One of the most important and powerful aspects to creating great images is the control of depth of field. Depth of field is the amount of the image that is in focus, both behind and in front of your main focal point. In other words, if your focal point is five meters from the camera, and there is acceptable focus one meter in front of that point and two meters behind it, then your depth of field is three meters. However, in photography, that distance is rarely quantified. Instead, the terms “shallow” and “deep” are used.
Shallow and 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. In a close-up portrait, a shallow depth of field draws the viewer’s attention to the subject’s eyes by having the rest of the image out of focus. Conversely, a landscape photographer may use a deep depth of field, so that the image is in focus from very close to the camera all the way to the horizon.
Depth of field is controlled primarily by the aperture of your lens. A wide aperture, f/2.8 or wider, will give a shallow depth of field. Using a narrow aperture, such as f/16, will create a deep depth of field.
Depth of field is also affected by the focal length of the lens and your relative position to the subject. Wide-angle lenses have a much greater depth of field than telephoto lenses. If you are using a 20mm lens at f/2.8 and your distance subject is 5 meters, the depth of field will be much greater than if you were using a 135mm lens under the same conditions. If you move closer, the depth of field will become shallower. If you move further away, it becomes deeper.
Returning to landscape photography, for most landscape images, the photographer is trying to lead the viewer’s eye into the image. A common way to do this is to use a leading line, which is an object within the scene that will lead the viewer’s eye from the foreground 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. A landscape photographer might use a wide-angle or ultra wide-angle lens, a position close to the start of the leading line, and a very small aperture, such as f/22.
Even under these circumstances, it is possible that a photographer still cannot get the entire image in focus. There is one more technique available to aid this: hyperfocal distance.
Hyperfocal distance is the point where all objects beyond the point of focus are in acceptable focus, all the way out to the horizon. A simple and effective rule of thumb is to focus two-thirds of the way to the horizon. This will generally give you optimum hyperfocal distance for the aperture. A little experimentation with the focal distance will go a long way to nailing the best hyperfocal distance.
Let’s look at the other side of the coin: shallow depth of field. A shallow depth of field is a great way to isolate a subject within the image. As stated earlier, a portrait photographer might use a shallow depth of field to draw attention to a subject’s eyes.
The typical portrait lens is the 85mm, f/1.4 or f/1.8. This is a very wide aperture lens and, when using it at those ultra-wide apertures, care needs to be taken to ensure 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 depth of field 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 bokeh. This is a term used 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.
So, there you have it—a brief guide to the wonderful world of depth of field. If you want deep depth of field, you are going to need slower shutter speeds, which leads to an increased likelihood of a camera shake during exposure.
Conversely, a wide aperture might provide a shutter speed that is too high for the flash sync, causing a rethink on the shot. Learn to balance out these potential problems, and you will soon be on the way to becoming a master in the art of depth of field.