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Basic Training: Focus on Focus

Focus is the act of bending light through a lens onto a plane, which clearly represents the objects on the other side of that lens. In the natural world, muscles in your eyeball perform this by causing the thickness of your lens to change, in order to project an image on your retina. In your camcorder, it's done by a series of glass or plastic lenses, whose distance from one another is changed by a rather complex collection of threaded tubes that move - some forward and others backward - to keep images sharp throughout the range of your zoom.

The distance that these lenses need to be from one another in order to produce a sharp image on your camera's CCD is well known, and in the film industry there is no looking "through the lens" to adjust focus; rather, focus is set with the aid of a tape measure and a set of tables. There are people, called "focus pullers," whose entire job - and they make careers from it - is to sit at the front of the camera and turn a knob from one preset point to another, to keep the objects in the lens in focus. They do this by looking at the distance of the subject to the lens and by using scales on the camera. Camcorders are much more convenient.

Today, camcorders rarely have focusing tables on the lenses. Most camcorders come with a viewfinder that allows the operator to see whether the object in front of the camera is in focus or not, and most camcorders today have automatic focus built in.

There are two types of auto focus: "active" and "passive." Both have their problems. Active auto focus sends out a beam of infrared light and measures the time it takes to bounce back from the subject. Passive auto focus measures the contrast between adjoining pixels in vertical lines, in much the same way that the old split-prism viewfinders worked in old 35mm SLR cameras. By racking the focus back and forth until the camera sees the vertical line properly, the camera finds the proper focus. If the subject moves, the camera will continue to look for a vertical line. For this reason, it's easier to focus on some things than others.

Many camcorders today have multiple "focus points" or areas on the screen where it's possible for the camera to find and focus on a vertical line. Think of these as "focus-sensitive areas" on the screen. The more you have, the greater ability your camera has to get good focus. But the camera can't always guess what you, the movie producer, want to have in focus. Is it the person in the foreground? Or the flower arrangement in the background? No matter how many focus points your camera has (check your camera's manual for how to set the different focusing options you may have), some oversight may be necessary from time to time.

• Brick walls
• Trees
• Chain-link fences
• Eyeglass frames

• Cloudless skies
• Blank sheets of paper
• Snow
• Very dark scenes with low contrast

• Windows
• Fire (such as candles)
• Black objects

For general video work of typical subjects in decent light, auto focus might be all some people ever need, but there are instances where manual focus is called for. Typically, these are instances where you want the focus to stay the same, no matter what's going on in front of the camera. One such instance is objects moving through the frame (imagine a commercial with a closeup of a hand holding a cell phone with an out-of-focus soccer game in the background). Composition may also fool the camera (judicious use of the Rule of Thirds may sometimes be at odds with your less-creative camera).