Focal Length - Stephan's Digital Garden

# Focal Length Planted: 05 November 2025 Last tended: 10 November 2025 One of the main characteristics of a camera lens, is its ==focal length==, usually represented in mm (e.g. 50mm). A camera lens is composed of different layers of glass pieces, with a front piece and a group of curved, optical lens elements behind it. While in theory a simple convex lens would suffice for getting an image, the many different elements are required to correct as many optical aberrations as possible (e.g. blur, distortion, color fringing, etc.) for a faithful reproduction of what is being captured. The interesting thing about lenses, is that contrary to how one might imagine it, light doesn't pass in a straight line through the lens. Instead, light entering a lens eventually converges into a single point, called the point of conversion; the focal point. The focal length is the distance between the focal point and the sensor (or film) of the camera. In case of a 50mm lens for example, this means the light is converging 50mm in front of the camera's sensor. Due to the way lenses focus light, the original image you would get is actually mirrored horizontally. The camera actually corrects it, by mirroring the image again. In case of DSLR cameras, this happens thanks to the optical path or mirrors and prisms in its body. With mirrorless cameras, it is done via its software. ^[https://photographylife.com/composing-upside-down-and-backwards] The focal length also has an effect on the field of view. The higher the focal length, the longer the distance between the focal point and the camera's sensor, which results on a steeper angle of the incoming light rays. Lenses with a shorter focal length are thus referred to as wide angle lenses, while lenses with a longer focal lens are referred to as telephoto lenses. The numbers very slightly on when a lens falls under the respective classification, yet generally speaking a lens with a focal length of ==less than or equal to 35mm is considered a wide-angle lens==, while a ==telephoto lens== is considered ==a lens with over 80mm of focal length==. ## Image Sensor Formats and Focal Length