It is one of the most important and powerful elements in photography and filmmaking. Creating great images and footage requires control of depth of field. Depth of Field can be somewhat of a mystery to those first starting out on their photographic journey. However it should be considered as one of the vital elements in building your knowledge of photography.
So before we go any further, we should define exactly what depth of field is. Put simply, it is the amount of the image that is in acceptable focus. This is both behind and in front of the point you actually focused on. Lets say your focus point was 5 meters. If there was acceptable focus one meter in front and two meters behind your depth of field is three meters. However in photography we rarely quantify that distance, Instead we generally use the terms shallow and deep to determine what we are looking for.
Shallow or Deep Depth of Field
A shallow depth of field, where there is little in focus front and behind the focal point is a very powerful creative tool. For example if taking a close up portrait, by using a shallow depth of field we can draw our viewer’s attention to the models eyes by having the rest of the image out of focus. Conversely, a landscape photographer may use a deep depth of field, with the image in focus from very close to the camera all the way to the horizon.
A shallow depth of field is also important in film making. It is one of the elements that go to make up the “cinematic look”. This has become much more accessible to film makers since the advent of video on DSLRs and Mirrorless systems.
Depth of Field and Aperture
Depth of field is controlled primarily by the aperture of your lens. A wide aperture, for example f2.8 or wider will give a shallow depth of field. Using a small aperture, maybe f16 will create a deep depth of field.
Focal Length and Position
This however is not the full story. DofF is also affected by the focal length of the lens and your relative position to the subject. Wide angle lens have a much greater depth of field than telephoto lenses for the same aperture. If you are using a 20mm lens at f2.8 and your distance subject is 5 meters, the depth of field will be much greater than if you were using a 135mm lens.
If you change your position to the subject your depth of field will also change. If you move closer, the DofF will become shallower, move further away it becomes deeper.
How do we Use Depth of Field
So, armed with the knowledge of what DofF is, let’s see how it applies in the real world. Returning to landscape photography, for most landscape images, the photographer is trying to lead the viewers eye into the image. A common way to do this is to use a leading line. This is basically an object within the scene that will lead the viewers eye from the foreground out to the horizon. For this to work, the leading line needs to be entirely in focus. If the start of the line is out of focus the viewer will not connect with it. To achieve this, the landscape photographer will generally use a wide or extreme wide angle lens. They will find a position close to the start of the leading line and use a very small aperture, f22 not being uncommon.
It is entirely possible that even under these circumstance he cannot get the entire image in focus, but fortunately to aid this, he has one further technique at his disposal, hyperfocal distance.
The hyperfocal distance is the point where all objects, beyond the point of focus are in acceptable focus, all the way out to the horizon. The hyperfocal distance can be calculated quite accurately if you know the distance to the horizon and have the equations to hand. However for many of us this is not usually the case. There is however, a simple and effective rule of thumb and that is to focus two thirds of the way to the horizon. This will generally give you the optimum hyperfocal distance for the aperture. A little experimentation with the focal distance will go a long way to nailing the best hyperfocal distance.
Why Use a Shallow Depth of Field
So lets look at the other side of the coin, shallow depth of field. A shallow DofF is a great way of isolating our subject with in the image. As we said earlier, a portrait photographer might use a shallow DofF to draw attention to the eyes of a model. The typical portrait lens is the 85mm f 1.4 or f1.8. This is a very wide aperture lens and when using it at those ultra wide apertures care would need to be taken that there is not too much out of focus.
One of the best tools on many cameras to combat this problem is the Depth of Field preview button. Pressing this will stop the lens down to the required aperture, allowing you to see how shallow the DofF is. If your camera does not possess one, the next best option is to take test shots before starting the main shoot. One of the side effects of wide aperture and shallow depth of field is the strangely named Bokeh. This is a term to describe the quality of the out of focus regions and can vary from lens to lens. When considering a lens for wide aperture shooting, Bokeh should be a primary consideration.
A Cinematic Look
Shallow depth of field is an often used technique in cinematography. It focusses the attention of the viewer solely on the subject rather than the frame as a whole. By the clever use of depth of field filmmakers can hint at the environment we are looking at without showing it in sharp focus.
So there you have it, a brief guide to the wonderful world of depth of field. As we said at the top, all photography is a balancing act and DofF defiantly falls into that area. If you want deep depth of field, you are going to need slower shutter speeds. This gives an increased likelihood of camera shake, and may require a tripod. Conversely a wide aperture might provide a shutter speed that is too high for the flash sync causing a rethink on the shot. However learn to balance out these potential problems and you will soon be on the way to become a master in the dark art of Depth of Field.