20.07.10
Photo Zone: Depth of Field
Professional photographer Martin Middlebrook explores depth of field and explains how to use it for maximum creativity in your images.
Depth of field (DOF) controls us as much as we control it. It directs our aesthetics, it needs time and light in quantities we cannot easily manage and rarely have, it forces restraint and generates freedoms, and it is all the more beautiful for that. Of all the mechanisms available to photographers, the control of aperture, and hence depth of field, is the most liberating.
It is greatly misunderstood, though, because of its variable nature. What is true of one lens is not true of another, what is true of one discipline is not true of another, and when you have come to terms with all of this, it matters not, because you are free to apply it however you wish. DOF describes how much of your image, front to back, is in precise focus, often called the ’zone of sharpness’.
Any element of your image that does not fall into this ’plane of focus’, will soften and eventually become blur (bokeh) the further away it falls from this plane. DOF relates to the size of chosen aperture. F32, small aperture equates to a large depth of field. F2.8, big aperture, equates to a small depth of field.
DOF relates to the focal length of the lens. DOF relates to the distance from the subject that the image is captured. DOF is usually greater behind the plane of focus than in front. There are three main controls that dictate DOF. They are aperture, focal length and distance from the subject. The relationship between all three provides the resulting DOF, but the possibilities are almost infinite, so it’s better to view this subject as a creative rumination, than a technical query.
When, many years ago I first tackled DOF for personal reasons, I grabbed my 90mm macro, bunged a roll of Fuji Velvia into my film SLR, set up a simple still life, and shot 36 consecutive shots, starting at F2.8 and kept shooting, decreasing the aperture by the half stops available, until I had run out of film. I can’t quite remember what my final aperture was, but it was a neat process that taught me all I needed to know about controlling DOF with aperture. Except it didn’t. In truth, it taught me very little indeed. All it taught me was how that lens would operate from that distance with that subject, and how it might control the aesthetics under those very specific circumstances.
The image of the common frog shown above is a case in point. It shows exactly the plane of focus with a 90mm macro, set at F2.8, with distance from the subject of about 2in. In doing so, it provides that the zone of sharpness is a matter of millimetres. But it also demonstrates what we mean by plane of focus. You can see in the diagram below that the zone of sharpness is a slice through the X dimension of space, a sliver of clarity that is all but lost before and beyond.
If I were to shoot this frog at F32, from exactly the same place, my guess is that half of the frog would have been in focus. If I were to photograph it at F32 from 6in away it would all have been in focus, but if I were to photograph it from 3 feet away again at F2.8, it would have all been in focus as well. As a rule, at any given aperture, depth of field will decrease as focal length increases. And the same applies as your distance from the subject decreases. I know how this approximates across all of my lenses, under almost any given circumstance, so I rarely have to check. So I know that, if I set F5.6 on my 17-40mm and I am shooting a landscape (with no obvious foreground element) at 17mm, then almost the entire scene will have front-to-back focus.
The temptation is to shoot at F16, but this may not be necessary. That is not to say that you won’t have more absolute focus by shooting at F16, but in many cases we simply won’t notice the difference. And yet if I shoot at F5.6 at 400mm at 10 feet from the subject, my DOF will be exceptionally shallow.
The point is that this relationship is so variable and so infinite, that really getting to know your lenses is the key to using aperture as a creative control. It’s so easy to do as well, especially if you use a digital camera. It isn’t going to cost you a roll of Velvia and development to boot, it’s going to cost about 10 minutes of your time. When you know how your lenses are going to operate across all available variables, then you can begin to make decisions that are based on creative factors.
As with all these things, it’s impossible to be definitive because of the following equation: Creative control = focal length + aperture ÷ by available light + ISO rating x speed of movement of subject.
Okay, so this clearly is not a real equation, but it indicates that, as with all things in photography, every element, be it available light or movement of the subject, has an effect on all other decisions; no decision can be taken in isolation, all elements are linked. There is not room enough here however to consider all variables, so I want to look at how and when you might choose to control absolute DOF, and the visual mechanisms you might employ to manage this.
PORTRAIT ISOLATING THE SUBJECT WITH A SHALLOW DEPTH OF FIELD
With a 50mm lens at F1.8, the zone of sharpness is through the eyes only. This isolates the subject, avoiding the distraction of background clutter. Additionally, we can take advantage of the blur that is created, so that highlights and shadow dance across the frame, adding lustre and energy, bringing the subject to life. It adds a sentimentality to the image that suits our subject — we connect more directly as a result.
ENVIRONMENTAL PORTRAIT PROVIDING CONTEXT WITH A LARGE DEPTH OF FIELD
Portraiture is more than just about having a beautifully focused frame-filling subject. Sometimes we are trying to weave a story into our images. We are hoping to provide a context that conveys a sense of place and time, of heritage and ethic to our subject. To do this we need additional information. Shot with a 17mm, I have achieved front-to-back focus with an aperture of only F4 in the image above right. I increased ISO and used flash. This combination was sufficient to provide enough light and DOF, in what was a very dark place, enabling me to tell a story.
