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Old May 5th, 2002, 05:12 PM   #1
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Critical Focus

I would like a clarification on exactly how this works if someone here can do so.

It has been mentioned many times that zooming in on a subject and then setting the focus to manual will allow the videographer to zoom in and out on that subject and never lose focus. Someone described this to me as seting the "critical focus."

The soccer field example on the other thread illustrates this perfectly. If a person in on the sideline just to one side of the center of the field, he can establish critical focus on the corner farthest from him. I know that he can then zoom in and out on that corner all day long and never lose focus. What about when a person pans and zooms to one of the other corners on the field? Will everything still stay in focus.

I don't know if tilting the camera up or down and zooming in on a subject closer than the original set point, in this case the corner of the field, will throw the setting off.

I am probably not explaining this very well. I know I can test this myself and to some extend I have, but good solid theory answers are sometimes much simpler than practice.

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Old May 5th, 2002, 09:20 PM   #2
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Greg:

As you have probably noticed, focus becomes much more of an issue when at the telephoto end of the lens than the wide angle end. This is because there is less depth-of-field on a telephoto setting, which refers to the range of focus available at a given focal length. Imagine all the soccer players lined up down the field in a straight line heading away from camera (raked a little bit so they aren't blocking each other). At a wide angle setting, the player five feet from the camera is in focus, as is the last guy forty feet away from camera. Zoom in on the guy five feet away, and the players in the background at the end of the line go out of focus. Now zoom in on a guy fifteen feet away. If you have pieces of the players in front or behind him, you may find that players up to 25 feet away are still in focus, but only a couple of players closer to camera stay in focus. This is because depth of field generally extends further away from the focus point than closer (towards camera).

OK, so we know that the more telephoto the lens, the less depth-of-field is available and thus focus becomes more critical. Hence the technique of zooming in to set focus and zooming back out--the focus setting on the subject is maintained, but the depth-of-field expands so that even if that person moves forward or back a bit they will stay in focus.

In your example, if you focus on the far corner, you have actually set the focus of the lens behind most of the players (unless they are taking a corner shot). That would be more apparent if you zoomed in tighter. It's best to ride the focus with the subject if you plan to zoom in at some point. DV cameras have a tremendous amount of depth of field compared to larger formats, and shooting outside in daylight maximizes depth of field so the technique of riding focus is something not a lot of videographers are forced to do, but a great way to practice is to set the lens at a telephoto setting and follow people around, switch from person to person to objects in the foreground etc. and get intuitive at finding focus manually.

And incidentally, tilting or panning does not affect focus.

Hope this helps!
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Old May 5th, 2002, 11:15 PM   #3
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Gee whiz, it's just so dang *cool* to have an SOC hanging out here with us. If I had a payroll, I'd put Charles on it. Heh.

;-)

Long live the Society,
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Old May 6th, 2002, 08:55 AM   #4
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Thanks guys!

What is an SOC? I have not heard of this acronym before.

Resources like this and DVD commentary tracks provide a lot of information about how things are done in the world of film making. I am currently watching Bruce Campbell's commentary on The Evil Dead. Very entertaining.

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Old May 6th, 2002, 10:49 AM   #5
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Why, the SOC is the Society of Operating Camerapersons, of course... a most prestigious outfit which you may explore at http://www.soc.org/ -- hope this helps,
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Old August 17th, 2002, 02:24 PM   #6
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In still photography I think the technique is called focussing on the 'hyperfocal distance'. This maximises the depth of field (normally one third in front and two thirds behind the selected point of focus). Pre focussing on the hyperfocal distance of the lens minimises the need for focussing.
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Old August 17th, 2002, 10:01 PM   #7
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Yes, that is a handy tool Ross. In the film world we sometimes refer to that as "working the split". It's useful if we are trying to hold focus on two actors that are different distances from the lens. The camera assistants use either old-school slide-rule type charts or Palm software to calculate the hyperfocal for a given situation.

The nice thing with digital is that you can dial in the correct setting by watching the monitor.
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