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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.