Circle of confusion

Blur circles created in the Gallery of blur in the image, so if the projection of a point a subject is before or behind the projection plane to be projected or when a point of light is sharply focused than the Airy disk by diffraction.

These two effects, in a blur, are modified at the entrance pupil in opposite directions, so that for a given aperture, the so-called critical aperture a minimal blur and thus a maximum resolving power results.

Depth of field

Growing in importance of the circle of confusion in the calculation of depth of field. From the lens of view arises in a focused illustration of a light cone whose apex in the case of a correct focus exactly hits the film plane. In a deviation from the tip is cut off or projected on the film plane, also. Therefore arise blur circles that are defined over a certain size for a given film format as blur (see Focus ).

Maximum tolerable circle of confusion diameter

The for the acceptance of sharpness maximum tolerable circle of confusion diameter for a camera is denoted by Z, where the size of Z from the mean resolving power of the eye results, which is the resolution limit in the ideal case in about one minute of angle and allows us to two points be seen as separate if they are at least 2 minutes of arc apart. Consider an image with a conventional viewing distance, so us the screen size appears at a visual angle of approximately 50 ° or 3000 minutes of arc. In a blur, the angle exceeds 2 minutes, so 1/1500 of screen sizes, so we will begin to recognize just a blur. As a finished picture usually is an enlargement of the resulting the moment of capture on the sensor or in the plane of the film image, the permissible circle of confusion diameter Z with 1/1500 is the film or sensor diagonal of the camera valued ( for exceptions see below).

The following table illustrates the maximum size of the blur circles depending on the recording format of the respective photographic apparatus:

In certain situations, other values ​​of Z should be used. Will not you look at the picture, for example, from a conventional viewing distance, which corresponds approximately to the image diagonal, then so also changes the permissible circle of confusion diameter. Let's stay for observation further away from the image, larger circle of confusion diameter can be allowed. Let's go closer to the image zoom to view details, so smaller circles of confusion are needed while still get a sharp image impression. Another exception arises whenever a detail from the original image is cut out and more nachvergrößert. In this case, Z will be quantified so that it is 1/1500 of the diagonal of the sensor or film area is, which was used for the inclusion of this image area.

This results in more contexts from the circle of confusion diameter valid during a specific recording format:

1 The degree of Z is crucial in determining the beneficial aperture, ie the smallest possible aperture, in which the depth of field is just maximized to the extent that the sharpness of the impression of sharpness is not generally reduces by at small apertures increasingly effective diffraction. Since, as can be seen from the table to increase the permissible circle of confusion diameter with larger sensor or film formats, a smaller aperture can be set on large format cameras without large loss of sharpness.

2nd hand or body movements at the moment of recording can lead to a blurring of the image. Exceeds the blur caused by camera shake, the degree of Z, it will remain visible in the finished recording.

3 The accuracy of self- focusing can be specified in the form of a circle of confusion diameter, with a focusing accuracy is desirable, which is significantly higher than the allowed valid circle of confusion diameter in the limiting case for the perception of blur. So if located in a small-format camera Z at 0.03 mm, and an auto focus sensor has an accuracy of the focus position, showing a circle of confusion of a maximum of 0.01 mm, so this focus is sufficiently accurate.