We have all seen 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 that are 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. In this article, we will look at how your camera settings and your lens can affect depth of field, and how you can manipulate those variables to help you create that look in your photos.
Depth of field is the range within an image that is in focus. This can be a very thin slice of the image, as when taking a macro photo, or can be the whole image, as in a landscape photo with the lens set at infinity focus. To truly understand depth of field we first need to understand the exposure triangle.
The Exposure Triangle
I have to admit one of the toughest parts of starting out in digital photography for me was understanding the exposure triangle. The exposure triangle shows the relationship between aperture, shutter speed, and ISO settings on your camera. These are the three things that affect the exposure of an image. I know most people just go with the Auto setting when they buy a shiny new DSLR camera and let the camera decide the settings. This may be fine for well-lit scenes with very little contrast, but you soon run into problems as you start to shoot in more diverse environments. The problem is that although cameras are pretty smart, they still can get it wrong. If you really want to take artistic control of your photos and not just shoot snapshots in Auto mode, you have to understand the exposure triangle. Luckily, you don’t have to know all the higher-level math behind the settings. If you are like me, you just want to know what each of them does and how one affects the other, right? So let’s look at each point on the exposure triangle in more detail.
The first setting in the exposure triangle is the aperture setting. Aperture refers to the opening formed by the blades in the lens that determine the amount of light hitting the camera sensor. This is what is referred to as the f-stop number. As you can see from the graphic, higher numbers mean a smaller diameter opening for light to enter the camera and fall upon the sensor. Lower numbers mean more light hitting the image sensor in the camera. Changing the aperture setting from, for example, f4 to f5.6 means you double the amount of light hitting the image sensor. The aperture setting also controls the focal plane in the image. With a smaller aperture like f16 or even f22, the image will be in focus from very close all the way out to infinity. When you open the aperture wide, you narrow the focal plane. This is how you take photos with the subject in focus and the background blurry. It leaves you with a very narrow focal plane.
If you shoot in aperture priority mode, the “A” setting on Canon cameras, you set the aperture and the camera will then adjust the shutter speed according to what the internal meter thinks is a proper exposure, given the ISO you have set. Many times the camera gets it right, but sometimes the camera will underexpose or overexpose your subject. That’s why learning the exposure triangle is so important. When you understand how it works, you can shoot in full manual mode to achieve the creative look you want. However, for many shots aperture priority will do a good job for you, and thinking about setting the shutter speed is one less thing you will have to worry about.
There are also times when it may be that you want to set the shutter speed to achieve a certain look, such as making flowing water look silky. We do this by setting the shutter speed, so let’s look at how that works.
Shutter speed refers to the amount of time the shutter is open and allowing light to strike the image sensor. Fast shutter speeds freeze action. Slower shutter speeds can smooth out running water or help with gathering enough light in night photography. Slow shutter speeds can also introduce blur in your images in creative ways. Below are examples of a fast shutter speed and a slow shutter speed.
As you can see, changing shutter speed can help you creatively when capturing an image. But when looking at the exposure triangle, you need to be aware that you need to adjust your aperture when you change your shutter speed to get an acceptable exposure. A slow shutter speed will allow more time for light to strike the camera sensor, which will increase the exposure and make the image brighter. Using a shutter speed slow enough to create the image of the waterfall above, the image would be completely blown out without stopping down the aperture to f-16. The other option is to change the camera into shutter priority mode and let the camera automatically choose the aperture for you.
ISO stands for International Organization for Standardization. This designation has carried over to digital photography from film, where the film speed referred to the light sensitivity of the film. Now, ISO refers to the sensitivity of the digital sensor to light. Higher settings mean the sensor is more sensitive to light. There is a trade-off, however. Digital sensors can also have digital noise induced by adding sensitivity. Typical DSLR cameras have the ability to set ISO from as low as 50 to 12,800 or higher. Unfortunately, as stated, a higher ISO setting will introduce grain and noise into your image. It’s best to experiment and see the maximum ISO setting your camera will allow and still produce an acceptable image. That way you will know the limits of your camera when it becomes necessary to increase your ISO, such as when shooting photos of the night sky.
