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Circle of Confusion |
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So you're having trouble sleeping and want to know about the Circle of Confusion, ok here goes! |
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The diagram below shows an object being photographed, in this case a point. In theory, at optimum focus the image in the photographic medium (film or digital sensor) should also be a point (the black lines in the diagram) but this is never the case. In truth the dot will be resolved as a small circle (the grey lines in the diagram). Now this is not the disaster it first seems. The human eye can only resolve detail down to a certain level, namely 4 lines / mm (or 4 dots / mm), giving a minimum resolution of 0.25mm. Any circles smaller than that will be percieved as points. |
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Before we go to the next step, we need to set out a fe standard otherwise everyone will be doing things differently and we'll all get different results. The photographic industry has settled upon a standard image being an 8 by 10 inch print, derived from a 35mm film, viewed from 12 inches distance. Now ask yourself that question I just know is on your mind. |
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How large can a dot be on a 35mm film such that it is still too small to be resolved by the human eye when viewed on the standard image (detailed above). |
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This maximum size is known as the Maximum Permissable Circle of Confusion and is the correct terminology. |
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Now the maths! an 8 by 10 inch print has a diagonal of 325mm. 35mm film has a diagonal of 43.27mm. This makes 35mm film 7.5 times smaller than the 8 by 10 print. |
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The maximum dot size on the print is 0.25mm, therefore it must be 7.5 times smaller on the 35mm film, giving 0.033mm. |
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0.033 is the widely regarded as the Maximum Permissable Circle of Confusion for a 35mm camera. |
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Click here to view the Maximum Permissable Circles of Confusion for other popular cameras. |
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Well done, you've made it this far. Sadly though, if you are still awake then your problem is more serious and you need medical attention. Otherwise.........sweet dreams! |
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