The main identifying feature of a lens is its focal length. Lenses with a single fixed focal length are known as prime lenses.
The focal length of a lens is a measure of how strongly it converges or diverges light. A lens with a short focal length is stronger than one with a long focal length. In other words, short focal lengths bends the rays more strongly, bringing them to focus in a shorter distance. Short focal length lenses have a wider angle of view. Conversely, a lens with a long focal length is weaker, and bends the rays more feebly, bringing them to a focus in a greater distance. Long focal length lenses have a narrow angle of view.
A lens with a focal length about equal to the diagonal size of the film format is known …Read More →