Using Depth of Field Creatively
There are several ways to control depth of field creatively in your images. Let’s look at a few ways to control depth of field and how the setting will affect your final image.
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 on 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 simple act of breathing can move your camera enough to change the focal point and ruin the image. 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.
Focal length– If shooting with a 50 mm vs a 200 mm lens, the 50 mm lens will have a greater depth of field for a given aperture setting. 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. This has to do with the physics of the optics in the lens and is beyond the scope of this article.
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, as explained earlier. 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 Canon DSLR cameras, there is a small button to the right of the lens mount that is called the 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.
Depth of Field In Landscape Photography
In photography, careful use of depth of field can be a very powerful tool. It can force viewers to focus only on the sharp areas by utilizing a shallow depth of field, so the background in the image is out of focus. This draws the eye to the area of the image that is in focus, putting your subject as the focal point of the image. This concept is most often seen in portrait photography, where the photographer uses it to create a background that is blurred so that the subject stands out as sharp. Our eyes are not comfortable viewing images that are out of focus. Our eyes tend to be drawn to the parts of an image that are sharp and in focus. Our gaze will focus on that part of the image, rendering the other unsharp parts of the image as blurry and not worthy of our attention. This use of a shallow depth of field is particularly well suited to portraiture. As long as the eyes in the image are sharp, most other things can be forgiven if they aren’t pin sharp. People tend to look at the eyes first, and so the eyes really need to be sharp in nearly all portrait photography.
Landscape photography is generally at the opposite end of the spectrum as far as the depth of focus we want to achieve. The vast majority of landscape images require a very deep depth of field, with all the elements in the scene sharp and in focus. This is due to the fact that landscapes generally are trying to emulate an actual scene as the eyes see it. Viewers are usually drawn into the image by its great depth of field, and they tend to look at more details.
As discussed earlier, depth of field is controlled in two ways. The most common way to control it is by shooting in aperture priority mode or setting the aperture in manual mode. The smaller the aperture (the larger the number, i.e. F22), the greater the depth of field. The larger the aperture, (smaller number like F2.8), the shallower the depth of field. The apertures in-between have a depth of field that is directly proportional to the aperture selected. Most lenses also have a sweet spot where the sharpness is best. This is due to the diffraction of the light as it passes through the lens elements.
Depth of field can also be dictated by the focal length of the lens, and the camera format for which the lens is used. For instance, a wide-angle lens always has a much greater depth of field than a telephoto lens. A very wide-angled lens such as a 14 mm lens has a depth of field so great that it doesn’t require much in the way of focusing. On the other hand, a 600 mm telephoto lens has an extremely shallow depth of field when zoomed all the way to 600 mm. Unless you are focusing on a subject very far away at 600 mm, the depth of field will always be very limited.
There are also macro lenses, which are made to be able to focus very close to objects. Once you start moving in and start focusing very closely, the focal plane will become extremely shallow and very narrow. As you move the lens closer to the subject, the focal plane will become razor-thin. In extreme close-ups, even the slightest movement will cause the image to go out of focus entirely. Although you wouldn’t use a macro lens for landscape photography, it does illustrate the depth of the focal plane and how it works.
So remember with landscapes, you want to use a smaller aperture (higher number) to increase the area that is totally in focus in your image. The greater depth of field will allow you to capture all the elements of the image in sharp focus. You will probably need to use a tripod though because as your aperture decreases your shutter speed will need to increase to get a useful exposure. You will need to increase the shutter speed yourself if you are shooting totally manual. If you choose aperture priority, the camera will choose the shutter speed for you after metering the brightness of the scene. Unfortunately, although modern digital cameras are very good, sometimes they still get it wrong. You should view the histogram on your view screen and make sure the histogram is not jammed against either end of the scale. Using the histogram is a subject for an entire article all on its own.
Now that you have a basic understanding of depth of field, and you want to take your photography journey to the next level, you might like the Photography Master Class. This video course will definitely help you on your journey to be a better photographer.