What Is Depth Of Field?
INTRODUCTION
Have you ever noticed how some shots isolate characters against a soft, dreamy background that fades away, while other shots remain sharp and focused across the entirety of the frame.
This is because of depth of field - a simple but powerful tool in a filmmaker’s arsenal. It draws your eye, creates mood, and, sometimes, tells a story all on its own. But how does it work? What makes one shot shallow and intimate, while another feels deep and expansive? What effect does this have on storytelling?
WHAT IS DEPTH OF FIELD?
Depth of field is the part of an image that appears sharp or in focus. Rather than being a single point of sharpness, there’s actually an area, or zone where objects will be rendered sharp by a camera’s lens.
The position of this zone is determined by what focus distance is selected on the lens - which is indicated by the little distance markings on the barrel.
For example, if a lens’ focus is set to 10’, everything from the nearest point of 8’ to the furthest point of 12’ from the camera, will be rendered in acceptable focus. Objects closer than 8’ or further than 12’ from the camera will be soft, blurry and out of focus. Therefore, we’ve got a depth of field of 4’.
However, depth of field doesn’t always stay the same. If the depth of field is a small number, more of the image will be blurred and we say that the depth of field is shallow. While, a larger number, where most of the image is in focus, is called a deep depth of field.
Filmmakers can alter how much of the image is sharp or blurred by adjusting three settings or variables: one, the aperture of the lens, two, the focus distance and, three, the focal length of the lens.
Aperture is the opening at the back of a lens that lets light into the camera. On cinema lenses this is controlled by rotating the iris ring on the barrel. The wider this hole is, the more light is let through and the lower the T-stop or f-stop number will be.
Cinema prime lenses will often have very wide apertures with stops around T/1.3 to T/2.8. Changing this to a narrow aperture, such as T/16, will let in much less light.
The aperture that cinematographers choose has a large influence on depth of field. Opening to a wide aperture like T/1.3 will create a very shallow depth of field and increase the amount of blur in the background.
Closing to a narrow aperture like T/16 will deepen the depth of field, keeping a greater portion of the shot in sharp focus.
The second variable that affects depth of field is the focus distance. In other words, how far from the camera the lens is focused to.
This will be set to the distance of the object that the filmmakers want rendered in sharpest focus, which is usually an actor.
The closer this distance is between the camera and subject, the shallower the depth of field will be. Moving the focus distance farther away increases the depth of field, bringing more of the scene into focus.
This is why close up shots, where the actors are very near the camera tend to have a shallower depth of field, whereas wide shots, where the subjects are further away will have a much deeper depth of field.
The final variable is the focal length of the lens - which determines how wide the field of view is. Long focal length telephoto lenses, that capture a much more zoomed in image, produce a shallower depth of field which isolates subjects against blurred backgrounds.
While wide angle focal lengths naturally have a deeper depth of field which keeps more of the frame sharp.
Summing up the camera settings, if you want a super shallow depth of field image, which is mostly out of focus, then you can shoot with a wide aperture, like f/1.4, a close focus distance, like 4’, and a long focal focal length lens, like a 85mm.
Whereas you can achieve a deep depth of field effect with most of the shot in focus by using a narrow f/16 aperture, a far focus distance like 30’ and a wide angle 14mm lens.
One final additional variable which may come into play is the size of the camera’s sensor. Smaller format cameras require the use of wider focal length lenses to get a comparable field of view as a telephoto lens on a large format camera.
Since, as we now know, wider angle lenses produce a deeper depth of field, this means that using a large format camera, like 65mm, will typically give a shallower depth of field, while using a smaller format, like 16mm, will give a deeper depth of field.
DEEP DEPTH OF FIELD
Now that we know how to manipulate depth of field, let’s take a look at some films that did just that. Perhaps the most well known use of deep depth of field was in Citizen Kane, which was shot by cinematographer Gregg Toland. He also used this technique in a few other films which he photographed, such as The Best Years Of Our Lives.
Their reason for shooting these deep focused shots stemmed from Toland and director Orson Wells opting for angles and compositions which would allow scenes to take place in single frames - without needing to cut to various characters and shots during a scene.
