A Focus on Focus (page 2)

Since your viewers will watch the sharpest part of the frame, you can direct and move their attention for dramatic purposes without actually moving anything at all. A "rack focus" or a "shift focus" is one way to do this. As the names suggest, you shift the focus from one part of the frame to another. For example, your scene shows the heroine reading on a park bench. The sun is shining and life is peaceful. Then the focus shifts to the previously out of focus background to reveal the stalker, staring at her. The audience is immediately made aware of something the subject isn't aware of and proximity is established and even reinforced, since the two people are in the frame at the same time.

Rack-focus can be quick or slow. An obvious example of a slow shift would have the camera go in and out of focus to show the character getting drunk or driving tired. Another way would be to have the scene slowly go completely out of focus to show the character passing out, perhaps from injuries. This can be done as a point of view shot (POV) or not, but the audience will understand what is going on if it is well done.

The same sort of trick can function as a transition. You can have the scene go totally out of focus and then cut (or even better, use a quick dissolve) to a new scene that starts out of focus and slowly sharpens. For instance, you could do this to show the hero waking up in a strange location.

Using focus thoughtfully to lead your audience's attention can be subtler than a pan or a zoom. Focus can be an almost invisible curtain that strikingly reveals your story. You can control where they look and what they see, but they are rarely aware of how you do it. Suddenly, what was right before our eyes all along becomes obvious and the impact can be magical.

Gene Bjerke is a freelance scriptwriter and author of Writing for Video.

Most lenses do not render everything in front of them sharply, which is why we need to focus. There is a zone of maximum sharpness and objects in front of or behind this zone are softer. The range of acceptable sharpness is the depth of field.

In general, the area of acceptable sharpness extends twice as far behind the focus point as it does in front. Besides that, there are three factors that determine the total extent of the depth of field.

• Focal length - Longer lenses have less depth of field than shorter lenses. Thus, when you zoom in, your depth of field gets shallower.
• Aperture - Any given lens will have greater depth of field as the aperture gets larger and the f/stops get smaller. Thus if you can adjust your f/stop, you can change your depth of field. Unfortunately, small cameras have inherently small apertures, so there is only so much you can do to get that artistically narrow depth of field, especially in low-light conditions.
• Distance - The closer the camera is to the subject, the shallower the depth of field. Thus to get a shallow depth of field, place your subject close to the camera.

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