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
Images for kids
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In this computer simulation, adjusting the field of view (by changing the focal length) while keeping the subject in frame (by changing accordingly the position of the camera) results in vastly differing images. At focal lengths approaching infinity (0 degrees of field of view), the light rays are nearly parallel to each other, resulting in the subject looking "flattened". At small focal lengths (bigger field of view), the subject appears "foreshortened".
See also
In Spanish: Distancia focal para niños
