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Changing frame speeds during filming


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#1 Edwin Feliu

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Posted 05 November 2018 - 04:20 PM

I read that Derek Jarman did this to create special effects during filming. Will my film be ruined if I change the frame rates during filming? I would do this to deliberately change the movie's tempo.


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#2 Roger Haney

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Posted 05 November 2018 - 08:42 PM

Well, since the shutter speed changes with the transport speed; this may cause issues with exposure. The shutter speed is roughly twice the transport speed. If the camera has an auto iris; there may be a lag until it corrects the aperture for the proper exposure. Going from 24fps(1/48th-sec) to 18fps(1/36thsec) May briefly cause under exposure. Going from 24fps to 36fps(1/72nd-sec) May briefly cause over exposure. If your camera has a manual only iris; I wouldnt try this with reversal film because it doesnt have the exposure latitude. Negative film stocks are more forgiving with over and under exposure. If it were me; I would get an intevelometer application and do it in post.
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#3 Stuart Brereton

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Posted 05 November 2018 - 09:12 PM

Going from 24fps(1/48th-sec) to 18fps(1/36thsec) May briefly cause under exposure. Going from 24fps to 36fps(1/72nd-sec) May briefly cause over exposure. 

 

It's actually the other way around. 24fps to 18fps would cause over exposure. 24fps to 36fps would cause under exposure.

 

Exposure changes due to frame rate can be compensated for by an Iris pull during the shot, which obviously has an impact on your DoF, or by a change in shutter angle. Some professional 35mm cameras can alter shutter angle automatically when the frame rate is changed. Baz Luhrmann used this technique in Romeo & Juliet (1996)


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#4 Nick Collingwood

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Posted 17 November 2018 - 03:21 PM

Many cameras like the Nizo 801 or Canon 814XL-S have a "slo-mo" button that can be pressed at any time while filming and it ramps up the framerate to 54fps or 36fps respectively. Then when you let go it goes back to your normal rate you had the camera set on (whether it's 18 or 24). The camera's auto-exposure meter will adapt but it may take a split second for it to adjust.

 

So your film definitely won't be ruined. Of course these cameras were built with that in mind. But maybe some cheaper cameras might not react as well when switching framerates on the fly.


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#5 Edwin Feliu

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Posted 21 November 2018 - 12:17 PM

I took Roger's advice and bought a Minolta 601-XL with built-in intervalometer. Not sure how it works, though. I assume increasing the intervals makes for slower time lapses. I think this was a good way around the issue.


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#6 Mark Dunn

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Posted 21 November 2018 - 12:39 PM

I took Roger's advice and bought a Minolta 601-XL with built-in intervalometer. Not sure how it works, though. I assume increasing the intervals makes for slower time lapses. I think this was a good way around the issue.

That's not what he meant. He meant software to alter the running speed in editing. A camera intervalometer does nothing like that- timelapses compress minutes, or hours, into seconds by exposing a frame every few seconds. I don't think that's what you wanted to do at all.

In fact increasing the interval makes time appear to run faster, not slower. At one frame per second, a minute passes in just over 2 seconds at 24fps. At one frame every 10 seconds, five minutes is compressed into just over a second and so on.


Edited by Mark Dunn, 21 November 2018 - 12:43 PM.

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