Ring confusion

The circles of confusion are the smallest points placed one beside the other which it is possible to distinguish on a negative , or more generally on the support of a camera. The diameter of these points was measured on the negative one as soon as the points appeared Nets and distinct on paper. The diameter (E) of these circles is called diameter of confusion.

It is given with the intersection of two factors:

the quality (granularity and resolution) of the support of the image (film or sensor), on the one hand, which fixes the possible circle of confusion minimum ,
required clearness on the other hand (function of the size of desired pulling and the distance from observation, but also of the vision of the observer…) who fixes the desirable circle of confusion maximum .

The eye has its limits, it is for example unable to perceive variations lower than 1 mm with a distance of 3 meters. Therefore, on a film 24x36, any ray luminous of diameter lower than 0,03 mm (under optimum conditions of contrast and luminosity) will be perceived like Net… on a final pulling of 8 x13 cm observed with 30 cm . Thus the circle of confusion was calculated a long time. It was defined in the beginnings of the Photographie by Zeiss and Sinar as being equal to 1/1730 X the diagonal of the format, i.e.:

For the Format 24x36: $\ frac {1} {1730} \ times \ sqrt {24^2+36^2} \ approx0,03 mm$
For a format 6x6: $\ frac {1} {1730} \ times \ sqrt {60^2+60^2} \ approx0,05 mm$

Nowadays, these values are always at the base of the scales of Depth of field of the objectives, whereas the sensors make it possible to obtain circles of confusion lower than 0,01 mm (case typical: cut pixel of 5 with 7 µm and Filter antialiasing generating blur on 1,5 pixel), generating an already observable blur (blur of 30 µm is 4 to 6 pixels) in extreme cases theoretical of the depth of field. In the same way, for recent films, the value of the circle of confusion could be gone down to 15-20 µm to benefit as well as possible from the definition of film on great pullings.

See too

Random links: