THE FOCAL LENGTH
Usually represented in millimeters (mm), focal length is the basic description of photographic lens. However the focal length is not a measure of the actual length of a lens, but a calculation of an optical distance from the point where light rays converge to form a sharp image of an object to the image sensor (or film plane) when the lens is focused at infinity. Put in a plain language, it is the distance measured from the optical center of the lens to the image sensor, where the optical center is the point within a lens, at which the rays of light from two different sources entering the lens are ‘assumed’ to converge, as illustrated in the figure below;
Fig. 1: illustration of focal length
The focal length carries two important details about a lens i.e. the angle of view (how much of the scene will be captured) and the magnification (how individual elements will be enlarged). Shorter focal length lenses provide a wider field of view, but offer less magnification. Conversely, longer focal lengths offer a shorter field of view, but provide greater magnification. In addition, the shorter the lens’ focal length, the shorter the distance needed to obtain the same field of view (FOV) compared to a longer focal length lens as can be illustrated in fig. 2 below
Fig. 2: Understanding the focal length
Role of focal length in photography?
The focal length number is an indication of how much of the scene is captured in the photo. The lower the number, the wider the view and therefore the more we can see. A higher number will show a narrower view, and thus the less we can see. A lens with 14mm focal length would be a very wide lens and capturing a lot of the scene while a lens with 400mm focal length is a very narrow lens that only captures only a tiny part of the scene, consequently resulting in large magnification such as witnessed in binoculars or a telescope.
The typical focal length of camera lenses is in the range of 10mm to 500mm, perhaps with the exception of specialty lenses that can extend outside this range. Cameras can have fixed lenses that have just one focal length or zoom lenses, which allow the focal length to be varied to different lengths. You will therefore see settings such as 20mm-50mm or 65mm-150mm in zoom lenses indicating the range you can set the focal length.
Fixed lens with 85mm focal length
Zoom lens with focal length range of 70-200mm, set at 100mm
Fixed Focal Length Lenses is not the same as Fixed Focus Lenses. While Fixed Focal Length Lenses have the ability to be focused for different distances, Fixed Focus Lenses are intended for use at a single, specific working distance.