Thin lens positive if the surface is concave, and negative if it is convex. This is an arbitrary sign convention; some authors choose different signs for the radii, which changes the equation for the focal length. Certain rays follow simple rules when passing through a thin lens, in the paraxial ray approximation: By tracing these rays, the relationship between the object distance "s" and the image distance "s′" can be shown to be which is known as the thin lens equation. In scalar wave optics a lens is a part which shifts the phase of the wave-front. Mathematically this can be understood
Focal cloud Focal cloud A focal cloud is the collection of focal points of an "imperfect" lens or parabolic reflector whether optical, electrostatic or electromagnetic. This includes parabolic antennas and lens-type reflective antennas of all kinds. The effect is analogous to the circle of confusion in photography. In a perfect lens or parabolic reflector, rays parallel to the device's axis striking the lens or reflector all pass through a single point, the focal point. In an imperfectly constructed lens or reflector, rays passing through different parts of the element do not converge to a single point but have different focal points. The