Field of view (FOV) is the angular extent of a scene captured by a lens on a given sensor, expressed in degrees. It is the more accurate way to describe how wide or narrow an image is, since focal length alone (50mm, 85mm, 200mm) only tells the full story when paired with the sensor size that lens is being used on. A 50mm lens on full-frame (36x24mm) has a diagonal field of view of about 47 degrees; the same 50mm on an APS-C body has a diagonal FOV of about 30 degrees; on Micro Four Thirds, about 24 degrees.
FOV is usually quoted as the diagonal angle, since that is the standard set by ISO 517. Horizontal FOV (across the long edge) is slightly narrower, and vertical FOV (across the short edge) narrower still. For a typical 3:2 aspect ratio frame, the horizontal FOV of a 50mm on full-frame is about 40 degrees, and the vertical FOV is about 27 degrees. Lens spec sheets typically list the diagonal value, but graphics tools and surveying applications sometimes list horizontal, which can cause confusion when comparing.
The relationship between focal length and FOV is non-linear. Doubling the focal length does not halve the FOV; it produces an angle whose tangent is half of the original. A 24mm lens on full-frame has a diagonal FOV of about 84 degrees, a 50mm has 47 degrees, an 85mm has 28 degrees, a 200mm has 12 degrees, and a 600mm has just 4 degrees. The shape of the curve means small changes at short focal lengths (24mm to 35mm) feel very different from small changes at long focal lengths (400mm to 500mm).
Understanding FOV in degrees matters most when working across sensor formats or when planning aerial, panoramic, and architectural shots where the included scene must be calculated in advance. Aerial photographers planning coverage area at altitude use FOV math to determine how many frames are needed to cover a given ground area. Panoramic stitchers use FOV to determine overlap percentage. Architects shooting building interiors choose lenses by required FOV to fit a known room dimension at a known shooting distance.
The same lens on different sensors produces dramatically different FOV. A 50mm on full-frame is a standard normal lens; on APS-C ((crop factor 1.5x) it behaves like a short portrait lens at 75mm-equivalent FOV; on Micro Four Thirds it acts like a longer portrait at 100mm-equivalent. The lens itself does not change, only its rendered FOV on a given crop sensor. This is why “focal length” without sensor context can mislead, and why hybrid shooters often think in terms of full-frame-equivalent focal lengths.
Ultra-wide and fisheye lenses push FOV to extremes that introduce distortion. A rectilinear 12mm on full-frame covers about 120 degrees diagonal with strong corner stretching; a circular fisheye covers 180 degrees mapped to a circular image. The Laowa 9mm rectilinear covers 135 degrees, the widest non-fisheye coverage commercially available. At the long end, super-telephotos like 800mm and 1200mm produce FOVs under 3 degrees, capturing only a tiny slice of the world but at very high magnification. The choice of lens, in any genre, ultimately comes down to required FOV plus aesthetic considerations like compression and bokeh, not focal length in isolation.