# Circle of confusion

Circles of confusion arising in Photography at blur in the image, so if the projection of a point of a subject before and behind the projection plane or if by diffraction one to be projected spot of focus than the Airy disk is mapped.

These two effects, which cause blurring, are opposite when the entrance pupil is changed, so that at a certain aperture , the so-called critical aperture , there is minimal blurring and thus maximum resolving power .

## Depth of field

Depth of field and circles of confusion

The circle of confusion is of particular importance when calculating the depth of field . Seen from the lens, a focused image creates a cone of light, the tip of which hits the plane of the film exactly if it is correctly focused. If there is any deviation, the tip is cut off or projected beyond the plane of the film. Therefore circles of confusion arise, which are defined as blurring from a certain size for a certain film format (see also sharpness ).

## Maximum tolerable circle of confusion diameter

The maximum tolerable circle of confusion for a camera for the acceptance of sharpness is denoted by Z, where the size of Z results from the average resolution of our eye, which in the ideal case is about one angular minute in the resolution limit and enables us to two points to be recognized as separate if they are at least 2 angular minutes apart. If we look at an image at a normal viewing distance, the image diagonal appears to us at a viewing angle of approx. 50 ° or 3000 angular minutes. In the case of a blur that exceeds 2 angular minutes, i.e. 1/1500 of the screen diagonal, we will just begin to recognize a blur. Since a finished image is usually an enlargement of the image created at the moment of recording on the sensor or in the film plane, the permissible circle of confusion diameter Z is given as 1/1500 of the film or sensor diagonal of the camera (for exceptions see below).

The following table shows the maximum size of the circles of confusion depending on the recording format of the respective camera :

designation Recording format diagonal Z
1 / 2.7 "digital camera sensor   6.3 mm 0.004 mm
1 / 1.7 "digital camera sensor   9.4 mm 0.006 mm
2/3 "digital camera sensor   11.2 mm 0.007 mm
4/3 type digital camera sensor 17.3 mm x 13 mm 21.63 mm 0.015 mm
APS-C digital camera sensor 22.5 mm × 15 mm 27 mm 0.018 mm
35mm format 24 mm × 36 mm 43.2 mm 0.03 mm
Medium format 57 mm × 57 mm 80.6 mm 0.05 mm
Large format 90 mm × 120 mm 150 mm 0.10 mm
Larger formats     > 0.1 mm

Other values ​​for Z should be used in certain situations. For example, if you do not want to look at the image from a normal viewing distance that roughly corresponds to the image diagonal, the permissible diameter of the circle of confusion changes. If we stay further away from the image for viewing, larger circle of confusion diameters can be permitted. If we go closer to the picture in order to consider details, smaller circles of confusion are necessary in order to still get a sharp picture impression. Another exception always arises when a detail is cut out of the original image and enlarged further. In this case, Z must be numbered in such a way that it is 1/1500 of the diagonal of the sensor or film area that was used to record this image section.

Further relationships result from the diameter of the circle of confusion valid for a certain recording format:

1. The dimension of Z is decisive when determining the useful aperture , i.e. the smallest adjustable aperture at which the depth of field is maximized just enough that the impression of sharpness does not generally reduce the impression of sharpness due to the increasingly effective diffraction with small apertures . Since, as can be seen from the table, the permissible circle of confusion diameter increases with larger sensor or film formats, a smaller aperture can be set on large-format cameras without a significant loss of sharpness .
2. Hand or body movements at the moment of recording can cause the image to shake. If the blurring caused by camera shake exceeds the dimension of Z, it will remain recognizable in the finished picture.
3. The accuracy of the focusing itself can be specified in the form of a diameter of the circle of confusion, the aim being a focusing accuracy that is significantly higher than the diameter of the circle of confusion, which is valid for the perception of blurring, allows in the borderline case. If Z is 0.03 mm in a 35mm camera and an autofocus sensor has an accuracy of the focus position that shows a circle of confusion of a maximum of 0.01 mm, this focusing is sufficiently accurate.