How to control the depth of field so your subject stands out sharply.
If you're like most videographers, you want to make your shots look as good as possible. Managing which areas of your shots are in and out of focus can be one of the most valuable tools in your arsenal. And controlling the depth of field ultimately determines your focus area. By understanding how your camcorder's lens operates, and with a little practice, you will be on your way to more interesting and creative shots.
What is Depth of Field?
Depth of field is the area of an image that appears in focus. Camera settings, the amount of light and your equipment's technical specifications are all factors that can impact depth of field, making it deep or shallow.
First, it is important to understand that whenever a lens is involved, there is only one plane of exact focus. There is, though, an area of acceptable focus in front of and behind this plane. In a shallow depth of field, only the subject and a small area in front of and behind it will be in sharp focus. A deep depth of field results in objects farther away from the focus plane lying within acceptable focus range (Figure 1).
Controlling the depth of field allows you to make your subject stand out sharply from the background, or to put as much of the shot as possible in focus. For example, when shooting your son playing violin in the midst of the string section, you may want him to stand out from the rest of the orchestra. On the other hand, you might want the entire orchestra to be in focus, which may not be a trivial task in low-light situations. Most consumer-level camcorders have very deep depths of field in most shooting situations.
What Affects Depth of Field?
While there are many ways to alter your camcorder's depth of field, there are really only two variables that determine depth of field: focal length and aperture. Focal length is a function of your camcorder's lens and is changed when you zoom in or out on a subject. You can, therefore, easily change the depth of field of your shot by simply zooming. The aperture, on the other hand, is a bit more complicated to control, but is easy to understand. The aperture or iris simply controls how much light gets into your camcorder. You can manually control aperture settings, but the shutter speed and the amount of light in a scene will always be an influence on these settings as well.
Let's begin by examining focal length. The focal length of your lens directly affects depth of field. When shooting a wide angle shot, the focal length of the lens is relatively short and will have a deeper depth of field. If you zoom in, the focal length gets longer, and the shot will have a more shallow depth of field (Figure 2).
Subject distance from the lens and from the background also plays a part in depth of field (Figure 3).
The aperture setting determines the size of the opening (iris) that lets light through to the CCD. A small aperture setting results in a deeper depth of field and a large aperture setting provides a shallow depth of field. Say, for example, that you're shooting a panoramic scene and want as much in focus as possible. You would use a smaller aperture to maximize the depth of field. If you want your subject to stand out from the background by being the only thing in focus, you would choose a larger aperture to decrease the depth of field. Keep in mind, a smaller aperture setting allows less light through, so you may have to make other adjustments to compensate, such as adding lights or slowing the shutter speed.
The amount of available light is one of the most important factors in determining the proper aperture setting, which then affects the depth of field. If the subject area is dimly lit, you'll need to use a wide aperture setting which will cause a shallow depth of field. If the subject area is brightly lit, you can set the aperture to a smaller opening, resulting in a deeper depth of field.
For outdoor shooting, you can create a more shallow depth of field by moving into the shade, which will allow you to use a larger aperture setting. Conversely, you can increase the depth of field by moving into the sunlight, which will require a smaller aperture setting. Indoors, controlling the amount of light can also help you control the depth of field. Adding additional light will allow for a smaller aperture setting, resulting in a deeper depth of field; lowering the light level allows for a larger aperture setting to give you a more shallow depth of field.
Shutter speed also affects how much light gets into the camera, which determines how wide you can open your aperture and therefore plays a part in depth of field. A high shutter speed lets less light through. Because less light gets through, you will need to open the aperture setting, which results in a more shallow depth of field. A slow shutter speed allows more light through. With more light, you can use a smaller aperture setting, which results in a deeper depth of field (see Figure 4).
Controlling Depth of Field
The camera and lighting variables already mentioned interact to control the depth of field. To retain proper exposure levels, you may need to make an adjustment to more than one setting at a time. For example, when using a larger aperture setting, you may have to increase the shutter speed to compensate for the additional light passing through the lens.
To utilize depth of field as a creative tool, get to know the different settings on your equipment. Find your camcorder's exposure or iris control and learn how to use it. Try using a higher or lower shutter speed setting to see what effect these have on the depth of field. In cameras with automatic exposure control (or ones with the ability to lock the exposure), adjust the shutter speed first and then set the exposure. Jot down the settings you use so you can recreate the effect later.
Remember that subject distance, combined with zooming, also plays a part in depth of field. Sometimes simply moving the subject, the camera or both while zooming in or out will give you the depth of field desired. Try various distances and zooms and note the results.
Controlling depth of field should result in more pleasing and creative shots, but there are some considerations to take into account. Among these are automatic focus and automatic iris.
- Automatic Focus: Many video cameras include an automatic focus feature. Because you measure depth of field from the point of exact focus, the depth of field will change as the focus changes. For example, when shooting a crowded scene, the automatic focus may search back and forth for something to focus on. As it does, different areas of the scene will drift in and out of focus. To avoid this, turn off the automatic focus and use the manual focus to determine for yourself which area you want to remain in focus.
- Automatic Iris: Many cameras have automatic iris (exposure) options. This feature allows the aperture to open up or close down automatically, depending on how much light is available. As mentioned earlier, the depth of field will vary depending on the aperture setting. For more precise control of the area you want to remain in focus, switch off the automatic iris and set the aperture opening manually. If your video camera doesn't have full manual controls, you can often lock down the exposure or select from a number of presets.