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Circle of Confusion Visual Explanation


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#1 Nick Centera

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Posted 15 April 2010 - 10:09 PM

Hey, I have been searching for a visual explanation of the circle of confusion and hyperfocal distance. I have been reading the Camera Assistant hb by Hart but I still do not understand what it is and why it is used? I am a more visual learner, so if you know of any visual representation of these ideas or have any advice, I would appreciate it. Thanks
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#2 Tom Jensen

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Posted 15 April 2010 - 11:19 PM

Do a Google search or look on Wikipedia. There are plenty of thorough explanations that will help explain it.
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#3 K Borowski

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Posted 16 April 2010 - 11:05 AM

A "visual" explanation would be an out-of-focus dot that appears as a soft, fuzzy circle, with the bokeh of the lens.


The only real "explanation" you can get for COC is a mathematical one.


Basically, an acceptable circle of confusion is when the circle gets so small that it appears to the human eye from a certain distance as a point instead of a circle. It's the criticalness of the application that determines how big the circle of confusion can be and still be "acceptable" for the viewer's needs.

This is how "hyperfocal" distances can be utilized to render sharp focus across a larger range. The range is defined based on how big the acceptable circle of confusion can be.


Speaking of "confusion" did this post confuse you? ;)

If you need a better explanation, let me know.
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#4 Tom Jensen

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Posted 16 April 2010 - 05:07 PM

Speaking of "confusion" did this post confuse you? ;)


Circle of Confusion: When the producer, director, 1st AD and DP huddle together.
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#5 John Sprung

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Posted 16 April 2010 - 06:16 PM

Edit: This should be a better version of the PDF:

Attached File  CofC_Layout1.pdf   159.54KB   89 downloads

OK, so light reflects off of objects in the real world, mostly in a diffuse manner, going in all directions. So, pick a point on an object and follow the light from that point to the iris in your lens. It forms a very narrow cone. Out the back of the lens comes another cone of the same light. When the outside world cone gets longer, the inside the camera cone gets shorter, and vice versa.

Real world points at the plane of critical focus turn into points on the film because that's where the point of the cone in the camera is. At other distances, the film plane cuts through the cone elsewhere, forming the infamous circle of confusion.



-- J.S.

PS: OK, I got a better version of the PDF up, but I can't get rid of the goofy one that came out sideways.... ;-)

Attached Files


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#6 Nick Centera

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Posted 16 April 2010 - 06:25 PM

Hey thanks for all the responses! That pdf attached really helps!
What I am thinking of relating it to in my head is when, for example, street lights in the distance get turned into the circles.

I guess my next question would be should one always stay in between a certain range for the CoC? I know its a dp's choice, but are there limits/standards that you usually don't cross?
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#7 John Sprung

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Posted 16 April 2010 - 07:08 PM

For stuff like out of focus lights in the BG, you can do whatever you want.

For stuff that's supposed to be reasonably close to focus, there are Depth of Field tables, based on particular sizes of circle of confusion, like 0.002" for 35mm and 0.001" for 16mm film. But those tables were calculated and published long ago, before we had computers. Today, you can shoot tests and decide for yourself where "good enough" ends. Here's a spreadsheet to let you make DOF tables with your own choice for CofC:

http://www.auner.net/misc/DOF/DOF.xls




-- J.S.
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#8 Nick Centera

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Posted 17 April 2010 - 10:11 AM

The table is great! I guess my next question, and I am sure sounds kinda dumb, but when you choose your CoC, you do not do anything physically to the lens right? Now I am wondering how to actually apply the tables to when I'm shooting. I understand the concept much better but not how to implement them?
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#9 Tom Jensen

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Posted 17 April 2010 - 10:39 AM

[quote name='Nick Centera' date='Apr 17 2010, 08:11 AM' post='321378']
The table is great! I guess my next question, and I am sure sounds kinda dumb, but when you choose your CoC, you do not do anything physically to the lens right? Now I am wondering how to actually apply the tables to when I'm shooting. I understand the concept much better but not how to implement them?
[/quote
You choose your CoC and then you use the appropriate depth of field and figure out focus from there. Don't get too wrapped up in this concept. Spend more time thinking about focus and depth of field. Some depth of field charts don't even give you a choice so it's built in.
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#10 Nick Centera

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Posted 17 April 2010 - 10:48 AM

quote
You choose your CoC and then you use the appropriate depth of field and figure out focus from there. Don't get too wrapped up in this concept. Spend more time thinking about focus and depth of field. Some depth of field charts don't even give you a choice so it's built in.
[/quote]

Thanks for the help! I have another question of topic from here but still on dof. All the new dslr cameras seem to have a very abrupt fall off. Maybe I am the only one to notice? But the dof on the video from them always seems unrealistic. Comparing it to 16 or 35 where the background gradually goes out of focus, on the dslr they seem to just jump. Any explanation?
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#11 John Sprung

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Posted 17 April 2010 - 04:22 PM

Any explanation?


Yes -- With bigger formats -- DSLR's are VistaVision size -- you need longer focal lengths to get the same angle of view. Put those longer focal lengths into the DOF spreadsheet, and the math shows you get less DOF. That's why on big still cameras, like 11x14 inch view cameras, you may be working at f/45 to get reasonable DOF from your ultra fast f/8 lens. That's why they have dinky little lenses, much smaller than the film, and our movie cameras use big lenses to make little images. It keeps going the other way, 2/3" video cameras are in between 16mm and super-8, and have loads of DOF. That makes them ideal for sitcoms, where focus pulling takes that are eight minutes long with loads of moves is a major feat.




-- J.S.
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#12 Nick Centera

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Posted 17 April 2010 - 04:33 PM

[quote name='John Sprung' date='Apr 17 2010, 02:22 PM' post='321425']
Yes -- With bigger formats -- DSLR's are VistaVision size -- you need longer focal lengths to get the same angle of view. Put those longer focal lengths into the DOF spreadsheet, and the math shows you get less DOF. That's why on big still cameras, like 11x14 inch view cameras, you may be working at f/45 to get reasonable DOF from your ultra fast f/8 lens.

So in essence, if you shot at say f22, you would get a closer dof compared to actual 35? Sorry, I should just go to wikipedia or something, but I am wondering what is angle of view?
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#13 K Borowski

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Posted 18 April 2010 - 02:55 PM

I can't find a good example, but does someone have access to a still from a movie online that has a good example of some different-sized bokeh?


That would probably be a good way to visually explain it. I'm going to search for some formulae in my 1949 Naval Photography guide.

That XLS depth-of-field calculator that was circulating around here about a year ago probably has them in it already. Anyone have a link?
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