The Pros And Cons Of Shallow Depth Of Field

INTRODUCTION

Ever noticed how some shots in movies have a blurry background, while in others everything is pin sharp across the entire frame? This is due to the depth of field of an image and is more often than not a conscious choice that is made by filmmakers. 

Shots with a super soft, out of focus background have what we call a shallow depth of field. While those that have large areas of the image in focus have a deep depth of field.

Let’s break this down a bit more as we dive into what depth of field actually is, the factors and settings that can change it, and then go over some of the pros for shooting with a shallow depth of field, as well as go over some of the reasons why shallow focus may actually be undesirable. 

WHAT IS DEPTH OF FIELD?

Depth of field is a measurement of the distance between the nearest point that a lens renders in sharp focus and the furthest object that is sharp. 

For example, one could shoot a close up shot of a character on a telephoto lens where the nearest point of focus are their eyes and the furthest point of focus are their ears. In this example, the distance between these two points, the depth of field, is a measly 3 inches. This is what we’d call shallow focus. 

In another example, a camera may shoot a long shot on a wide angle lens where everything from the foreground to the horizon is in sharp focus. In this example the distance between those points is so far that we just call it infinity. This is what we call deep focus. 

Based on those examples, we can establish that there are a few different variables that change how much depth of field an image has. In fact there are three variables: the focal length, the distance to the in-focus subject and the aperture of the lens.

Shots captured with a telephoto lens that has a long focal length - such as a 290mm zoom have a much shallower depth of field than shots that use a wide angle lens - such as an 18mm lens - which will create a deeper depth of field. So one way to create a background with more blur is to choose a longer focal length.

The second variable for depth of field is determined by where the focus distance is set. The nearer to the camera that the subject is and the tighter the shot, the shallower the depth of field will become. This explains why when you shoot an extreme wide shot that focuses on the horizon most of the frame will be sharp.

Finally, the third variable that filmmakers can use to change the depth of field is the aperture or stop of the lens. The wider open the iris on the back of a lens is, the lower its T-stop will be and the shallower the depth of field it will produce. 

One reason why fast lenses such as T/1.3 cinema lenses are desirable are because cinematographers can shoot them wide open to create a background full of soft bokeh. 

When a long focal length lens, a subject close to the camera and a fast aperture are all combined - much to the horror and disgust of the focus puller - the depth of field that a camera captures will be very shallow.

Inversely a wide focal length lens, a subject far away and a closed down stop will mean that the depth of field will be very deep and the focus puller can relax.

There’s also a fourth variable, the sensor size, which doesn’t directly affect the image's depth of field but does affect it indirectly. Shooting on cameras with a larger sensor size produces images that have a wider field of view. To compensate for this extra width, cinematographers will either shoot on longer focal length lenses to produce a comparable field of view, or are forced to physically move the camera closer to maintain a similar frame. 

As we now know, those two actions, using a longer focal length and focusing on a subject closer to the camera will both make the depth of field shallower. 

PROS OF SHALLOW DEPTH OF FIELD

The biggest cliche about images with a blurry background is that they look ‘cinematic’. The idea of a ‘cinematic’ shot can’t only be tied down to a specific image characteristic. I mean, obviously there have been plenty of gorgeously shot pieces of cinema that don’t use a shallow depth of field. 

However, sometimes cliches have an inkling of truth to them. To understand the link between images with a shallow depth of field and cinema, we need to go back to the days before digital cinema cameras. 

In the early days of video, most cameras had little sensors, wider lenses and slower apertures. While movies captured on 35mm film used a larger film plane, longer, faster lenses. 

So the ability to capture images using a shallow depth of field was technologically limited to filmmakers that shot for the big screen, while deeper focus had associations with the less highly regarded video format.

Although this has now changed, with advances in digital technology making it easy for even entry level cameras or smartphones to simulate a shallow depth of field, I’d argue that there’s still an unconscious mental association that persists between a shallow depth of field and ‘cinematic’ movies in the mind of the audience.

With that out of the way, I’d say that the single greatest practical use of shooting with a shallow depth of field is because it allows filmmakers to control what they want the audience to see and ‘focus’ their attention to.

The smaller the depth of field, the less information in a frame will be in focus and the more power the focus puller has to show where the audience should direct their gaze. 

