The focal length of a lens is a measurement of distance—often misconstrued as a constant measurement between a lens element and the sensor, but in fact, it’s a measurement from the nodal point to the sensor. That nodal point, also called the “entrance pupil,” is the point in a lens where the light rays entering converge on their way through the optics to the sensor.
Focal lengths are most often provided in millimeters, but old-school large format film photographers are used to hearing their lenses described in inches too. (An inch is about 25mm, so that standard 210mm view camera lens is often called an 8-inch lens.) Most of us, though, are familiar with lenses in millimeters—like 50mm, 100mm, 24-70mm and 70-200mm. The former are fixed focal length lenses, known as primes, while the latter are variable focal length lenses we all call zooms.
Those are the technical definitions, but what do they mean in practice? In layman’s terms, the focal length translates to a wider or narrower angle of view—and this is what we photographers are actually thinking of when we consider compositions before choosing lenses by focal length.
Angle Of View
Sometimes called field of view, a lens’s angle of view is measured in degrees in both vertical and horizontal axes, and represents whether the lens shows more of the scene in front of you or less of it; a wide view or a narrow one. For instance, a 180-degree angle of view would mean that a lens can take in everything—from turning your head 90 degrees to the left to 90 degrees to the right and everything in between. (Some ultrawide fish-eye lenses, in fact, do provide 180-degree angles of view and sometimes more—seeing everything in front of and some of what’s behind the lens.)
This is why lenses are categorized as they are—from “wide angle” to “normal” and “telephoto.” The shorter the focal length of a lens, the wider its angle of view. And the longer the focal length, the narrower the angle of view. In practice, the telephoto lens’s narrow angle of view enlarges subjects. Positioned at the same distance from the camera, a subject photographed with a longer focal length will occupy more area on the sensor than the same subject at the same distance photographed with a shorter focal length providing a wider angle of view.
Choosing The Right Lens
Armed with the above knowledge, photographers can make more informed choices about which lenses they use in a given circumstance. To capture more of a landscape scene or an architectural interior, for instance, photographers choose a wider angle of view provided by a short focal length lens such as 18mm or 24mm. Photojournalists often like to provide some context in their images, so a 35mm wide angle allows in a bit more of the scene than a “normal” 50mm prime and its angle of view that approximates what a human eye takes in. Portrait photographers often want to isolate their subjects, so a narrower angle of view such as that from an 85mm, 105mm or even 135mm lens is preferable. Sports shooters and wildlife photographers want to enlarge the subjects they’re photographing as much as possible, so they choose super-telephoto lenses of 200mm, 400mm and more for their smaller field of view.