What does depth of field actually mean?

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What does depth of field actually mean?

Anybody who takes up photography will sooner or later stumble across the following term: depth of field. But what does it actually mean? And just how deep can the depth of field be?

The term is used to describe the depth to which images remain sharp in the field of view. A deeper depth of field thus enables us to keep both near and distant objects sharp in focus. This is especially important for landscape or architectural photography, although also for macro photography for example.

The aperture is a crucial factor in controlling the depth of field. The smaller the opening, the deeper the depth of field. However, the focal length and the shooting distance, or the distance between the camera and the subject, also have an influence.

Sharp point and blur circles

Strictly speaking, a lens can only focus on one single point. It is only at this point that the reproduced image of an object will be perfectly sharp. In all other regions, which are either nearer or further away from this point, these points will be reproduced as discs or blur circles. The smaller the discs, the more our eyes will perceive them to be sharp. When using larger apertures, it is often only possible to recognise a few sharp points, but there will be even more blur circles – this is also described as the Bokeh effect (popular in portrait photography). If the size of the aperture is reduced, these blurred circles become smaller and the sharp zone expands in the image.

The second parameter that influences depth of field is the magnification level. This is calculated from the focal length and the shooting distance. There is a rule of thumb for this: The further the distance to the object, the deeper the depth of field. A shorter focal length (wide angle) increases the sharpness of the image. In contrast, if the subject is located nearer to the camera, the depth of field is shallower; when taking a photograph with a (macro) telephoto lens, the sharp zone is often only a few millimetres deep.

Incidentally: The camera's sensor size also has an influence on depth of field. This is because the smaller the size of the sensor, the shorter the focal length required. It is well known that when using a 35 mm lens on an APS-C camera we can achieve the same image dimensions as with a 50 mm lens on a full-frame camera – the depth of field is also correspondingly the same.

Depth of field in comparison

The illustrations demonstrate how the depth of field can change according to the size of the aperture, the shooting distance and focal length.

Changing the aperture

Focal distance 70 mm
Focus at distance of 10 m
The smaller the size of the aperture, the deeper the depth of field.

Changing the shooting distance

Focal distance 70 mm
Aperture f/8
The shorter the distance between the camera and the subject, the less sharp the background will be in the image.

Changing the focal length

Aperture f/8
Focus at distance of 10 m
A deep depth of field can be achieved using a wide-angled lens.

All focal lengths are based on a small image format of 24 x 36 mm. Smaller sensors require shorter focal lengths or larger shooting distances to reproduce the image full frame. This means that cameras with APS-C and Four Thirds system sensors, as well as compact cameras, will have larger depth of field zones using the same aperture number.

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