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Measuring for focus - perpendicular distance or trigonometry?


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#1 Mei Lewis

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Posted 26 August 2015 - 07:19 AM

I've worked on a few smaller productions where people have used a tape measure to pre-measure distances to actors' marks so they can ocus to them during a take.

 

From memory (it was a while ago) they measured from the mark on the camera to the actors' mark, so the line of measure is usually off at some angle from the perpendicular to the film plane.

 

I think the distance scale on lenses specificy perpendicular distance between the film plane and the plane of focus. That would be shorter than the measured distance typically.

 

Do focus pullers use trig to figure out the distance they should pull to on the lens, having only measured the direct distance? Or do they measure perpendicular distance?


Edited by Mei Lewis, 26 August 2015 - 07:20 AM.

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#2 Kyryll Sobolev

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Posted 26 August 2015 - 08:45 AM

I have not seen any 1st ACs use or calculate trig on a regular basis - it will never be time effective

ACs use intuition and experience to adjust

While you can calculate the precise distance to the film plane, on the day it wont matter as much - the actor may overshoot the mark, or lean in further, or the dolly will have to adjust because someone popped into the frame, etc

You could use trig if the camera is way up on the crane and subject is too far for cinetape to read - use laser disto to get length to subject and height of the camera
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#3 Stuart Brereton

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Posted 26 August 2015 - 10:01 AM

I've never seen a focus puller measure directly to the mark on the ground. Usually they measure to a point directly above it, normally at about the same height as the actor's head.


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