It makes it possible to more easily isolate a certain character or detail in a frame. The more you isolate a character from the background, the more they stand out and become the central point of the story. A shallow depth of field therefore empowers filmmakers to visually tell stories from a more subjective viewpoint. 

Depending on the context, a shallow depth of field can also be used for other, more creative, applications. Because a super shallow, drifting focus makes images feel dreamy, it can be used as a tool to chronologically differentiate certain scenes from others - such as using it as part of a different visual language for flashback scenes.

Shots that drift in and out of focus may also be used as a deliberate technique to emulate a less controlled environment and make certain sequences like action feel faster, more panicked and more dynamic.

From a purely technical point of view, shooting a lens wide open also lets in more light and makes it easier to achieve exposure in darker shooting environments. This also means that smaller lighting setups will be needed for scenes in darker spaces, like night exteriors - where shooting at a deep stop is rarely practically possible. 

Another technical point is that cinematographers choose certain lenses over others because of their visual characteristics and how they render an image. The wider the aperture and the shallower the focus, the more pronounced these characteristics, such as their bokeh and focus falloff, become. 

It’s almost seen as a bit of a waste to shoot a beautiful, vintage set of lenses at a deep stop. As you close down to around T/8 or T/11 most lenses will become increasingly sharp across the frame and will be more difficult to differentiate from each other. So for those who want to create a level of soft texture to the images, shooting at a faster stop is prefered.      

CONS OF SHALLOW DEPTH OF FIELD

While shooting with a shallow depth of field is wildly popular in the film industry, there are also some reasons and situations where it may not be desirable.

I mentioned before that shallow focus can be used to tell stories by guiding the audience’s gaze towards a specific part of the frame, but inversely a deeper focus can also be used to tell a story in a different way.

Shooting a film with a deep stop, where more of the frame is in sharp focus, allows the audience to peruse the environment and pick out information from it themselves - rather than having those details spoon fed to them with shallow focus by a filmmaker. In this way a deeper focus presents stories in a way that is subtly more objective.

Another persuasive case for a deeper depth of field is that it allows you to see more of the set and environment that the character is in. I remember a focus puller that I used to work with who would voice his surprise, especially at younger DPs, who would always shoot every shot with the aperture wide open and make the background as blurry as possible. 

Why travel all the way to a beautiful location, or spend loads of money constructing an incredible set, only for the audience to not see any of it because the background is so out of focus?

Deeper focus shots that see the location are a useful tool for the audience to place where exactly the character is in their minds. 

Moving on to the practical side, and being fully transparent, that focus puller may have advocated for a deeper depth of field because it makes their job of keeping the subject sharp much easier. The shallower the depth of field is, the less margin for error focus pullers have to accurately pull focus and maintain a higher ratio of shots that are usable.

This is why if there is a particularly challenging focus pull, the DP may chat to the 1st and stop down the lens a couple of stops to help achieve more accurate focus. If you’re short on shooting time, sometimes it’s better to sacrifice a smidge of buttery smooth bokeh in order to maximise the number of takes that will have usable focus. Rather have four usable takes for the director to work with in the edit than one take that is in focus that has a shallower depth of field.

Another case where a deeper depth of field may be preferred is when shooting a two shot. As the name suggests this is a single shot with two people in the frame. Sometimes these two characters may be placed at different distances apart from the camera. When shooting with a shallow depth of field, this may mean that only one of the people can be rendered in sharp focus, because the depth of field doesn’t extend far enough to the second character.

A solution to this is to shoot with a deeper depth of field and get the focus puller to do what is called splitting the focus. This is where the lens is stopped down and focused to a distance in between the two characters - so that the depth of field extends nearer to get the first person in focus, and further to get the back person in focus at the same time.

Before I mentioned that shooting wide open accentuates the interesting optical qualities of the lenses, however, for certain films the look may be more suited to shaper images. The more that a lens is stopped down, the deeper the depth of field becomes and the crisper and more accurately resolved the image will be.

This is particularly useful when shooting with certain old, wide angle anamorphic lenses such as the Cooke Xtal Express set. The wide focal lengths in this set have a huge amount of focus falloff when shot wide open with the ‘sweet spot’ of the lens only rendering sharp focus in the very centre of the frame. 

So to minimise actors looking soft on the edges of a shot and to sharpen up the lens to an acceptable level, some DPs prefer to shoot these lenses with a deeper focus at a stop such as T/5.6 or T/8.