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What is the formulae for calculating circle of confusion?


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#1 Allyn Laing

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Posted 11 January 2006 - 11:30 PM

I have been searching through books for the formulae of working out circle of confusion, but are unable to find it anywhere can anyone help by directiong me to a website or telling me how.

Also, I am very green to working out depth of field charts how do you read them? I see them but cannot understand them

Allyn.
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#2 David Mullen ASC

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Posted 11 January 2006 - 11:45 PM

You don't really calculate circles of confusion, you pick an established figure based on the degree of enlargement of the image to some extent (I assume someone once did some tests to determine these figures). This is one reason why depth of field will always be a somewhat nebulous concept because an image may end up being presented in a great variety of screen sizes. You would probably pick the most critical situation for calculating depth of field.
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#3 Allyn Laing

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Posted 12 January 2006 - 12:20 AM

You don't really calculate circles of confusion, you pick an established figure based on the degree of enlargement of the image to some extent (I assume someone once did some tests to determine these figures). This is one reason why depth of field will always be a somewhat nebulous concept because an image may end up being presented in a great variety of screen sizes. You would probably pick the most critical situation for calculating depth of field.


Most learning manuals i have been reading on cinematography tell you to firstly work out what circle of confusion you want to work with before you calculate your depth of field. I do not know or understand the process of these numbers and how they correlate to depth of field

The reason why it is critical for me is that I am shooting a 16mm at the moment and want to keep a constant C.O.C within scenes for various moods. Can you point me in the direction of these figures or tell me more about them?
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#4 Stephen Williams

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Posted 12 January 2006 - 04:57 AM

I do not know or understand the process of these numbers and how they correlate to depth of field


Hi,

If you choose a smaller coc you will have less DOF.

DOF is only a calculation of what is acceptable focus to you! At what point do you say something is not sharp enough to be concidered in focus? 16mm on SD television could look sharp enough with plenty of DOF but when blown up, there is not as much DOF as you were expecting.

I have telecined in SD, then switched the telecine to HD, the DOF shrinks before your eyes!

Stephen
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#5 Mark Dunn

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Posted 12 January 2006 - 06:06 AM

David's spot on as usual. You pick a circle of confusion and then work out the DOF based on that. Or better still, you look it up in Samuelson's or ASC manual, (or use a Kelly calculator?).
As for reading the tables, for each aperture with a given lens, at each distance setting in the table there's a 'near' and 'far' distance to tell you what's in focus. My Samuelson's tables also give the hyperfocal distance for each stop- set the lens there and everything's in focus from half that distance to infinity.
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#6 Dickson Sorensen

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Posted 12 January 2006 - 09:17 AM

to quote the ASC manuel "It should be understood that the determination of depth of field involves a subjective sensation that requires taking into account the conditions under which the final projected image is viewed."

Though I have noticed that the more respondents to a question posted the larger the circle of confusion becomes.
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#7 Allyn Laing

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Posted 12 January 2006 - 10:57 AM

Though I have noticed that the more respondents to a question posted the larger the circle of confusion becomes.


Such a true statement :D
I found this whilst searching earlier for the answer it may be handy for first timers with the same question, it contains the formulae for working out depth of field and good links:
http://en.wikipedia..../Depth_of_field
thankyou

Allyn

Edited by Allyn Laing, 12 January 2006 - 10:58 AM.

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#8 Laurent Andrieux

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Posted 12 January 2006 - 12:19 PM

The wikipedia article about the circle of confusion gives a good definition for it actually. It only speaks of still photography but it's the same reasoning for motion picture film.

As mentionned above, it depends a lot on the screen size.

But, as you mentionned the fact that you wanted to keep the same CoC all along a shooting, that doesn't really matter since the whole film will be seen on the same screen... but...

It's very difficult, if not impossible to keep the CoC all along a scene, because it depends on factors such as the image's contrast and definition, for instance, and that is a big problem for your concern.

You see, it will change according to your lighting (soft or contrast). Outside, the fact there is a cloud passing in front of the sun changes your CoC, so...

2 other situations have to be pointed out : the contrast of the set behind an actor, and it's distance. They will give different "feeling" of depth of field ; and the actor's skin and make up. A soft make up can make the actor look soft if the shot is cutting with an actor with an "harder" skin, in a reverse shot for instance.

Also different focal length on a zoom or different lenses in a series are not always the same for what is about definition...

I think you should may be more think in terms of contrast, definition etc. If you want to give a look along a scene, and create different moods, use filters for instance, different light contrasts, color casts etc. This will be more noticable as an effect and it's easier to work with, than trying to evaluate a CoC (how will you do that, BTW ?) and try to keep it the same along a scene, and then change it to create a different mood for another scene etc.



Just a 2 cents...
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