 No image is formed during such a test, and the focal length must be determined by passing light (for example, the light of a laser beam) through the lens, examining how much that light becomes dispersed/ bent, and following the beam of light backwards to the lens's focal point.For a "thick" lens (one which has a non-negligible thickness), or an imaging system consisting of several lenses or mirrors (e.g.a photographic lens or a telescope), the focal length is often called the "effective focal length" (EFL), to distinguish it from other commonly used parameters