Nearly all modern video cameras have options for auto and manual focus. Auto-focus lenses work by finding vertical lines and lining them up. This means it's relatively easy for an auto-focus camera to quickly and accurately focus on something like a line of trees, but very difficult for it to focus on a blank white wall. Auto focus typically also looks for an object in the center of the frame to focus on, which can be difficult if you're composing using the Rule of Thirds (as you should be). For situations like this, manual focus is called for. Manual focus is sometimes more accurate, but it is difficult to do in situations where subject-to-camera distances change.

Any part of a shot can be out of focus, as long as a person's eyes are in focus. For people who are wearing glasses, it's acceptable to focus on the frame, because it becomes a substitute for the eyes. But rule number one must be "keep the eyes sharp."

In watching dramatic TV series that often contain low-light situations, you will occasionally see a shot that's not exactly in sharp focus. Having a not-perfect shot is something you may have to deal with from time to time. But keep in mind that focus is one of the most critical aspects of your production, right up there with lighting. As high definition becomes the standard for video production, focus will be even more important.

Contributing Editor Kyle Cassidy is a visual artist who writes extensively about technology.

Ever since people began using optical devices, they've had trouble focusing them. The viewfinder, while convenient, has always been smaller than optimal. This leads landscape photographers with gigantic 8x10 view cameras to hide under a dark cloth with a magnifying glass pressed onto the focusing glass to make sure the image is razor sharp. On vintage camcorders, a magnifying glass was usually built into the eyepiece, making the little one-inch screen seem a little larger. Videographers today often bring larger monitors with them on location.

Focus is especially critical in HD video. These days, it's typical that you're performing your focus on a 2.7-inch LCD screen, and your viewers are going to be watching it back on a 52-inch LCD screen. Being off by a fraction of an inch can be extremely noticeable. Expensive, high-end professional camcorders have long had a pair of useful focus-assist tools that have recently found their way onto consumer gear: these are Magnify and Peaking. Both of these assist in manual focusing.

Magnify (also called expanded focus) is exactly what you'd expect: when selected, the viewfinder "zooms in," doubling or tripling the magnification and allowing you to view the focus critically. This is actually better than holding a magnifying glass up to the viewfinder, because you're using your whole viewfinder to view a small area of the image.

Peaking highlights the area that's in focus. Very often, peaking shows the image as black and white with colored dots over the in-focus area for quick interpretation: denser dots indicate sharper focus.