The focal distance is a characteristic value of lenses, optics and concave mirrors over which incident light rays or electromagnetic waves are concentrated at the focal point. The focal distance is the distance between the principal plane of an optical lens or convex mirror and the point of focus, the focal point.
In lenses, the focal plane is on the film material or, in digital cameras, on the image sensor. The focal length determines the image section that is captured by the lens via the angle of view. In the case of parabolic antennas, it is the angle of the incident microwaves. This applies equally to photography, microscopy, telescopy, antenna technology and radio telescopy. An increase in the focal length leads to a narrower angle of view, a reduction to a wider angle of view. In photography, this is referred to as a telephoto or telephoto lens, or a wide-angle or wide-angle lens. Normal focal lengths are those that lie between 35 mm to 45 mm and correspond to an angle of view of 45 °. This is also the focal length of the human eye.