Depth of Field (page 2)

Aperture (Iris)

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.

Special Considerations

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.