enough to 25.

If you have some reasonable way of recording and monitoring what you shoot, you should be able to test and play back on the set to be sure if you have a problem. A great many light sources such as incandescents and modern flourescents don't follow the mains frequency, and work fine without any extra math or magic. I've seen problems with neon signs and advertising displays, though. Particularly, some of the LED chasers use their own frequencies, so you may have to experiment with them, or just get them off the set.




-- J.S.
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#5 Paul Bartok

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Posted 29 August 2011 - 09:52 AM

Ben, very likely the best choice would be to shoot at 24 fps, and forever after just pretend that it had been 25. Your footage will run 4% fast in the UK, as does everything we make here and sell over there. No frame interpolation, no blending, no Twixtor or any of that. Just present the exact same frames you shot, only at 25 instead of 24.

There are two ways to deal with pulsating light sources, magic frame rates, or magic shutter angles. If you pick a magic frame rate, you can use whatever shutter angle you want to get the motion blur and exposure you want. If you have to use a non-magic frame rate, you're stuck to pick one of the shutter angles that are "magic" for that frame rate.

Light sources that pulsate due to AC power produce light on both the positive and negative half cycles, so they pulsate at twice the power frequency, 100 Hz in the UK, 120 Hz here in the US. Your goal is to get the same amount of light in every frame.

The first way to do that is to choose a frame rate that divides evenly into the pulse rate. That way each complete exposure and pulldown cycle of the camera corresponds to an integer number of light pulses. So, the shutter always opens at the same point in the light pulse cycle, and closes after admitting the same amount of light. That allows you to use any shutter angle you want. 24 works here, because 120 = 5 x 24. 25 works there, because 100 = 4 x 25.

For over or under cranking, the possibilities are:

120 = 1 x 120 = 2 x 60 = 3 x 40 = 4 x 30 = 5 x 24 = 6 x 20 = 7 x 17.143 = 8 x 15 = 9 x 13.333 = 10 x 12 .... = 20 x 6, etc.

If you can't use one of the magic frame rates, you're stuck with the more difficult approach. You have to find the shutter angles that, at the required frame rate, have the shutter open for an integer number of light pulses. Suppose you must use 25 fps with 120 light pulses per second. In that case, you want a shutter angle A such that

(1/25) x (A/360) = (N/120) where N is an integer. The possible N values are 1, 2 ,3, and 4, because at N = 5, the exposure becomes longer than the cycle time, so your shutter angle would have to be over 360 degrees. Working the math, we get:

A = 75 x N

So, the possible angles are 75, 150, 225, and 300. (The next one would be 375, but that's impossible.)

It should be clear that magic frame rates are the easy way, and 24 is plenty close enough to 25.

If you have some reasonable way of recording and monitoring what you shoot, you should be able to test and play back on the set to be sure if you have a problem. A great many light sources such as incandescents and modern flourescents don't follow the mains frequency, and work fine without any extra math or magic. I've seen problems with neon signs and advertising displays, though. Particularly, some of the LED chasers use their own frequencies, so you may have to experiment with them, or just get them off the set.




-- J.S.


I think he said it all simple rule if your in America its 60Hz so shoot at 1/60
most PAL Countries it should all be 50Hz so stick to 1/50
unless something's changed it's 60 for US and 50 for Australia,UK, etc.
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Broadcast Solutions Inc

Aerial Filmworks

Abel Cine

Visual Products

Willys Widgets

Tai Audio

Opal

Metropolis Post

The Slider

Ritter Battery

FJS International, LLC

Glidecam

Rig Wheels Passport

rebotnix Technologies

Wooden Camera

CineTape

Paralinx LLC

Technodolly

Media Blackout - Custom Cables and AKS

CineLab