Lens Properties

The range across the sensor or film surface onto which the subject is exposed is expressed as an angle, called the angle of view. Wide-angle lenses with their short focal lengths have a wide angle of view, which means the exposure range is wide.

Conversely, telephoto lenses, which have long focal lengths, have a narrow angle of view, making the exposure range narrow. So a wide-angle lens is used to take a wide area of a subject nearby whereas a telephoto lens is used to take only part of a subject located further away. A single zoom lens, meanwhile, can function as a number of lenses with different focal lengths, enabling you to smoothly alter the angle of view and quickly frame the shot. You can select your lens to create the effect of distance or depth of field, or to suit the location and surrounding conditions.

When you focus on a subject, there is part of the subject that is in focus and parts in front and behind which are not in focus. This range in which the object is seen to be sharply in focus is called the depth of field. If the focal length is kept the same, the depth of field gets deeper (the range in which the subject is sharp gets wider) as the aperture is stopped down, and it gets shallower (the range in which the subject is sharp gets narrower) as the aperture is opened. Even when the aperture stop is the same, the depth of field gets shallower as the subject distance gets shorter, and deeper as the subject gets further away. Furthermore the depth of field is deeper with a short focal length wide angle lens, and shallower with a long focal length telephoto lens.