# can some body explain about circle of confusion in detail?

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### #1 sandeep sony

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Posted 16 May 2010 - 01:01 PM

can some body explain about circle of confusion in detail?
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### #2 Simon Wyss

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Posted 17 May 2010 - 01:20 PM

vijayveluvolu, look:

Had the camera obscura an infinitely small hole, the picture were infinitely sharp. No circle of confusion but an infinite number of points. Unfortunately, the picture were also infinitely dark since no light can enter an infinitely small opening.

We are forced to have a hole of finite diameter, so geometrically we must deal with fascicles of light which end as little circles, not points.

Now come the lenses. At the price of light losses, colour and distance aberrations of higher grades, we are able to correct for making the hole wider. There's a relative opening, the effective lens diameter calculated against the focal length of the lens. There's gain in going beyond 1:1, it's called resolving power. There's loss in such wide openings, such as depth of (sharp) field. Things become very complicated when you take closer looks. What remains is the circle of confusion. You cannot have everything in focus.
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### #3 Brian Pritchard

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Posted 18 May 2010 - 03:26 AM

This is the definition of Cirle of Confusion from the Ilford Manual of Photography.

Lens Aperture and Depth of Field.
If the focussing scale is set to give sharp focus at, say, 15 ft., then objects at 20 ft. and 10 ft. respectively will be less well defined, and objects at 25 ft. and 5 ft. respectively will probably be very much out of focus. There will, however, be a range around 15 ft. over which definition will be tolerably good. This range, known as depth of field, increases very markedly as the aperture of the lens is stopped down. Depth of field can only be quoted for a given degree of permissible unsharpness, and this is defined in terms of “circle of confusion.” Points of light in planes other than that which is sharply focussed are reproduced as circles which are cross-sections of the pencils of light coming to a focus either behind or in front of the sensitive surface. If these circles are small enough under the conditions of viewing to be considered as points, then subjects in the same plane will be rendered sufficiently sharply.
‘Ilford manual of Photography’ Edited by James Mitchell 4th Edition
Published 1949 by Ilford Ltd and Henry Greenwood & Co Ltd

I hope this helps
Brian
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### #4 Jim Hyslop

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Posted 22 May 2010 - 10:16 AM

Vijay, they say a picture is worth a thousand words, so have a look at this Wikipedia article, in particular the first diagram.

Putting aside all the technical jargon, a "circle of confusion" basically means "it's not in focus." Looking at the diagram, the point in the top image is closer to the lens than its focus distance, so the image converges behind the focal plan, resulting in a circle on the film (or sensor) rather than a point. What you see is a blurry circle instead of a sharp point. That circle is the circle of confusion. In the middle image, the point's image converges exactly at the film plane, so you see a sharp point. In the bottom image, the image converges in front of the focal plane, resulting in another blurry circle.

If you have any more specific questions, please feel free to ask them. "Explain in detail" is pretty vague :-)

--
Jim
p.s. As per forum rules, please make sure your first and last names are clearly indicated.

Edited by Jim Hyslop, 22 May 2010 - 10:17 AM.

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### #5 Dirk Tomson

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Posted 18 August 2011 - 08:32 AM

an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. It varies depending on camera type, aperture and focusing distance, although print size and viewing distance can also influence our perception of depth of field.
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### #6 Thomas Del Ruth ASC

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Posted 05 January 2013 - 01:27 PM

It's an area of 12' in diameter around the camera...
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### #7 Simon Wyss

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Posted 05 January 2013 - 02:33 PM

Isn’t it this forum?
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