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25 fps film in the US



Vince Forcier wrote:
> 
> Hello all,
>     We have a client that wants to shoot 25 FPS in the US with some
> flourescent light sources. I know that there are flicker issues when
> shooting 24 FPS w/50hz lights. Are there similar problems shooting 25
> FPS w/60 hz lights. Our client is waiting for an answer, and we need
> something more definitive than "it might work, why don't you shoot a
> test". I figure if anyone can answer this they probably exist in the
> collective brain trust of the TIG. Thanks.

The answer is you need to shoot 25 f.p.s. with a camera that has a 150
degree shutter, and then you will not have any flicker problems. Your
exposure will be 1/60 of a second.
The Long convoluted reasoning follows.

Yes Problems. What kind of Fluorescent? I believe Kinoflos are safe at
any speed, something about their flicker rate being about 250,000 times
a second, but I would check that out to be sure. If it is a standard
household style fluorescent, then it flickers on and off 60 times a
second. or effectively 120 On and off periods per second. Your frame
rate needs to divide evenly into 120 for no flicker problems with a 180
degree shutter.

exposure time equals shutter angle divided by 360  times 1 over the
frame rate.

24 fps and a 180 degree shutter equals   180/360 times 1/24 = 1/48 which
is acceptable for Fluorescent and NON Flicker Free HMI. This assumes
that your power is stable and running at 60 hz.

25 fps and a 180 degree shutter = 1/50 of a second. Flicker.

25 FPS and a 187.5 degree shutter = 1/48th of a second. NO Flicker at 60hz.

There are very few cameras that have a 187.5 degree shutter opening.
Panavision and Mitchell spring to mind.

If you shoot at 25 FPS with a 150 degree shutter angle however. using
the above formula
shutter angle/360 X 1/Frame Rate you get 150/360 X 1/25 = 1/60th this is
HMI safe.

Arri SR 3, and Aaton XTR prod have these shutter capabilities. 
Also in 35mm  Sync sound cameras. Panavision, the Arri BL-4 and later
cameras, and I think Moviecam. Please check with your rental house or
equipment manufacturer to make sure the camera has this shutter angle capability.

You could always shoot at 24 and transfer at 25, adjust the pitch of the
sound lower by 4 percent as well.

Steven Gladstone
Cinematographer - DiSTONE Productions
http://home.earthlink.net/~veenotph

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