Depth of field

Depth of field is the range of distance that is in focus in a photograph. The sharpness of the image is highest for a certain distance at which the lens is focussed, in front of and behind it decreases continuously. There is no clear limit beyond which the image suddenly becomes blurred, so figures on depth of field should be treated with some caution. We will come to how they are usually calculated, but this is not important for the understanding and practical use of depth of field.

The limited depth of field is either an undesirable or a welcome effect, depending on the intended look of the picture. It is undesirable if important parts of the picture do not appear sufficiently sharp. It is welcome as a means of image composition:

• It can blur less important or even disturbing foregrounds or backgrounds.
• It allows to direct the viewer's attention, because sharpness attracts attention.
• A contrast between sharp and blurred areas in the picture intensifies an impression of three-dimensional depth.

Playing with sharpness and blur is a permanent favourite in photography, whether digital or analogue. This page is about the technical, rather craft aspect of depth of field.

Depth of field depends on four factors:

• Aperture – a smaller aperture = larger f-number results in greater depth of field.
• Focal length – a smaller focal length results in greater depth of field.
• Sensor size – a smaller image sensor results in greater depth of field.
• Distance – a greater distance to the subject produces greater depth of field.

The changes in depth of field are not linear, e.g. halving the focal length will more than double the depth of field.

The dependency on the aperture is most important in practice – there is room for manoeuvre in almost every shot. Focal length and distance can allow adjustments depending on the lens and the shooting situation, the size of the image sensor is always invariably set by the camera.

With smartphones, the aperture is not variable as it is with large cameras and the image sensors are very small, so you will always get a fairly large depth of field with them and cannot play with it. They now offer electronic alternatives, usually called portrait mode to blur the background of people pictures with image editing software after it is taken.

The depth of field can vary greatly, be as narrow as a few cm or even mm, and can become almost arbitrarily large. Portraits can have pin-sharp eyes and already be noticeably out of focus at the tip of the nose. With other camera settings, on the other hand, a landscape photo can have foreground a few meters away and be sharp from there to the horizon.

Here are two examples of the possible extreme differences:

Wide depth of field | All factors work towards a larger depth of field e.g. in a landscape shot with a good compact camera:

• small aperture, f-number 11
• short focal length, wide angle 28 mm equivalent
• small image sensor with crop factor 2.7 (so-called 1 inch sensor, a common size of 13.2×8.8 mm in size)
• focused at 5 m shooting distance.

These settings bring a depth of field of less than 1 m to infinity.

Narrow depth of field | A realistic shooting situation where all four factors provide a shorter depth of field is a close-up shot:

• large aperture, f-number 2
• medium focal length of 50 mm
• full-frame camera with KB sensor
• 0.5 m shooting distance.

The result is a depth of field of just 1 cm. The focus was on the 50 cm mark, which was actually about 50 cm from the camera sensor.

In case you are wondering where the exact figures suddenly come from: There are formulas in optics to calculate the blur. They provide figures for how large a single, tiny dot of the subject is reproduced in the photo, depending on the four influencing factors. Where the resulting blur reaches the resolution of the human eye, one draws the line between sharp and blurred for such calculations.

How far the depth of field extends forward or backward from the focused distance also varies widely. Any rule of thumb about how much of the depth of field is in front or behind is only valid, if at all, under certain narrow assumptions about camera type, focal length, distance.

You can see it already in the two examples shown above: Around a focused distance of 5 m, the landscape photo has a depth of field of 4 m to the front and infinity to the back. In the close-up photo, the depth of field extends from a focused distance of 0.5 m to about the same distance from the front and to the back.

A closer look through more numerical examples shows that this is generally applicable:

• At very shallow depth of field, which will occur with any focal length, aperture, and sensor size at a very short shooting distance, about half of the depth of field range will be in front of and behind the focused distance. It's another matter that your camera may not be able to focus close enough to achieve that.
• As the depth of field increases, the more will be behind and the less will be in front of the focused distance. As the shooting distance increases, the depth of field eventually reaches infinity, i.e. much further behind the focused distance than before it.

The potential control over the depth of field is an advantage of cameras with large image sensors, especially full-frame cameras. In addition to the pure numerical values for a greater theoretically calculated depth of field, the fact that an out-of-focus background simply looks softer and more pleasing with a large sensor plays a major role.

For cameras with a smaller sensor, i.e. a crop factor greater than 1, the depth of field increases – assuming the same focal length equivalent and f-number. They have a depth of field like the camera with a larger sensor and an aperture that is larger by the crop factor.

Example: A camera with crop factor 2, focal length 25 mm (=50 mm equiv.) and aperture 4 has the same depth of field as a full-frame camera with the same focal length 50 mm and aperture 8.

Smartphones have the smallest sensors and therefore a greater depth of field by nature.