Focal Length (photographic)
The focal length of a photographic system, like a camera, can be described as…
In more detail, it is the distance over which parallel rays of light entering the lens are focused. A lens system with a shorter focal length has higher optical power than one with a longer focal length. The shorter focal length bends the light rays more strongly, focusing over a shorter distance.
Where a subject is infinitely far away, a long focal length (low optical power) creates higher magnification but a narrower angle of view. Where a shorter focal length (high optical power) is in use it creates lower magnification and a wider angle of view.
Some photographic lenses may have an “effective focal length” of a specified length, but not measure that long. This is because the arrangement of lens elements in a zoom lens can foreshorten the actual focal length. For example, a quoted 200mm focal length can have the same result as an actual focal length of 200mm but be physically shorter. The type of lens elements that create this effect are called “telephoto” elements. We refer to a lens with with this configuration as a telephoto lens.
In a zoom lens the focal length can change. As some elements are moved apart (or together) the physical separation of those elements cause the focal length to change. Some lenses will get longer as the focal length is increased as the elements are moved apart. However, some lenses maintain the movement entirely within the lens body although a similar movement is occurring to separate elements when the focal length increases/decreases.
Some useful facts:
- Focal length is measured in millimetres.
- The focal length of a photographic lens is not the same as the focal length of an optical lens.
- You cannot take an actual measurement of the photographic focal length of a photographic lens. Focal length is accurately measured by the manufacturer and does not relate to physical measurements on the outside of the lens body.
- Many zoom lenses have telephoto elements. This allows lenses to change focal length and remain shorter than a specified focal length and still achieve the same results.