Depth of Focus: The Optical Phenomenon

One of the best ways to deal with a distracting background is to set up your shot so that the background is out of focus. Increasing the distance between camera and subject allows use of a longer focal length (telephoto) lens, which will reduce the depth of field and blur the background. Keeping objects both near and far in focus calls for the opposite approach; move the camera in and shoot with as wide a focal length as possible. This is why many news videographers choose to zoom with their feet--moving the camera instead of the zoom lever--when following action.

This predictable optical phenomenon, known as "depth of focus," varies with not only the focal length and distance of the subject from the lens, but also with the iris setting. The f/stop you set the lens at affects not only exposure, but also how much of your shot is going to be in focus. Shooting with the iris wide open, generally around f/2.8 on most video lenses provides a far shallower depth of field than shooting the same scene with the lens stopped down. Shooting at smaller iris settings also makes it easier to maintain focus when following action.

Many shooters consider the iris setting to be a technical function, dictated by the camera's electronic needs, instead of a creative function the videographer can employ to influence the look of a scene. We'll take that up in another Sharp Shooters' Tips.