Andy Harbin Photography
Capturing the beauty of The Cotswolds and surrounding areas

So you are having trouble sleeping and want to know about the Circle of Confusion? Ok you asked for it!

Firstly may I start by saying that this isn't something you will generally need to worry about. Whilst it is used in the calculation of depth of field and hyperfocal distance etc you will only need it if you intend to calculate these things yourself using formulae. If you are anything like me you will use an App on your smart phone or some other low cost device / 'calculator'. However, the following will give you some idea of what it is and how it is derrived.

The diagram below shows an object being photographed; in this case a simple point. In theory, at optimum focus, the image on the photographic medium (in this case 35mm film, but could be a digital sensor) should also be a pin-sharp point. In reality this is never the case as the dot will be resolved as a small circle (albeit it very small). Now this isn't the disaster it first seems. The human eye can only resolve detail down to a certain level, namely 4 dots per millimetre or a resolution of 0.25mm. Any circles smaller than that will be perceived by us humans as simply points (or circles so small they have no descernable width).

Before we go any further we need to set out a few standards otherwise everyone will do things differently and get different results.

The photographic industry has settled upon a standard image as being an 8 x 10 inch print, derived from a 35mm film and viewed from a distance of 12 inches away.

Now the point (if you'll excuse the pun) all of this is getting to is this...How large can a point be on a 35mm film and still be too small to be resolved by the human eye as anything but a point on the 'standard image' defined above? This maximum size is known as the Maximum Permissable Circle of Confusion.

Now a bit of maths.

An 8 x 10 inch print has a diagonal of 325mm. A 35mm film has a diagonal of 43.27mm. This makes 35mm film 7.5 times smaller (325 / 43.27 = 7.511) than our 8 x 10 print.

The maximum resolution on the print we know is 4 dots per millimetre or 0.25mm, therefore on the 35mm film it will be 7.5 times smaller or 0.033mm.

0.033mm is widely regarded as the Maximum Permissable Circle of Confusion for a 35mm film camera.

Below are some Maximum Permissable Circle of Confusion values for popular makes of camera.

CofC = 0.03:

Canon 1DS mkII, Canon EOS 5D.

CofC = 0.023:

Canon 1D mkIII.

CofC = 0.02:

Fijifilm S5 Pro, FijifilmS3 Pro, Nikon D2Xs, NikonD40X, Nikon D70, Nikon D80, NikonD200, Pentax K100D, Samsung GX-1L, Samsung GX-10, Sony Alpha 100.

CofC = 0.019:

Canon EOS 400D, Canon EOS 350D, Canon EOS 30D.

CofC = 0.015:

Olympus E400.

CofC = 0.007:

Fujifilm S9600, Fujifilm 9500, FujifilmS6500, Fujifilm F20, Fujifilm F30, Panasonic Lumix DMC LX2