# Circle of Confusion

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### #1 Jase Ryan

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Posted 06 July 2008 - 08:06 PM

Can someone please help explain the Circle of Confusion in simple language? I read about it on Wikipedia and understand it a bit, but not completely. If you can make it simple for me that would be great!

Thanks,

Jase
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### #2 Matthew W. Phillips

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Posted 06 July 2008 - 08:27 PM

Can someone please help explain the Circle of Confusion in simple language? I read about it on Wikipedia and understand it a bit, but not completely. If you can make it simple for me that would be great!

Thanks,

Jase

It's a really fancy and confusing way of saying "smallest discernable detail." It is the real world equivalent of what a pixel (picture element) is to a computer image file. Also referred to as "Circle of least confusion."

Note: the smaller the "discernable detail", the sharper the image appears. Just like with pixels, the smaller the pixel, the sharper the image. (IE: Blu-Ray is sharper than NTSC because it has smaller pixels)
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### #3 Dominic Case

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Posted 07 July 2008 - 09:24 PM

In any imaging system, the actual circle of confusion is the size of the "blob" that represents a point of light from the original object.

In an ideal system a point of light would be represented by a point on the image. In the real world, no lens focusses perfectly, no film emulsion has infinitely small grain, and no CCD has dimensionless pixels.

In particular, however good all these things are, no lens can have everything (at all distances) perfectly in focus, and so out-of-focus points of light will be represented by blobs that are roughly circular (more often, they are the shape of the iris in the lens). Think of the cone of light coming from the lens, focussing to a nearly-perfect point just before or just after the emulsion. It will form a disk shape on the actual emulsion (or CCD surface).

Now, the other thing is that nobody needs a point of ight to be a point of light. When you are looking at an image of a certain size, blown up a certain amount, at a certain distance, you can only resolve detail down to a certain size. Everything smaller than that size will appear as a single dot. So the slightly out-of-focus parts of the image don't matter, as you can't detect the out-of-focusness.

The smallest size you can detect as being bigger than a dot is called the acceptable circle of confusion.

Strictly, the size of the acceptable circle of confusion varies with viewing dimensions (as explained above). If 16mm is going to be blown up to 35mm and projected to the same size as a 35mm image, then the accpetable c-o-c for the 16mm image must be smaller than it would be for a 35mm original, for example. However, mostly, c-o-c's are defined according to a standardised or "normal" set of viewing sizes or enlargements.
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### #4 billdillasc

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Posted 08 July 2008 - 11:05 AM

The circle of confusion is the largest picture element we will consider as part of a focused image. It is one method of quantifying optical image sharpness, principally used at the time of original photography. If the image size, focal length, and focus distance stay the same, the depth field will be greater with a smaller circle of confusion.

One important note: there is no such thing as a 16mm or 35mm depth of field chart. There is only a depth of field chart for a given circle of confusion.

Hope that helps.

Bill Dill, ASC
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### #5 John Sprung

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Posted 08 July 2008 - 12:37 PM

If you look at the geometrical theory of optics, a point at the distance your camera is focused on gets represented by a point on the film or chip. A point at any other distance gets represented by a circle instead. The farther you go from the plane of critical focus, the bigger that circle gets, and the softer the image gets.

Now comes the subjective part: To calculate a depth of field table, we have to choose how big a circle we're willing to call "good enough". Traditional tables for 35mm use 0.0010" diameter. As Dominic says, that may or may not be a reasonable choice for what you're shooting. For 16mm, 0.0005" is usually used. On digital cameras, the circle of confusion is quite a bit larger than the photosites. For instance, the vertical photosite pitch on the Genesis is 0.00024" (horizontally you have the color stripe issue....).

I've never heard of anybody doing it, but you could calculate tables with a really large circle if you wanted to guarantee having something way out of focus -- for instance, if you want to be sure that the audience won't recognize an actor until you rack to reveal who it is. You could also work the formulas the other way around, shoot a test with objects at various distances, and derive the circle of confusion for the ones that look right on the edge to you.

-- J.S.
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### #6 billdillasc

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Posted 08 July 2008 - 01:21 PM

The circle of confusion is the largest picture element we will consider as part of a focused image. It is one method of quantifying optical image sharpness, principally used at the time of original photography. If the image size, focal length, and focus distance stay the same, the depth field will be greater with a smaller circle of confusion.

One important note: there is no such thing as a 16mm or 35mm depth of field chart. There is only a depth of field chart for a given circle of confusion.

Hope that helps.

Bill Dill, ASC

Let me refine my answer a little:

The circle of confusion is the smallest point of light in the focused part of an image.

Shorter is always better.

Bill Dill, ASC
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