In so doing, they layered information, having characters and actions play out both in the foreground as well as the extreme background. Such as this famous composition which places the parents and banker in the foreground of the shot, while simultaneously the young Kane, whose future is being discussed, plays in the background.
This then cuts to a three shot, another example of deep focus, where all the characters in the foreground, middleground and background are rendered sharp in a single composition, rather than the traditional method of cutting between single close ups of each character. This way we feel the performances and emotions of all the respective characters play out in real time.
Using a deep depth of field is also great for contextualising the space. If everything is soft and blurry we get far less of a sense of the environment that the character is in and don’t feel the full vastness of the landscape. Having a deeper depth also lets audiences explore the frame with their eyes and peruse the reactions of various characters in the background.
Coming back to our depth of field variables, how did Toland practically pull off these deep focus shots in Citizen Kane? He started by selecting a wide angle focal length - in this case a 24mm prime lens which was the widest commonly available focal length of the time.
To further increase the depth of field, he turned to the second variable, aperture. Rather than shooting at a stop of f/2 or f/2.8 which was popular at the time, he closed down the aperture all the way to between f/8 and f/16 for all the deep focus shots in Citizen Kane.
Shooting with a wide angle lens and a deep stop enormously expanded the depth of field, creating images with crisp, deep focus.
However, there’s one practical challenge that came as a result of stopping the lens down so much. The smaller the aperture the less light is let into the camera.
When this was combined with the slow film stocks of the time (Citizen Kane was photographed on Super-XX emulsion with an ISO of only 64) this meant that an enormous amount of artificial light was required to expose the image.
Tolland used a number of high output arc lights to have enough illumination. When these high output fixtures are blasted directly at actors, they tend to create hard shadows against walls.
Many older movies with interior scenes which were shot with a deep depth of field have these trademark hard shadows, due to the high output lighting which was needed to shoot on slower film stocks with a closed down aperture. In comparison to many of today’s films which are shot with a shallower depth of field and softer light.
Despite shooting with a wide lens and a deep stop, many of these shots are still not pin sharp across the entire frame. Either the foreground or background subjects are slightly soft. However, to spread the depth of field as much as possible, Tolland used the third variable of setting the focus distance.
To do this he used a technique called splitting the focus. Instead of placing the focus on the foreground subject and letting the background fall away out of focus, he would instead use the depth of field as a whole.
He would place the focus somewhere in between the back subject that needed to be sharp and the front subject. Normally the focus distance would be placed somewhere in the middle, yet slightly favouring the front subject, since the depth of field is more limited at the front distance than at the rear.
SHALLOW DEPTH OF FIELD
While a deep depth of field expands the context of our gaze, using a shallow depth of field has the opposite effect of limiting, focusing and guiding the eyes of the audience to a specific part of the frame.
Throwing the background out of focus is a very simple but effective device that immediately lets filmmakers show the audience what the subject is that they want them to look at - whether this is an object, or, usually, a person.
When a shallow depth of field is paired with a close up to draw our attention onto one person, it creates a more elevated, subjective emotional tone, forcing us to sit and empathise with a character.
Compared to wider, deeper depth of field shots which are often a bit more objective and don’t hold quite the same emotional power.
One example of a film that uses a shallow depth of field is If Beale Street Could Talk. Director Barry Jenkins and DP James Laxton used shallow focus as a tool to isolate his two main characters, putting them at the emotional centre of this cinematic world.
Laxton used all of those variables we discussed to maintain a blurry background. Him and Jenkins used this to create as immersive an experience as possible, which positions the audience in the shoes of the character.
He chose a large format cinema camera, the Alexa 65, which was paired with DNA primes lenses that could be shot wide open with a fast T/2 aperture. The large format sensor allowed him to use longer focal length lenses which made the depth of field shallower.
This meant that it wasn’t only the close ups that had a very blurred background, but even the wide shots had a very shallow depth that could be used to isolate our focus onto specific characters in the frame.
Such as this wide shot, which was done on a long focal length Hawk 150-450mm telephoto zoom. Combining the depth of field variables of a large format camera, a long focal length lens and a wide open T/2.8 aperture, meant they could shoot a wide shot with a deeper focus distance and still blur everything in the frame except the two characters who were the, literal, focus of the shot.