- Apr 11, 2012
Controlling Depth of Field
If you’ve watched feature films, commercials, or music videos closely, you’ll notice that focus is often used creatively to control the viewers’ attention. Sometimes the camera will start out of focus and then slowly roll into clarity to reveal a subject. Other times the camera might rack in-between a person in the foreground to then find a person farther in the background.
A driving force in the popularity of DSLR cameras for video is the image sensor superiority these cameras offer. The large sensors allow for greater control in depth of field. When used properly, your footage can take on more cinematic qualities.
What Is Bokeh?
You’ve likely noticed that the way your camera blurs objects is different than how your eyes behave. In fact, there’s actually a Japanese word for this stylized blurring. It’s called bokeh (pronounced boh-kay or boh-kuh).
What’s the big deal? Well, many find the use of blur appealing. It can also be quite useful when you want to keep the focus on your subject and let the background go soft. The blurring often appears most in the brightest areas of an image (such as headlights or small lights in a skyline). However, bokeh can be used to simplify just about any background.
There are two ways to easily create bokeh in an image:
- Using a prime lens or a very fast zoom lens, you can open up the lens wide. Using a low f-stop (typically f/2.8 or lower) can really bring out a bokeh blur (Figure 4.6).
Figure 4.6. A theater marquee in San Jose, California, combined with passing traffic turns into an abstract shot when the camera is thrown far out of focus.
- Using a longer (telephoto) lens, you can compress the action of your shot. Instead of being close to your subject, move farther back and zoom in. This can create isolation due to a shallow depth of field (Figure 4.7).
Figure 4.7. While hiking through a forest on Grand Bahamas I came across an intricate spider web. The scene was very low light, and my zoom lens was not very fast. By zooming all the way in from a greater distance, I threw the background out of focus and simplify it, which really helped the strands of the web to stand out.
Foreground and Background
If you want to properly use bokeh or a shallow depth of field, you need to think about your foreground and background, and how you compose your shots. I’ll explore composition in Chapter 5, “Composing Shots.” For now, here are a few simple techniques you can try:
- When setting up an interview, portrait, or talking head, don’t position the subject against a wall. Instead, look for a more open space. Try shooting in a long hallway, a conference room, or even just through a doorway. Stacking the scene works well to create depth (Figure 4.9).
Figure 4.9. Positioning my subject at the top of a staircase allowed me to compose the shot so there were areas in the background that stretched off into the distance (and became defocused).
- If you’re shooting a large crowd or busy event, tweak the aperture settings on your camera. Open the aperture a bit and set your focus so the front of the group is in focus and the back starts out soft. As people move through the scene, they’ll reveal themselves (Figure 4.10).
Figure 4.10. By zooming in from a greater distance, as well as shooting at an angle, I created a well-defined focus plane for this shot. People farther away are out of focus but reveal themselves as they move towards the camera.
- If you’re recording two people talking to each other, increase the distance between them. Then try using a zoom lens and adjust your zoom level to taste. This lens will make the two people appear close to each other but set one of the two more out of focus than the other.
As you become more experienced, you’ll want to try rack focusing (changing your focus from one object to another while recording). For example, you might want to go from a focused image to a blurry one as an animated transition. Or you might want to keep a moving subject in focus as the subject walks through a shot. Just keep in mind that rack focusing is not easy to do.
Modern lenses are actually more difficult to rack focus because lenses manufactured today are designed to autofocus more quickly. To do this, manufacturers set the lens so the focus ring only needs a small turn to move through all the focus positions. This feature is great for shooting stills because it’s much faster to shoot when the camera is in control. But it makes it more difficult for video.
For this reason, I seek out older lenses for my video kit. Stopping into just about any local camera store, you’ll find a used department with older prime lenses. Many of these lenses have focus rings that turn almost a full 360 degrees, which makes focusing by hand much easier because the ring is not as sensitive. If you can’t find an older lens that matches your camera (often a problem for Canon shooters due to changes in the lens mount system), you can purchase an adapter ring. If your lens has physical rings to control both aperture and focus, you can use an adapter from companies like Fotodiox or Novoflex (Figure 4.11).
Figure 4.11. An older Nikon prime lens is attached to a newer Canon 7D using an adapter ring from Fotodiox. Lenses need manual aperture and focus rings to work properly with the adapters.