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:
  1. The aperture of the lens.
  2. The focal length of the lens, or the combined lenses if a supplementary lens is added.
  3. The distance of the lens from the subject.
  4. The circle of confusion.
Control over the depth of field is a vital tool for photographers. It enables us to emphasise important subjects and suppress unwanted items. Greatest depth of field occurs when:
  1. The aperture is high.
  2. The lens is of low focal length.
  3. The subject is distant.
  4. The circle of confusion is large.
Although it is possible to calculate the depth of field from the Hyperfocal distance, the resultant figures are not as accurate. The advantage of calculations using the Hyperfocal distance is that they are much easier to do on a pocket calculator. The depth of field values for different lens designs, particularly those of telephoto and inverted-telephoto design, will differ slightly because the distance between the object side and the image side principal points will vary accordingly. Many lenses have Depth of Field scale markings which enables an estimate of Depth of Field to be made. However these scales are only relevant to the film format for which the lens was designed, and would give erroneous results when using, for example, a lens from a 35mm camera on a camera with a smaller film format. Any mathematical calculations of depth of field are based upon geometric optics that assume a perfect lens and a perfect film or camera. Practical experience is the only true guide to the performance of a lens. Depth of Focus, a term which is often incorrectly used for depth of field, relates to the area behind the lens. It is how far the film/sensor can be moved from the position of accurate focus whilst producing an image of sufficient quality.

Circle of Confusion

The 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.
The resolution of the image does not change with the size of a print but it does for a closer or farther viewing distance. The smallest size of the Circle of Confusion may be limited by the physical size of the digital sensor elements. In addition, diffraction has an effect when very small apertures are used. With computer monitors it must be remembered that their resolution is less than that of a print so that this must be taken into account when assessing Depth of Field. Establishing the circle of confusion is the first step in arriving at an estimate as to the performance of a lens in so far as Depth of Field is concerned.

Hyperfocal distance

The Hyperfocal distance is the distance from the lens to the nearest part of the subject that is in acceptable focus when the lens is focused upon infinity. If we focus the lens on the Hyperfocal distance then the subject will be in focus from half the Hyperfocal distance to infinity. The lenses on some cheaper, fixed focus, cameras are designed to be focused on the hyperfocal distance in order to give the greatest depth of field for the average subject.