The circle of confusion is a term in photography relating to the fuzziest a point can be and still be called "in focus" (this is related to the depth of field). This value often calculated as the largest circle on the film that will still be seen as a point when enlarged to 8"x10" and viewed from a normal viewing distance (2-3 feet).

Film size is important in this calculation because it relates to the amount of enlargement necessary to get to 8x10. The larger the film size, the larger the tolerable circle of confusion. A circle that is 0.026 mm on 35mm film when enlarged to the same size as a 6 x 4.5 cm (roughly twice the size of the 35mm) will be about 0.05 mm.

The other part of this that comes into play is the quality of the human eye. The human eye can distinguish 5 lines per millimeter at a distance of 25 cm. If the minimum resolution of 5 lines/mm at normal viewing distance for an 8x10 print then maximum size for a point to still be regarded as a point is 1/5th of a mm - about 0.200 mm. If actually using an 8x10 size film (see large format) this means the circle of confusion would be 0.200 mm. However the 35mm film is about 7.5 times smaller and thus needs a circle of confusion 7.5 times smaller too: 0.2 mm / 7.5 = 0.026 mm

Using the "Zeiss formula" the circle of confusion is calculated as d/1730 where "d" is the diagonal measure of the film in millimeters. For 35mm film (43mm diagonal) this comes out to be 0.024 mm. The exact value of 1730 is up for some dispute - its a ballpark figure. The important part is to realize where this comes from. Taking that 5 lines per millimeter and an 8"x10" which has an 330mm diagonal we get 1650 lines for the diagonal.

The circle of confusion of 0.026 mm is "average" - an average person taking a photograph with an average (not professional) camera on average film (print film - not professional grade) processing the film at an average photo-store (with typical photo printing machines operated by technicians of average talent). If you want the image to be sharper consider a smaller circle of confusion - this is often the case with images hung in a gallery where people look a bit closer or using a digital camera and inkjet printer where a shaper image is necessary to correct for the lower resolution of the system.