Monday, 01 January 2018 14:46

The depth of field

Written by Riccardo Berti
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Knowing and using depth of field (PDC) in photography is essential. Knowing how to perfectly control the extension of the sharpness plane allows the photographer to sharpen the subject of the photo even if arranged on different visual planes and to give the composition a documentative and aesthetically more appreciable value. In this short chapter we try to understand what depth of field is, how to use it in the composition of photography and what are the parameters that influence it.
 
Definition of depth of field (PDC)
Taking a cue from Wikipedia we can define the PDC as follows:

"In photography, the sharp depth of field or simply the depth of field (abbreviated as PdC or DoF from the English Depth of Field), represents the area in which the objects appear focused in the image.The pdc is in very close relationship with the circles of confusion, depends on the aperture of the diaphragm and the distance between objects and optics, depending on the focus and the angle of view of the lens (or focal length) "

This definition is correct, but perhaps a little complex to understand for those approaching the world of photography. Trying to simplify the speech we can say that the depth of field in photography represents the band of sharpness for the human eye between the front and back to the subject in focus. In photographic shooting the width of this band depends on three main factors:

  1. The aperture of the diaphragm
  2. The focal length
  3. The focusing distance

Let us look at point by point how they influence the depth of field and how they can be exploited in the composition of our photography.

PDC and aperture diaphragm

Profondità di campo in funzione del diaframmaLeaving the focal length and the distance from the subject unchanged, we can repeat what has already been written in the chapter concerning the opening of the diaphragm, which for convenience of reading below: 

At a larger aperture (f followed by smaller numbers) corresponds to a smaller depth of field and vice versa. This means that on a horizontal plane the subjects that will be in focus towards the horizon with an aperture of, for example, f2 will have a smaller distance than if we set the diaphragm to f22.

As we can see from the image the larger the lens aperture (f2) with the distance of the subjects unchanged and always using the same focal length, and narrower is the band of sharpness.

PDC and lengths focal pitch
The depth of field is mainly influenced by the focal length. The higher the focal length, the smaller the depth of field. This means that, in general, telephoto lenses have a narrow depth of field while wide angles have a high depth of field. In reality, the PDC does not depend directly on the focal length, but on the use that we make of the different focal lengths. The zoom and telephoto lenses are used to shoot distant subjects while the focal length for close subjects, in fact if we compose the same photograph with two different focal lengths we would see that depth of field more or less would be the same.

 PDC and the focus distance
The distance from the point of focus is the other factor that affects depth of field. At the same aperture and focal aperture we will have a greater PDC further away from the subject in focus.

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