Theatrical Lighting Lenses
A lens curved on one side and flat on the other. The more pronounced the curvature of the convex side, the closer to the lens will be the point at which light rays entering the lens from the convex side will converge. The distance from the lens to this point is called the focal length.
A plano-convex lens is described by its diameter and focal length. For example, a 6″x9″ lens will have a diameter of 6″ and a 9″ focal length. The shorter the focal length, relative to the diameter of the lens, the wider the beam of light; thus, a 6″x12″ lens will emit a beam of light 3/4 the width of the 6″x9″ lens. When two plano-convex lenses are used “belly-to- belly”, their effective combined focal length is halved. For example, two 6″x9″ lenses belly- to-belly will have an effective focal length of 4!”. Fixtures using plano-convex lenses typically project sharp-edged images.
Plano-convex lenses with the flat side cut away in steps. Step lenses are optically similar to plano-convex lenses, but lighter and less prone to cracking from the heat. The light from a step lens is usually not as even as that from a plano-convex lens.
Fresnel lenses, as opposed to step lenses, are cut away from the front. They are extremely thin and therefore efficient and less likely to crack from heat Unlike step lenses, each of which has a single focal length, each concentric ring of a Fresnel lens has a different diameter and a slightly different focal length. Fixtures using Fresnel lenses project soft-edged images.