Depth of Field
Depth of field is the distance between the nearest and furthest parts of the subject that are all in acceptable focus at the same time. The criteria that affect depth of field are:
- The aperture of the lens.
- The focal length of the lens, or the combined lenses if a supplementary lens is added.
- The distance of the lens from the subject.
- The circle of confusion.
- The aperture is high.
- The lens is of low focal length.
- The subject is distant.
- The circle of confusion is large.
Circle of ConfusionThe circle of confusion can be described as a point on the final print that appears in focus to the average human eye when that print is viewed from the required distance. Providing that the circles of confusion are small enough, the eye will accept them as points, and the image will be acceptably sharp. The maximum permissible circle of confusion depends upon the viewer’s eyesight and the distance from the print. For normal viewing this distance is assumed to be 25 cm/10 in which equates to the diagonal of a 20.5cm x 25.5cm/8″ x 10″ print and it is generally accepted that the individual can distinguish at least 4 lines per mm which would represent a dot size of 0.25mm. The circle of confusion can be calculated from the amount that the image has to enlarged to a 20.5cm x 25.5cm/8″ x 10″ print. For example, a 35mm image has to be enlarged 7.516 times, whereas a Nikon DX has to be enlarged 11.461 times and a 20.5cm x 25.5cm/8″ x 10″ negative does not have to be enlarged at all. So for a 35mm camera and 4 lines per mm the calculations are as follows:
- Circle of Confusion = 0.25/7.516 = 0.0333mm.
- The calculations for the Nikon DX and 4 lines per mm would be:
- Circle of Confusion = 0.25/11.461 = 0.0218mm.