Depth of Field

What is depth of field?

Depth of field is the range of distances in a scene that look acceptably sharp in a photo. A shallow depth of field keeps only a thin slice in focus, like a face with a melted background, while a deep one keeps everything from the foreground rocks to the mountains crisp.

A lens can only focus perfectly at one distance. Points nearer or farther turn into small blur discs called circles of confusion, and depth of field is simply the zone where those discs stay too small for you to notice. On a full-frame camera that limit is usually set at about 0.03 mm on the sensor, which assumes a print viewed at a normal distance.

What controls depth of field?

Three things matter: the aperture, the focal length and how far away you focus. A wider aperture (a smaller f-number), a longer lens and a closer subject all make it shallower. Distance is the strongest of the three, since depth of field shrinks roughly with the square of it.

Some numbers make this concrete. A 50 mm lens at f/2, focused on someone 3 m away, keeps about 43 cm sharp. Stop down to f/8 and that grows to almost 2 m. Focus at about 10 m with the same lens at f/8, the hyperfocal distance, and everything from 5 m to the horizon looks sharp.

Sensor size matters indirectly. To frame the same shot, a small sensor needs a shorter lens, and a shorter lens gives deeper focus. That's why phone cameras struggle to blur a background optically even with bright apertures like f/1.8.

The one-third rule and other misconceptions

Photography guides often say the sharp zone extends one third in front of the focus point and two thirds behind it. That's only true at one middle distance. Up close, for macro and product shots, the split is almost even. At the hyperfocal distance the zone behind the subject runs on to infinity.

Bokeh is a different thing from depth of field. It describes the quality of the blur, the shape and smoothness of out-of-focus highlights. The word comes from Japanese, and the spelling with an h was popularized in 1997 by Mike Johnston, then editor of Photo Techniques, so English speakers would say it right.

Faking shallow focus

Phone portrait modes estimate a depth map from two cameras or a neural network and then blur each pixel by its distance from the subject. You can spot the fake at its edges, where hair, glasses and the gaps between fingers come out sharp in the wrong places.

Tilt Shift Effect fakes it by position in the frame, which works when distance runs from the bottom of the picture to the top, such as a street seen from a balcony. Keep Width around 0.15 and Blur around 10 for a convincing shallow slice. For a soft background behind one subject, Blur Image with an inverted Ellipse mask keeps the face sharp.

Further reading

• Understanding Depth of Field in Photography (Cambridge in Colour)