Creative Field Techniques - Depth of Field
A good photographer makes several decisions when setting up for a shot including the aperture setting. Adjusting the aperture can reduce or increase the amount of light that hits the camera’s sensor. This seems straightforward, however changing the aperture will also change the depth of field which has a significant impact on an image. Aperture settings are expressed as numbers with the letter ‘f’ in front of them, such as f4 or f16. The smaller the number, the wider the aperture, as illustrated above. F4 is a wider aperture than say, f8 and f8 is wider than f16. The wider the aperture, the greater the amount of light that comes through. For example, adjusting the aperture from f16 to f8 increases the aperture size, thus increasing the light entering the camera. However, this change will also greatly affect depth of field.
What is depth of field? Depth of field or DOF is the distance between the nearest and farthest objects that are within focus and adequately sharp throughout the image. A shallow DOF is a relatively small distance whereas a deep DOF is a relatively large distance. Take a look at the image above where the foreground tree is several feet away from the trees in the background. For this, a deep DOF works well. Now look at the bird image below, shot on the Anhinga Trail in Everglades National Park. Here a shallow DOF is better because it allows the bird to be sharp and in focus but keeps the background out of focus.
The following images provide a comparison between f8 and f16 aperture settings. Noticeably different between the two is the background appearance. A wider aperture (f8) and shallower DOF made the background look blurry in the top image, where as more detail can be seen when using a narrow aperture (f16) or greater DOF in the bottom image.
In addition to aperture, two other factors that can alter depth of field, and for the most part can be controlled by the photographer.
Distance between the camera and the subject. Both images below were shot with an aperture of f8. In the image on the left, the distance to the bird was 18.5 inches and in the image on the right, 28.5 in. Once again, notice the blurred background in the first image with less distance. Thus, the farther away you are from the subject, the greater will be the DOF.
Focal length. In short, the greater the focal length (as with a telephoto lens), the lower the DOF. The shorter the focal length (as with a wide angle lens), the greater the DOF. The river scene above was shot with a wide angle lens at a focal length of 22 mm, whereas the bird image was taken with a telephoto lens at 240 mm. An aperture of f11 was used for both.
When should you use a shallow DOF?
Shooting wildlife. When photographing wildlife, we often want the emphasis placed on the animal and not its surroundings. The background in the image below was cluttered with grass. With a wide aperture (f5.6) and big focal length (400 mm), the DOF was shallow enough to blur out the grass, thus minimizing the contrast noise.
Shooting far-away landscapes with no obvious foreground. This is not so much about wanting a shallow DOF but rather wanting to zoom in on far away scenes. Nevertheless, this is also a lesson where a deep DOF is unnecessary. The image below with the large sunrise over the silhouetted cypress trees is a good example of that. Because the trees were so far away, I used a telephoto lens at 330 mm to fill the frame. However, with a great distance between me and the subjects (trees and sun), DOF is high enough to make a sharp image.
Below is another good example where you have a high perspective overlooking a hilly landscape, shot in Iowa farm country. Because everything was so far away, there is no significant foreground leading into the scene. A wide-angle lens would simply make everything appear far away and too small. With a telephoto lens, the greater focal length tends to compress the scene and with the great distance between me and the subject, the entire image appears in focus.
When should you use a deep DOF?
When you have lots of wildlife to photograph all at once. This could be a small flock of birds or a family of bears, or it could be a large scene like the one below. If you have more than one wildlife subject in the frame and one is behind the other for example, a narrower aperture will increase the DOF and help keep both in focus. And the greater the distance from the subject, the greater the DOF. Keep that in mind when photographing wild grizzly bears!
Landscapes with prominent foregrounds. These types of scenes are typically shot at focal lengths from 11 mm to 24 mm. Take a look at the scene from Lake Michigan below. This scene, like the first image above has significant foreground and background elements, so having all of them in focus or a deep DOF is important here.
A couple more things to know; first, most cameras today have a “DOF” button, usually located in the front near the lens mount. When pressed, the scene through the viewfinder will become a preview to how the image will look with the current DOF in place. Frankly, I never use this feature because to me, the preview never looks like the image after it has been taken. But check it out on your camera, you may find it useful.
Second, some photographers use a semi-automatic camera mode called "aperture priority" (designated as 'A' or 'AV' on the mode dial). This may be useful when you want to maintain DOF from one shot to the next. The camera will effectively change ISO and/or shutter speed, while you lock in the aperture setting. But before you let your camera make exposure decisions for you, check out my previous blog on why this is not always the right way to go.
As you can see, getting the right amount of light is not the only consideration to make when adjusting your exposure settings. Depth of field or DOF has a profound effect on an image and therefore, it is important that you understand how to control it to get the optimal DOF every time. To recap; three things affect DOF, aperture, distance and focal length. Now that you understand this, go out and experiment but more importantly, learn to recognize scenes that look best with shallow DOF and those that look best with deep DOF. Be shallow minded at times, but don’t be afraid to go deep when necessary! Here's one more shallow scene!
If you are interested in learning more about camera or editing techniques, I offer individualized instructions in the field as well as in Photoshop. Please visit my website to learn more about these workshops or contact me at email@example.com with any questions you may have.