Fresnel lenses consist of concentrically aligned ring-shaped lens segments. The optical effect of these lenses is comparable to the effect produced by conventional lenses of corresponding shape or curvature. Fresnel lenses are, however, considerably flatter, lighter and less expensive, which is why they are frequently used in luminaire construction in place of converging lenses. The optical performance of Fresnel lenses is confined by aberration in the regions between the segments; as a rule the rear side of the lenses is structured to mask visible irregularities in the light distribution and to ensure that the beam contours are soft. Luminaires equipped with Fresnel lenses were originally mainly used for stage lighting but are now also used in architectural lighting schemes to allow individual adjustment of beam angles when the distance between luminaires and objects varies.