Let's focus on focusing!
Focus is a matter of perspective, and many filmmakers have taken some artistic license playing with focus. But for the most part, the human eye is most satisfied looking at an image (moving or still) that is sharp and clear as real life.
Essentially, the art of focusing involves bending light. Sounds magical, right? In a way, it is. Light travels in beams. When you put a lens in front of an object, the convex nature of the lens causes the light beams coming from the object to bend at varying angles, depending on what angle the beams enter the lens. Where those bent beams meet on the other side of the lens is where your image appears most in focus—think “crossing the streams” from “Ghostbusters”!
Understanding the sheer physics of how your camera focuses on an image isn't really necessary to take a good photo or shoot good video. It does help to know about it though. It's understanding focus settings and styles that is key to shooting quality work. Here is a brief primer on some typical camera focal settings and styles.
1. Autofocus. It does all the work for you! Depending on your camera, it involves an internal sensor or system of sensors that measures available light relative to distance from a subject, and focuses based on what it senses. There are two types of autofocus in cameras—active and passive. Active autofocus “actively” uses sound waves or infrared to detect the distance from an object, whereas passive autofocus measures contrast once the light beams have passed through the lens.
2. Trap Focus. This focal technique is a blend of manual and autofocus that's great for sports photography or shooting still images of fast-moving subjects. With trap focus, the camera will only capture an image when the camera senses that it is in focus. You set the parameters, so you can have more control over a shooting situation that may not be directly under your control.
3. Rack Focus. This technique involves switching the focus from one subject to another—most commonly between a subject and its background—on video or film. It requires a short depth of field to work, which means anything that's not the subject in focus will be blurry. It's a hallmark of some artistic filmmaking.
4. Follow Focus. Much like trap focus with still photography, this is a great technique for shooting video or film of a moving object. It basically involves adjusting the focus on a moving subject as you film it. In a professional setting, to achieve follow focus, you'd need a rig that you can hook up to your camera that acts like the gears of a bicycle to allow a camera operator to delicately and minutely adjust the focus. Autofocusing on a moving subject isn't always the most accurate, especially with a short depth of field, so this more manual technique is preferable.
5. Soft Focus. This is a style of focusing that gives a subject a dream-like appearance, softening blemishes and lines by ever-so-slightly and selectively blurring an image or shot. That isn't to say it’s out of focus exactly, as just unfocusing your camera lens doesn't create this effect, but rather it is created by using a specific lens designed for this purpose. Soft focus lenses were invented and popular in the 1930s and 1940s—the heyday of the glamour shot!