Focal Length

Instructional Video Production 4(1+3)

Lesson 05: Camera Features and Effects

Focal Length

A camera fitted with a fixed lens does not allow us to frame different kinds of shots from the same position. We will have to physically move the cam­era closer to the object to get a closer shot, and vice versa. But a camera fitted with a zoom lens can get shots anywhere from an ECU to an ELS, from a single camera position. A zoom lens can become a short-focal length lens (wide angle) by zooming out, or it can become a long-focal length lens (telephoto) by zooming in.

Focal length is the distance from the optical centre of the lens to the point where the image is seen in focus by the lens. This means that when we zoom out, the distance between the optical centre and the point where the image is in focus decreases. When we zoom in, the distance between the optical centre and the point where the image is in focus increases.

All lenses, zoom or fixed, have measurements that designate their focal length. One fixed lens may be a 25-mm lens, whereas another a 50-mm one. A variable-focal-length lens may have a range of 10 mm to 100 mm.

The longer the focal length, the closer the shot and the bigger the image. A 150 mm lens will show greater detail, but less of a scene, than a 25 mm lens. A zoom lens that is in the 100 mm position will have a tighter shot than one in the 50 mm position.
Lenses or zoom lens positions that show shots wider than the range of the human eye are called wide angle; those that show view roughly as the eye sees them are normal; and those that magnify the shots are usu­ally referred to as telephoto. The normal lens creates the most natural perspective.

Zoom lenses come with ranges. A zoom range is the degree to which we can change the focal length of the lens. If the camera lens is marked 3 x, it means that we can zoom in (or increase the focal length) three times. If it is marked 20 x, it means that we can increase the focal length 20 times. Wide lenses come with a range in the negative. For example, if a wide lens is marked -2 x, it means we can decrease the focal length by two times.

While consumer camcorders have a limited zoom range, professional cameras have options to attach extenders either for telephoto (for closer shots from greater distances) or wide lenses (for wider shots from limited distances). These extenders are quite heavy sometimes and are detachable.

Last modified: Saturday, 21 April 2012, 6:48 AM