However, there is a supplementary angle to this image that I cannot emphasise enough. So often you hear photographers saying that they shot at F4 to isolate the subject, but this is often a half truth at best.
When you are photographing a red fox at dusk with a long lens, this combination alone forces you to shoot at F4. You need every bit of available light to get any kind of reading at all, let alone distill the subject at 1/125th. The result is a crisply focussed subject with wonderful background blur, by dint of pure physics. But we could have shot it at F16 by uprating to ISO 800 and provided additional information that could help in telling a different kind of story. Given the available light, our cameras will choose the largest apertures, but this may not be the best way of capturing our subject. Be flexible!
LANDSCAPE:
WIDE DEPTH OF FIELD OR SHALLOW COMPRESSED IMAGES
Landscape photography is more confusing because you do not have to stick to a set of prescribed rules. Compression is a valuable tool in landscape photography. It’s not always about shooting the whole horizon with a wide-angled lens set at F22. And even when you do, you may not need such a small aperture. By shooting with a 400mm lens set at F5.6, you can achieve front-to-back focus, giving a wonderful sense of compression, a stacking of layers like a pop-up book.
ARCHITECTURE
Typically when photographing architecture, front-to-back focus is essential. Small apertures are the order of the day. ISO 50 at F16, 17mm and 3.2 seconds. Depth all round!
MACRO
For those close-ups, big apertures guarantee beautiful bokeh, which can suffuse the whole image with an ethereal resonance. However, with macro, DOF can be too shallow to get all elements in focus. While F2.8 will give you this dreamy quality, it’s important that you record sufficient detail to complete the picture. Use your depth of field preview if you have one, know your lenses, or take test shots.
USING MECHANISMS:
DRAW THE EYE INTO THE FRAME
Photography exists for many reasons, but at its heart, it is a method of visual communication. And to allow us to maximise the potential of any image, we use mechanisms that allow us to harness the potential of the frame. Some of these techniques are very literal, such as placing specific tangible objects within the frame to tell the story for us. Others are more suggestive, however, such as compositional elements that draw our eyes to the subject. By minimising the point of focus and using a foreground element to draw the eye into the frame (see below), I have been able to break all compositional rules by placing the subject at the top of the frame. But the out-of-focus elements that sharpen through space, draw your eye to the subject. It’s a good example of using DOF to aid composition.
ADDING DEPTH AND COLOUR TO YOUR IMAGES
Blur is not just about isolating the subject, it is as much about adding extra dimensions and vivacity to our images. By shooting with a shallow DOF and deliberately placing out-of-focus elements in front of our subject, we can animate an otherwise flat scene.
Photographing Groups:
I include this fairly arbitrary subject because it is something we do a lot of and that we so often get wrong — and that includes me. Group shots — you need F11 and to increase your film speed/ISO accordingly. Don’t hold on to quality and lose light and DOF, you will kick yourself. You can never arrange a group of people such that their plane of focus is equivalent. And we don’t always appreciate at the time that 50% of the group will be subtly out of focus. Finally we have a shot where everyone is smiling, they are all looking at the camera and no one is blinking. It’s in the can and we stop shooting. But I will guarantee you that, if you have shot it at F5.6 with any lens longer than 40mm, some people will be out of focus.
Anticipating and allowing for movement: Controlling DOF is not all about changing the aesthetic nature of your images. DOF provides a zone of sharpness within which we want to place our subject. If your subject is moving towards or away from you, it makes sense to increase the zone of focus to allow for this movement. The shot above is a good example of anticipating zone of movement. The weather was perfect; I could have shot almost anything at ISO 50. To get this shot, though, I needed bundles of speed and a sufficient zone of focus to guarantee a truly sharp image. To achieve this I had to shoot at 1/1,000th and F8. So, I uprated to ISO 640 on a bright sunny day.
Sometimes you have to compromise on absolute quality to achieve your goals. However, by lying on my front I was still able to introduce a blurred foreground, and the drop-off of focus behind the subject still isolates it nicely. Isolating the subject by moving it away from a background:
If we accept that DOF is often twice as great behind the subject as the area of focus that lies in front of it, we are often presented with a problem. We want to achieve great background blur, but the background overpowers the scene. Wherever possible, move your subject closer to you and further away from the background. The background stays in the same place, and you don’t move, but by moving the subject you can isolate it better.
BE CREATIVE
Once you have got to grips with how depth of field changes across apertures and lenses, then you can begin to view aperture as your greatest creative weapon. By shooting through obstructions with a shallow depth of field, we can animate our images, or we can simply abstract them, so that they move away from literal interpretations of events and towards art. Or we can shoot wide with bags of focus, enabling us to introduce a narrative into our picture taking, so that we can allude to a sense of place and character.
Photography is visual communication, so the choices we make should pertain to resolving the best image for the story we wish to tell.
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