When DSLR video exploded onto the indie filmmaking scene a decade ago, film festivals were soon awash with shorts with ultra-blurry backgrounds. Now that we have some distance from that first novelty of large-sensor cinematography we can think more intelligently about how depth of field – be it shallow or deep – is best used to help tell our stories.

First, let’s recap the basics. Depth of field is the distance between the nearest and farthest points from camera that are in focus. The smaller the depth of field, the less the subject has to move before they go out of focus, and the blurrier any background and foreground objects appear. On the other hand, a very large depth of field may make everything from the foreground to infinity acceptably sharp.

Depth of field is affected by four things: sensor (or film) size, focal length (i.e. lens length), focal distance, and aperture. In the days of tiny Mini-DV sensors, I was often asked by a director to zoom in (increase the focal length) to decrease the depth of field, but sometimes that was counter-productive because it meant moving the camera physically further away, thus increasing the focal distance, thus increasing the depth of field.

It was the large 35mm sensors of DSLRs, compared with the smaller 1/3” or 2/3” chips of traditional video cameras, that made them so popular with filmmakers. Suddenly the shallow depth of field seen in a Super-35 movie could be achieved on a micro-budget. It is worth noting for the purists, however, that a larger sensor technically makes for a deeper depth of field. The shallower depth of field associated with larger sensors is actually a product of the longer lenses required to obtain the same field of view.

Once a camera is selected and filming is underway, aperture is the main tool that DPs tend to use to control depth of field. A small aperture (large f- or T-number) gives a large depth of field; a large aperture (small f- or T-number) gives a narrow depth of field. What all those early DSLR filmmakers, high on bokeh, failed to notice is that aperture is, and always has been, a creative choice. Plenty of directors and DPs throughout the history of cinema have chosen deep focus when they felt it was the best way of telling their particular story.

One of the most famous deep-focus films is 1941’s Citizen Kane, frequently voted the greatest movie ever made. First-time director Orson Welles came from a theatre background, and instructed DP Gregg Toland to keep everything in focus so that the audience could choose what to look at just as they could in a theatre. “What if they don’t look at what they’re supposed to look at?” Welles was apparently asked. “If that happens, I would be a very bad director,” was his reply.

Stanley Kubrick was also fond of crisp backgrounds. The infamous f/0.7 NASA lenses used for the candlelight scenes in Barry Lyndon were a rare and extreme exception borne of low-light necessity. A typical Kubrick shot has a formal, symmetrical composition with a single-point perspective and everything in focus right into the distance. Take the barracks in Full Metal Jacket, for example, where the background soldiers are just as sharp as the foreground ones. Like Welles, Kubrick’s reasons may have lain in a desire to emulate traditional art-forms, in this case paintings, where nothing is ever blurry.

The Indiana Jones trilogy was shot at a surprisingly slow stop by the late, great Douglas Slocombe. “I prefer to work in the aperture range of T14-T14.5 when I am shooting an anamorphic film like Raiders,” he said at the time. “The feeling of depth contributed to the look.” Janusz Kamiński continued that deep-focus look, shooting at T8-T11 when he inherited the franchise for Kingdom of the Crystal Skull.

At the other end of the aperture scale, the current Hulu series The Handmaid’s Tale makes great creative use of a shallow depth of field, creating a private world for the oppressed protagonist which works in tandem with voiceovers to put the viewer inside her head, the only place where she is free.

A director called James Reynolds had a similar idea in mind when I shot his short film, Exile Incessant. He wanted to photograph closed-minded characters with shallow focus, and show the more tolerant characters in deep focus, symbolising their openness and connection with the world. (Unfortunately the tiny lighting budget made deep focus impossible, so we instead achieved the symbolism by varying the harshness of the lighting.)

One production where I did vary the depth of field was Ren: The Girl with the Mark, where I chose f/4 as my standard working stop, but reduced it to as little as f/1.4 when the lead character was bonding with the mysterious spirit inside her. It was the same principle again of separating the subject from the world around her.

Depth of field is a fantastic creative tool, and one which we are lucky to have so much control over with today’s cameras. But it will always be most effective when it’s used expressively, not just aesthetically.