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I know that when I want to calculate Depth of Field by hand one of the variable elements in that equation is the Circle of Confusion. In layman's terms, what is the "Circle of Confusion," how do I calculate it, and is there any other ways that it applies to my photography aside from calculating DoF?

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    \$\begingroup\$ Short answer: "A great name for a beginner's photo club." \$\endgroup\$
    – Jerry Coffin
    Feb 20, 2011 at 15:27
  • \$\begingroup\$ "No direct application" apart from informing our choices of sensor, lens, aperture settings, post-processing techniques (like sharpening methods), and amount of enlargement! \$\endgroup\$
    – whuber
    Feb 21, 2011 at 19:26
  • \$\begingroup\$ I know you've gotten suspicious looks for asking questions before ;) but it is great to see someone with so much experience still willing ask questions and learn. First rate people always want to learn more. \$\endgroup\$
    – Itai
    Feb 22, 2011 at 15:08
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    \$\begingroup\$ @Itai: Thanks! Most of the time I actually do know the answers to the questions I ask, but in this case my understanding of CoC is fuzzy at best... :-) \$\endgroup\$
    – Jay Lance Photography
    Feb 22, 2011 at 22:46
  • \$\begingroup\$ Highly related: photo.stackexchange.com/a/30984/4848 \$\endgroup\$
    – BlueRaja - Danny Pflughoeft
    Dec 31, 2012 at 23:21

3 Answers 3

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This is often a source of confusion which most people get backwards, so understanding this is delicate:

When a light entering a lens is not in focus, a point on the subject is focused into a circle on the image plane (sensor/film). This circle IS the circle of confusion. The more out of focus a point is, the larger the circle of confusion becomes. This depends on focus distance, subject distance and aperture. It does not depend on the capture device resolution or viewing conditions.

The circle of confusion used in DOF calculations is the maximum allowable circle of confusion which is considered in acceptable focus. This is dictated by the size of the display medium and by viewing distance because of the way human vision resolves details.

Historically, most DOF tables use a standard COC which corresponds to unaided viewing of an 8"x10" at 14" away for someone with 20/20 vision, although I am sure other magic numbers are used sometimes.

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    \$\begingroup\$ One minor proviso: in a lens that has any coma (which is all of them, of course), the circle may be rather (or even quite) non-circular toward the corners of the picture, especially if (or example) produced by a high-contrast point of light. \$\endgroup\$
    – Jerry Coffin
    Feb 20, 2011 at 5:50
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    \$\begingroup\$ Another "point" to remember is that lenses can't focus perfectly, so even at the plane of focus a point is resolved as a dot and the size of that dot is known as the circle of least confusion. This puts a kind of a limit on the useful resolving power of both film and digital sensors. \$\endgroup\$
    – David Rouse
    Feb 20, 2011 at 18:01
  • \$\begingroup\$ @Jerry - Yes, I know. Just keeping things simple for the sake of shortness as it does not affect DOF. \$\endgroup\$
    – Itai
    Feb 20, 2011 at 20:38
  • \$\begingroup\$ Sooo... How do I calculate CoC? \$\endgroup\$
    – Jay Lance Photography
    Feb 22, 2011 at 22:50
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a lens is actually in focus at one point. as you move further from that point, the image blurs more. The amount of blur at a given distance from the focal point depends on focal length of the lens and the distance to the focal point.

The circle of confusion is the largest area blur that is indistinguishable from a point. It's dependent upon sensor resolution and film acuity. It also depends on viewing conditions and print size. The numbers you see used in calculations are just accepted standards.

It tends to be an easier concept for large format photogs as they deal with focal planes more.

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Given a lens with a given aperture and an object at infinity so all rays entering the lens are assumed to be parallel, the outer most rays from the axis of the lens focus at a distance closer to the lens than do rays entering the lens closer to the center of the lens. The closer rays focus further down the axis to form a sharp image.So, be it that not all rays from a distant object focus at the same point, the insertion of the focal plane within this range of confusion will allow only one point on the object side to be in great focus and objects further away and objects closer to the lens than the sharp focused point will be blurred. hence the depth of field is defined or better yet observed. small aperture smaller depth of field the big blur. Since an object focuses at one point the circle of confusion is minimum and is called the circle of least confusion.

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