Circle of confusion

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About the circle of confusion

Missing image
Cirles_of_confusion_lens_diagram.png
The depth of field is the region where the size of the circle of confusion is less than the resolution of the human eye. Circles with a diameter less than the circle of confusion will appear to be in focus.

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 is 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 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. One should note that with an 8"x10" camera, the value for the circle of confusion would not need any adjustments and thus is 0.200 mm.

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 sharper image is necessary to correct for the lower resolution of the system.

Accepted values for circle of confusion

Film formatFrame size1CoC
Small Format
APS-C222.5mm x 15.0 mm0.019 mm
35mm36 mm x 24 mm0.026 mm
Medium Format
64556 mm x 42 mm0.043 mm
6x656 mm x 56 mm0.049 mm
6x756 mm x 69 mm0.055 mm
6x956 mm x 84 mm0.062 mm
6x1256 mm x 112 mm0.077 mm
6x1756 mm x 168 mm0.109 mm
Large Format
4x5102 mm x 127 mm0.100 mm
5x7127 mm x 178 mm0.135 mm
8x10203 mm x 254mm0.200 mm

See also

External links

Footnotes

1:
The frame size is an average of cameras that take photographs of this format. Not all 6x7 cameras (for example) take frames that are exactly 56mm x 69mm. Check with the specifications of a particular camera if this level of exactness is needed.
2:
This format is commonly found on digital SLRs.

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