Focal length
The focal length tells how strongly a lens, a curved mirror or another optical device brings light together or spreads it out. If rays of light go straight into a lens and the focal length is positive, the focal length is the distance to the point behind the lens where the rays will be brought to a focus. If the focal length of the lens is negative, the focal length is the distance at which the light has spread to twice its original size. Light from a large distance produces an image at F (focal point). The distance from the centre of the lens to F is the focal length.
Focal Length Media
The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror.
Sketch of human eye showing rear focal length Template:Prime and EFL
Illustration of the relationship between optical power and focal length
