Circle of Confusion

[donlfaulkner]

I think I must be in it.

Every description I read about the circle of confusion only leaves me more confused.

I surmise that it has something to do with DOF.

Can someone explain what it is and how it is used, in preschool terms, without all the mathmetical mumbo jumbo?

[jeffkohn]

The CoC is the largest size that a circle can be on the sensor/film, and still look like a point in the final print. It's used in the equation to estimate depth of field.

[henrysamson]

Go out at night and focus on some distant lights. Now throw them out of focus. You will see these 'points' of light become circles. This is what happens to your entire subject when you focus and defocus a camera. Every point in your subject spread out into a circle, it is just easier to see with points of light. These are the "circles of confusion". Now, if your camera has a depth of field preview button that stops the lens down give it a try and watch the circles get smaller as the lens is stopped down. This is why smaller apertures have greater depth of field. You have managed to shrink the out of focus circles of confusion.

So, as Jeff noted, the amount of depth of field you have in a photograph is determined by how large a circle of confusion is acceptable. The smaller the format (with digital the size of each pixel on the sensor determines what is acceptable) usually the smaller the circle has to be as the final image will be enlarged. Large format contact printing allows for fairly large circles as there is no enlargement at all. The size of these circles is easily calculated if you know the distance to the subject, the distance the lens is focussed, the aperture (f stop) and the focal length of the lens.

This acceptable size is used to determine the depth of field markings on your lens. If you have two lenses of identical focal length for the same format and the markings on one lens seem indicate that one has more depth of field at a certain aperture, it does NOT. The manufacturer just decided that larger circles where acceptable.

I hope I did not confuse you more.

[donlfaulkner]

So there are/can be more than one circle of confusion for any given photo?

[henrysamson]

Originally Posted by donlfaulkner

So there are/can be more than one circle of confusion for any given photo?

Yes! The term is thrown around in the singular when discussing and calculating the acceptable size but actual images have an infinite number of them unless everything is in perfect focus.

EVERY point in your subject that is not in exact focus projects a circle of confusion. The term "circle of confusion" is perhaps poorly chosen. I am going to avoid saying that it just confuses the issue. The more out of focus a point is, the larger the circle it projects.

Have you ever seen a portrait against a very light (and out of focus) background? Noticed the way the background seems to bleed into the subject? Perhaps thin wisps of hair disappear altogether? This is caused by the circles of confusion from the not in focus background*. They spread out and bleed into the subject. So there are an infinite number of circles of confusion from the out of focus background. The larger the aperture and the longer the lens, the more pronounced this effect is.

* Flare and diffraction can also play a role but in most cases it is primarily cause by the out of focus background.

[jeffkohn]

EVERY point in your subject that is not in exact focus projects a circle of confusion. The term "circle of confusion" is perhaps poorly chosen. I am going to avoid saying that it just confuses the issue. The more out of focus a point is, the larger the circle it projects.

I agree the usage of the term can be confusing (sorry, that's not meant to be a pun). Your definition is correct in the general sense, but most of the time when photographers discuss "circle of confusion" they're talking about depth of field. The definition I gave above is what people generally mean when discussing CoC in relation to DOF formulas. Technically the term that should be used is "circle of confusion limit"; but nobody says that, they just say "circle of confusion".

So using the term CoC to describe any point that isn't in focus can cause misunderstanding (among photographers, if not optics experts ). For instance when discussing what happens to the out of focus background in a picture, I would be more likely to use a term like "blur circle" which I think is more descriptive.

For those interested in the nitty-gritty details, this wiki article is pretty good: Circle of confusion - Wikipedia, the free encyclopedia

[donlfaulkner]

Thank you so much, Hank and Jeff, for simplifying this for me.

So is this tied to the sensor/film size? That is the only parameter that seems to change the circle of confusion on the online depth of field calculator.


Jeff I re-read the link you provided and seem to understand their explanation a bit better.

It usually takes me looking at a subject several times and from several different angles before I begin to understand said subject.

Thanks guys for your patience with me.

[henrysamson]

Originally Posted by donlfaulkner

So is this tied to the sensor/film size? That is the only parameter that seems to change the circle of confusion on the online depth of field calculator.

Exactly! With film, Kodak suggests a maximum circle of confusion size of 1/1000 of the focal length of the NORMAL lens as acceptably sharp for the format when computing depth of field. It seems the online chart uses a slightly smaller circle for 35mm film. With digital I would assume that is just has to be smaller than a pixel on the sensor or at least that would be the lower limit.