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Simple focus question (hopefully)


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#1 Kristian Schumacher

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Posted 18 February 2009 - 05:05 AM

Hi everyone,

I feel kind of stupid for asking this, as I am 90% sure of the answer. I just need to be 100% Is focus distance measured from the film plane?

Thanks in advance,

Kristian
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#2 Ian Cooper

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Posted 18 February 2009 - 05:14 AM

Hi everyone,

I feel kind of stupid for asking this, as I am 90% sure of the answer. I just need to be 100% Is focus distance measured from the film plane?

Thanks in advance,

Kristian


Yes...

...apart from some video lenses where it's measured to the front element.
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#3 Kristian Schumacher

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Posted 18 February 2009 - 06:36 AM

Thanks a bunch, Ian.

Kristian



Yes...

...apart from some video lenses where it's measured to the front element.


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#4 Michael Collier

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Posted 18 February 2009 - 02:13 PM

...apart from some video lenses where it's measured to the front element.



Although that's more academic than practical, since there are no witness marks on the barrel and the numbers are spaced apart too far to use a tape measured focus.





(its better to feel stupid now, than to feel stupid during dailies of soft footage)

Edited by Michael Collier, 18 February 2009 - 02:13 PM.

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#5 Satsuki Murashige

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Posted 18 February 2009 - 04:43 PM

I just need to be 100% Is focus distance measured from the film plane?

Generally true with film cameras and cinema lenses, providing that the flange focal distance of the camera's lens mount has been set correctly, and the lens has been calibrated correctly. You should really test to make sure that eye focus through the viewfinder matches the witness marks on the lens barrel before taking the camera out to shoot your project.

Some exceptions would be if you're using still lenses (since they're not calibrated), or if you're using diopters in front of the lens (since they change the focusing distance of the lens), or if you're using behind-the-lens filters in a Bolex or Panavision camera (since the thickness of the filter will alter the flange focal distance). Also, some Panavision lenses have two witness marks on the barrel, one for shooting wide open and one for shooting stopped down. I think that's purely a Panavision thing, but I'm not sure.

With video, as Ian said most ENG lenses are loosely calibrated to measure from the front of the lens. On the other hand, 2/3" cinema lenses like the Canon cine-style zooms focus from the focal plane, and some like the Digiprimes actually focus 17mm behind the focal plane. I understand that this is because the focal plane marking on most 3-chip 2/3" cameras is placed at the beamsplitter prism which splits the image into three colors for each of the CCDs which are 17mm further back into the camera body, and that to be more accurate the focus should be measured from the CCD itself.

Edited by Satsuki Murashige, 18 February 2009 - 04:45 PM.

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