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Lens size, depth of field, focusing


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#1 Scott Bryant

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Posted 15 June 2007 - 02:56 AM

Generally speaking i know that DOF is primarily dependent on aperture setting, however i know some lenses tend to be so wide that they don't require focusing. If i follow this logic to its end (which may be a mistake) i reason that if a lens becomes wide enough, depth of field increases to infinity, so at every point in the frame the image is in focus. I know DOF and focusing are related but am i coupling them together inaccurately? If my logic is right, how wide does one need to get in order for focusing to not be an issue? I'm sure there is an equation to describe it based on focal length glass size and aperture however i haven't really found anything useful. Now to the meat of my question (and apologetically long post), i am using an Angenieux 12-120. How low of an aperture can i go and get away with not worrying too much about focusing? Can this even be possible? I know you have to get in a general ballpark but can i just spin the focus over to infinity or the 1.5 m mark from behind the camera and still be okay? Sorry about the long post for the rather simple question and i thoroughly appreciate anyone who has the dedication or will power to get through this whole thing.
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#2 David Mullen ASC

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Posted 15 June 2007 - 10:11 AM

Generally you would use the Hyperfocal distance if you were trying to hold as wide a range in focus as possible.

A 12mm lens, with a CoC figure of .0010", focused at 3', stopped down to f/5.6, will have a range of acceptable focus from 1' 7" to Infinity -- is that good enough for you?

The short answer is that with a wide-angle lens stopped down, focusing is generally easier -- but it's hard to shoot an entire movie that way.

You should worry about focus -- it's part of the filmmaker's tools and cinematic grammer. Avoiding it is just being lazy unless you have a good reason, like creating deep-focus compositions with important information happening far and near, for symbolic effect.

Otherwise, you get into the problem that consumer DV has, where almost everything is in focus due to the small target area. It's a bit unattractive. Focus is a form of information delivery, so if everything is in focus, then everything competes with everything else for attention.

Just use a Depth of Field chart to see the range of focus for different combinations of focal length, f-stop, etc.
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#3 Scott Bryant

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Posted 16 June 2007 - 09:18 AM

yeah i still use focus on most of the scenes. i was just asing this for scenes with handheld camera movement to make it easier. if its steady and on a tripod then i really work on focusing properly. this still may be lazy but i'm just not experienced enough to pull the focus that precisely yet. as always thank you very much david. your info is always helpful and i'm honored you take the time to answer questions from amateur auteurs.
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#4 David Mullen ASC

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Posted 16 June 2007 - 10:20 AM

With enough depth of field, you can shoot handheld and just roughly pull-focus and be close enough. During a handheld move, I get some different focus marks in rehearsal (either by eye or by tape with an assistant) and then just practice how much to turn my wrist on the barrel for different positions, so if I'm walking around a room and the focus is generally out there at 10', let's say, but then I walk up to someone in a close-up at 3', I can just turn the focus ring a certain amount by feel and know it's close enough because the wide-angle lens and depth of field covers any slight mistake.

If you are going to get close, it's good to just see how close you can get before you hit minimum focus on the lens, memorize that shot size, so that when you walk up to something tight, you turn the focus ring until it bumps at the minimum focus end and frame the shot knowing how close you're allowed to get.
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#5 Scott Bryant

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Posted 16 June 2007 - 11:59 PM

Thanks so much for the advice.
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