A common term in photography, depth of field is important to consider when creating any photograph. It is used at varying degrees to place either everything in the image into a sharp focus or to narrow the focus and highlight a subject, allowing other elements to be blurry.
Photographers use depth of field to create certain effects and draw the viewer's attention to particular elements of the scene. It is important to understand how the aperture setting on your camera, the focal length of the lens, and the distance of your subject affects a photograph's depth of field.
What Is Depth of Field?
Depth of field is the amount of your image before and beyond your focus point that will be in focus. You should understand what depth of field is because it will tell you whether or not your subject and background can be sharply focused at the same time.
Depth of field is determined by three primary factors:
- Subject distance
While the camera can actually only focus on one tiny point in space, the depth of field determines how much of the image is in "acceptable focus" to the human eye.
- In subjects such as landscapes, a large depth of field is often desired so that the entire scene appears to be in focus.
- In subjects such as portraits, a small depth of field is often used to blur the background and reduce distractions from the main subject of the image.
The Effect of Aperture/F-Stop on Depth of Field
The primary control of depth of field is the aperture, or f-stop, setting on your camera. Apertures range from f/1.8-f/64 and each lens you place on your camera will have a different aperture range which is indicated on the lens itself.
What Is Aperture?
Aperture describes an adjustable opening inside your camera lens that controls the amount of light striking the film or digital sensor. As the size of the aperture changes, the angle of light striking the film or sensor also changes. It is this angle change—much like eyeglasses change the angle of the light—that creates a change in the depth of field.
Aperture is measured by f-stops on your camera controls. F-stop settings represent a ratio derived from the size of the lens opening and the focal length.
Aperture has historically been confusing for new photographers (and some established photographers) because of the apparent conflict in its description: a small f-stop is a large aperture opening and a large f-stop is a small aperture opening. Because a smaller aperture limits the amount of light entering the lens, a large f-stop (smaller opening) also requires more light to properly expose an image.
A simple way to remember the relationship between F-Stop/Aperture and Depth of Field is:
- Large f-stop = Large depth of field = More light needed
- Small f-stop = Small depth of field = Less light needed
This means that:
- Larger f-stops, such as f/11, will require slower shutter speeds or more light and produce images with larger depths of field (more of the scene is in focus).
- Smaller f-stops, such as f/4, will allow faster shutter speeds or less light and produce images with shallower depths of field (less of the scene is in focus).
The Effect of Lens Size on Depth of Field
The focal length of your lens plays a big part in determining the depth of field (DOF) for your images as well.
How Focal Length Affects Depth of Field
Think of your lens strength as a limiting factor for your aperture capabilities. The higher the magnification factor, the smaller the depth of field will be, even with large f-stop settings.
The depth of field progression for a 70 to 300mm lens:
- 70mm = largest DOF
- 100mm = large DOF
- 200mm = small DOF
- 300mm = smallest DOF
This effect is especially pronounced in macro photography where the close proximity to the subject and high focal lengths result in depths of field that are sometimes less than an inch.
The Effect of Subject Distance on Depth of Field
Much like lens strength, subject distance plays a big part in determining the possible depth of field in an image. The closer you are to your focal point or subject, the less depth of field is possible.
How Distance Affects Depth of Field
To illustrate this effect, hold your hand at arm's length in front of your face. Even when focusing on your hand you can probably see a good bit of the surrounding environment in a reasonably clear focus.
Slowly move your hand toward your face until you reach the half-way point. Notice how much less of the area surrounding your hand is in focus. Continue moving your hand closer until it is as close as your eyes can focus on it and notice that very little of the area surrounding your hand can now be seen.
This same effect occurs with your camera lens.
- This effect, combined with high magnification factors, results in the tiny depth of fields seen in macro photography.
- It also makes the huge depths of field in many expansive landscapes possible when using a lower magnification factor lens.
See the Effect of Depth of Field for Yourself
It is easy to do a test so you can experience how to control the depth of field and get a visual for its effect on your photographs. To do so, it is best to use a tripod as shutter speeds will vary.
- Set your camera on the manual or aperture priority setting and do not change the focal length of your lens.
- Focus on your subject and set the camera at the smallest f-stop possible (for example, f/3.5). Take a photograph.
- Without moving the camera or the focus point, set the camera for a middle-range f-stop (for example, f/5.6 or f/8). Take a photograph.
- Again, without moving the camera or focus point, set the camera for the largest f-stop possible (for example, f/11 or f/16). Take a photograph.
Compare the three photographs side by side and notice how more of the scene falls into focus as you decrease the size of the aperture opening (use a larger f-stop). Also, notice that your shutter speeds have slowed down with these larger f-stops.
Putting Depth of Field Control Into Everyday Practice
Take this new knowledge with you and consider it in every photograph you take. It will give you greater control of your images and can be used for various effects.
Photographers will use depth of field to their advantage in various situations:
- Landscape photographers often use large f-stops to increase the depth of field in a scene.
- Portrait photographers often use small f-stops to decrease the depth of field in a scene, drawing the viewer's focus to the subject's eyes and face. Note that with large groups, you need a depth of field that will get every person in focus.
- Sports photographers will often use small f-stops to decrease the depth of field and allow the focus to be on key athletes while blurring the background. This also helps decrease shutter speeds in order to stop the fast action.