Focal length, almost always measured in millimeters, is the distance between the sensor and the focus point, where the image appears sharpest through the lens, and determines the magnification of the lens.
Focal length directly affects the field of view, or crop, of a photo. Smaller focal lengths have a wider field of view, and larger focal lengths have a smaller field of view. For example, a “wide angle lens” (a lens with a low focal length) will show more of the scene, and a telephoto (higher focal length) lens will show less.
Every lens has either a fixed or variable focal length. A fixed length (or prime) lens has a fixed “field of view” or zoom level, whereas a lens with a focal length range (for example, 24-70mm) is what’s known as a zoom lens, with an adjustable field of view.
Generally, lenses of lower, fixed focal lengths (like a 20mm lens, often referred to as a pancake lens) will contain fewer lens elements. As the focal length grows, more lens elements are introduced, and the image must pass through more pieces of glass to the image sensor in the camera.
Lenses with fewer lens elements are usually capable of producing sharper images with less distortion than lenses with more elements. More elements also reduce the amount of light reaching the sensor.