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Understanding the Circle of Confusion

written by: Misty Faucheux•edited by: Rhonda Callow•updated: 5/4/2010

Living in your own personal state of circle of confusion over the term itself? Don't worry, we've all been there. But, understanding the circle of confusion can assist you in taking photographs that appear sharp even when enlarged.

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    Defining Circle of Confusion

    The circle of confusion is a term used to describe how much an object must be blurred in order for a person’s perception to deem the object no longer sharp. There is no set point where this happens. But, basically, things become less sharp the closer the image is to the lens or the farther away it is. When the circle of confusion becomes perceptible to the photographers eyes, then the object is no longer considered sharp anymore.

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    Calculating Circle of Confusion

    The circle of confusion is a depth of field issue. Depth of field is the part of the picture that seems to be in focus. Basically, the ability of a human eye to see fine details is limited. And, generally this is considered one minute of arc or about 1/16 mm for someone with 20/20 eyesight. Most photographs, however, don’t even have this much detail. Photographers generally shoot for photographs within the 1/6 mm range.

    Source: http://www.cambridgeincolour.com/tutorials/depth-of-field.htm Now, why is this important? Well, when taking photographs, you and camera manufacturers need to consider how large most photographs get blown up to. Whether digital or printed, most photographs get enlarged to the equivalent of a 5x7 photograph or 5 times larger than the standard negative. So, to figure out the most common circle of confusion range, camera manufacturers will take the detail range (0.166 in decimal form) and divide that by the general enlargement of 5 times the negative. This works out like this 0.166/5 = 0.0333.

    This figure means that the circle of confusion for most digital cameras is 0.033. But, circle of confusion ranges can go down to 0.05 for low-end cameras to 0.039 for high-end cameras. Basically, the closer a camera’s circle of confusion is to this 0.0333 number, the sharper the detail.

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    Figuring Out Proper Distance

    Another thing to consider is that the farther away an object is, the sharper it will appear. If a large photo is being used on a building, it will appear very detailed. But, on closer inspection, you may actually be able to see the individual printing dots if you were standing right next to it.

    It’s important to understand circle of confusion because it allows photographers to figure out the depth of field that is necessary to create a sharp, focused photograph. Photographers will need to know the proper distance that is needed to reduce the occurrences of irregularities in light and focusing. While experienced photographers may be able to figure this out quickly, amateur photographers will have to experiment.

    Set up an object, and photograph the object at different distances and with different lenses. Zoom in and out. Once you figure out what the proper circle of confusion is for your camera, it will better enable you to take photographs that are sharp and properly focused.