Focus is much more than just getting a technically sharp image.
Lighting, composition and exposure are just a few of the variables you must control on a shoot. It requires a high degree of personal multi-tasking to shoot it right. It is tempting to just depend on your camera's automatics to simplify your life and, in many cases, they will do the job just fine. But by maintaining control of one or more of these details yourself, you can make a sort of magic happen. Take one of the most fundamentals for instance: focus.
You versus The Machine
Most autofocus systems work well for most scenes. The autofocus typically assume that the center of interest will dominate the center of the frame. This will be true most of the time, but what if you decide to put your main subject off center? You may find that the background is in sharp focus, but your subject is a little soft. The way to make sure your audience focuses on what you want is to make sure that what you want is what is in focus. This is as much a creative decision as anything else and machines are not good at creative decisions. People are.
The creative use of focus depends on being able to control what is and is not in focus. Part of creative focus control is in knowing what you want to be sharp and what you don't.
Getting the Best Focus
You can direct your viewer's attention by making the main subject the sharpest part of the frame, no matter where in the frame that subject appears. To get the widest creative range for your focus, open the iris (aperture) up to the widest f/stop (see the sidebar), use the shutter to adjust the exposure and zoom in on your subject. Now, adjust your focus manually. This is easier if you can find some part of the subject with definite edges. The eyes and eyelashes are the best places to focus if you are doing a close-up of a person. You can also use anything with fine details, such as hair or a definite pattern in clothing. Anything with texture is also a good candidate for focusing, just make sure it is part of where you want your viewer's attention.
When you zoom back out to frame your subject for the shoot, your subject will be sharp and the rest of the frame will vary depending on your depth of field. Usually, you'll want the foreground to be sharp and the background softer. Very short lenses, like those found on small consumer camcorders, have a deep depth of field, so your background may be as sharp as the foreground and compete with it for attention. One solution is to move your subject farther away from the background. Another is to zoom in, changing the lens to a longer focus. If these are not practical options, you can put the point of focus just far enough in front of your subject so that the background goes soft.
In some cases, you may want the opposite. Peering at your subject through some foreground object, such as leaves or flowers, can add a bit of mystery or atmosphere to a shot. This works best if the foreground object is out of focus and your subject, even if partly obscured, is sharp. This is strictly a manual-focus situation.
Video is a medium of motion. This sometimes means that the camera-to-subject distance changes during the shot, either because the subject or the camera is moving (or both). Even if the camera is not tracking backwards or forwards, you may pan or tilt on a scene that is at an angle to the camera, such as panning down a shelf of objects. In such cases, you may need to follow-focus, that is, adjust the focus as the shot progresses. At other times, you might want to change the focus in a static shot where neither the camera nor the subject is moving.
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.
Sidebar: Depth of Field
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.
Sidebar: Focus Controls
It is a rare camcorder that allows you to focus by manually turning a lens element directly. In many cases, you might press a button or dial a wheel to focus. Even if you can adjust the focus using a ring on the barrel of the lens, it usually only transmits an electrical impulse that runs the motors that control the focus. A true mechanical focus is best. Usually you can tell what you have by the feel, by the fact that the focus ring has a limited range (i.e. it doesn't go around and around in the same direction) and by the distance markings on the barrel. These markings allow professionals to rack focus almost instantly without even looking in the viewfinder. Using the focus motors on most cameras does not allow such lightning and accurate adjustments. Unfortunately, manual lenses are expensive, and therefore rare, in sub-$5,000 camcorders.
Sidebar: Aperture Ratios and f/stops
The size of your camera's iris or aperture is measured as a ratio called an "f/stop." As a ratio, it is obvious that 1/22 (0.045) is much smaller than 1/1.6 (0.625). On a camera's lens barrel or on your camcorder's LCD, the ratio is dropped and only the denominator remains: numerically 22 is larger than 1.6. So, although it is counterintuitive, you open up your iris and make it larger by selecting lower f/stop numbers.
Sidebar: Autofocus Tricks
One trick some folks use to set the focus on their camcorders is to put the camera in autofocus mode, let it focus the shot and then turn off the autofocus, effectively locking it down. Some cameras even have a button you can press and release to focus without switching into autofocus mode. Don't let some snob tell you that this is cheating: if it works for you, it's good enough for us. Whatever you do, in most situations, it is still highly recommended that you get your camera out of autofocus mode while shooting, to avoid those unintentional times when the camera hunts around for something to make sharp.