Auto Focus: Good or Bad?

Focus is an incredibly effective tool for directing the audience's attention.

The eye is always attracted to the part of the image that is in focus. This means that a knowledge of focus, depth of field, and lenses is essential if the camera operator and director want to use the camera creatively as a tool for storytelling.

There are a number of techniques that camera operators use to capture the action in focus.

• The most popular method is called follow focus. This technique requires the camera operator to continually change the focus as he or she follows the action, constantly turning the focus ring to keep the subject sharp whenever it begins to soften. However, focusing is not always as easy as that. With practice it can become a reflexive action. That does not mean it becomes easy.

• Zone focusing refers to when camera operators are assigned a "zone" on the field of play or at a news event. They calculate the aperture so that basically everything will be in focus when the player or other subject moves into their zone. They generally do not try to shoot the subject when they are out­ side of their zone.

• Prefocusing is when the camera operator focuses the camera on the subject before beginning to record. An example would be when a talk show host wants to show the audience a still photograph. Usually the host will place the photo on a stand located on his or her desk. Because one of the cameras has usually prefocused on the stand, the photo is instantly framed and in focus. Prefocusing is also required whenever a zoom lens is used.

Many lenses also include macro systems, which are designed to provide a sharp image almost up to the actual lens surface. However, it is difficult to avoid a camera shadow when lighting such close subjects.

As the subject gets closer to the camera, the depth of field becomes shallower, so focusing is more critical. How noticeable a fall-off in sharpness is will depend on the amount of fine detail and tonal contrast in the subject.

AUTO-FOCUS

Many video cameras are fitted with an automatic focusing feature. This is a useful device for the occasions when the camera operator is preoccupied with following the action, is not sure where the subject is going to move to next, and might get the focusing wrong in the heat of the moment. But why not switch to auto-focus all the time so that we can forget about focusing altogether?

If the camera operator relies entirely on the automatic focus features in the camera, the resulting images will vary greatly. The auto-focus system simply sharpens the focus in a selected zone of the shot, irrespective of what is appear­ing there. So it needs to be used with care.

While auto-focusing can be very accurate, here are some situations that can be problematic: .

• When the subject is not in the center of the picture. If two people are placed on either side of the frame, auto-focus may sharpen on an unim­portant distant object in the center of the screen and leave the subjects soft-focused.

• The camera operator may want to focus on someone near the camera and a distant person at the same time, but auto-focus may sharpen on one or the other-or neither.

• If shooting a subject through a foreground framework, such as a fence or branches, the system will focus on this foreground rather than the more dis­
tant subject.

• If following someone who is moving around within a crowd, the auto-focus system is likely to readjust itself continually, focusing on nearby people instead of the intended subject.

• When the camera operator zooms to change the size of the shot, the auto­ focus system may refocus as the shot is recomposed.

• If the camera operator is shooting a distant view and anyone (or anything) moves into the shot, closer to camera, the system may refocus on the second­ary subject instead and defocus on the real subject. If, for example, the cam­ era operator is panning around a scene and moves past a foreground tree, the system may readjust to focus on the tree.

When dealing with any of these situations, the best solution is to switch to manual focus to avoid the problems. Nevertheless, when used wisely, auto­ focus can be a useful tool.