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When to rack focus and when not to?


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#1 Lisa Browne

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Posted 12 September 2007 - 11:58 PM

Hi,

I'm new to film, and I was wondering when do you rack focus for a person moving and when to just refer to a depth of field table?

For example, let's say that you've measured your actor and he is 6 feet away from the camera. The f-stop is 4, so according to the depth of field table, given the actor is 6 feet away and the f-stop is 4, the near is 3'6" and the far is 21'5.

So does this mean the actor can move anywhere between 3'6 and 21'5 from the camera and still remain in focus? Is there still need to rack focus?

I guess my problem is figuring out the relationship between the depth of field chart and racking focus?

Thanks for any help! :)

Lisa Browne
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#2 Chris Keth

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Posted 13 September 2007 - 12:33 AM

You almost always pull to follow the actors. DoF basically tells you how much (or, usually, little) room for error there is.
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#3 John Brawley

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Posted 13 September 2007 - 07:37 AM

Hi,

I'm new to film, and I was wondering when do you rack focus for a person moving and when to just refer to a depth of field table?

Lisa Browne



Hi Lisa.

The DOF tables are designed around an amount that is accetably IN FOCUS. Focus is always continuous. Once outside of that range the focus just doesn't suddenly go totally out of focus. It gradually softens with distance. There is a range that's acceptably in focus, depending on subject distance, focal length and aperture. This range is determined by the circle of confusion or really, the shooting format. It means in reality that it's different for 35mm, 16mm, 2/3" CCD's etc.

So as long as you're using the right COC then you should trust the charts to hold focus for the specified range. Racking focus can sometimes draw attention to itself...which can also be good sometimes..it's another tool for us cinematographers to lead the audience around the frame. But sometimes racking focus can be an extra annoyence, especially in wider shots where it's more likely you'll have a shot that can hold focus anyways. And many lenses "breathe" when you rack the focus. That means the image size shifts. Drawing even more attention to itself !
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