We have all seen those photos with the subject in focus while the background is creamy and blurry. This is called bokeh. This term comes from a Japanese word and means the blur produced in the out of focus areas of an image produced by the lens. Understanding depth of field can help you produce those dramatic images. Once you have an understanding of depth of field, you can then control it and produce the effect whenever you want, within certain limitations.
Depth of field is the range within the image which is in focus. This can be a very thin slice of the image, as when taking a macro photo. It can also be the whole image, as in a landscape photo with the lens set at infinity focus.
This image is an example of a fairly shallow depth of field. Note how the background is blurry while most of the spider web is in focus.
As you can see, the trees and the light coming through the branches are out of focus in the background. The part of the web in the foreground is out of focus as well. Only the main subject of the spider web is sharp and in focus. This was shot at a relatively wide open aperture of f5.6. Using a lens capable of an aperture of f2.8 or even f1.8 gives you even more shallow depth of field.
Here is a landscape photo taken with a small aperture. As you can see, the whole image is sharp and in focus.
This image was shot with the aperture set at f16. So, by these photos, you can see how aperture affects the depth of field. Now let’s get into understanding depth of field and how you can manipulate it to get the effect you want in your images.
There are several ways to control depth of field creatively in your images.
The distance to the subject- This is obvious to understand when you look at shooting macro photos vs. shooting landscapes. In macro photography, your lens is very close to the subject. When you focus your subject, there is only a razor-thin area of the image front to back that will be in focus. That is why tripods are absolutely necessary for shooting macro photos of still life. The act of breathing can move your camera enough to change the focal point. On the other hand, when shooting a landscape if you focus on an object on the horizon, the entire image will be in focus because the focal point is so far away.
The lens focal length– If shooting with a 50 mm vs a 200 mm lens, the 50 mm lens will have a greater depth of field. You can use this to your advantage if you only have a zoom lens with you. To take a portrait with a zoom lens that has a blurry background, you can move away from your subject and zoom in to fill the frame. You will notice that now the background is blurred while the subject is sharp and in focus.
Use aperture to control depth of field– Probably the way most of us create those blurry backgrounds is with a wide aperture. This is where a good quality lens pays for itself. Many kit lenses will only open the aperture up to f4.5 or f5.6. You want to have a lens with a maximum aperture of f2.8 or even f1.2 to get great blurry backgrounds in your portraits. The wider the aperture (lower the f-number) the shallower the depth of field. The smaller the aperture (larger f-number) the deeper the depth of field and more of your image will be in focus. It may be easier to remember that as the f gets smaller, so does the area in focus. When the f gets bigger, the larger the area in focus becomes. Just be aware that you cannot change the aperture totally as you wish. That’s because when you change the aperture, you change one value of the exposure triangle. (The exposure triangle is the subject of another post on its own!) So when you change the aperture, you either need to change the shutter speed or ISO to compensate and get the correct exposure. If you shoot manual, you will have to do this yourself. If you shoot in aperture priority mode, you set the ISO and your camera will choose the correct shutter speed.
On my Canon cameras, there is a small button on the right of the lens mount that is called a depth of field preview button. It allows you to see how the final image will look at the chosen aperture. Consult your camera manual to find out if your camera has this capability.
Understanding depth of field is important if you want to have creative control of your images. The best way to learn how these things affect your depth of field is to get out your camera and practice. using different lenses and settings, find out how they make your images look. Play around with both the distances to your subject and the aperture settings. Attach your zoom lens and take photos of an object from the same point as you zoom your lens at different zoom settings. Notice as you zoom in how the background blurs. Use different apertures on each lens to see how setting the lens at its widest aperture affects the background vs. a middle setting such as f8, or even higher at f11 or f16. Use aperture priority mode and see what the camera changes the shutter speed to as your aperture changes. If you have a DSLR try setting the aperture and shutter speed manually to see how these affect each other. Doing this can also reveal creative settings that you can use later. If you find something you really like, write the settings down in a notebook to remind you later.
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