Fresnel Lens and Parabolic Reflectors
Light Collimation
Since its commercial introduction in 1929, the Fresnel Lens has been an important lighting tool for still photography, cinema and stage, because of its fantastic light quality and focusing ability. But why does a Fresnel lens produce such great light? To understand this, we need to have a basic understanding of lens optics, to see what's actually going on.
Convex Lens
A traditional convex lens works by taking the incoming parallel light rays on one side, and refracting them on the opposite side, causing the light to converge into a single point. This point is known as the focal point, and the distance to which is the focal length. Conversely, when you shoot a light source through the lens from that same focal point, the lens collimates the light rays coming out the other side, organizing them into clean, parallel rays. Conventional lenses used a spherical curvature, which caused spherical aberration, because the light rays did not converge properly at the focal point. As technology improved, lens manufacturers started using aspherical lenses to correct this issue. Most camera lenses purchased today use aspherical curvature.
Diagram of a Convex Lens
Fresnel Lens
A Fresnel Lens is a flattened version of a convex lens. It was developed in the early 19th century, as a way to reduce the amount of glass material needed to build a convex lens (specifically for large lighthouse lenses). A Fresnel Lens uses the cross-sections of the curvature of a convex lens, but flattens them onto planar surface. That's why the surface of a Fresnel Lens contains a series of concentric rings - each ring has a slightly different cross-section curvature, corresponding to the curvature of a convex lens of equal diameter.
Construction of the Fresnel Lens
As a result, you get the same collimated light rays, without the weight or large amount of glass material needed for a convex lens. It is these collimated rays that give light shaping tools like the Fresnel lens their distinctive quality - smooth and even coverage, rich detail and beautiful contrast. Fresnel-based lighting tools also use a focusing system to move the light in and out of the focal point. When the light is at the focal point (the "spot" position), the light is focused and hard, with the least amount of spread. In the "flood" position, the light spreads out wider and has softer shadows.
Fresnel Lens with Light Source at Focal Point ("Spot" Position). The light rays become collimated, with more contrast and less spread.
Fresnel Lens with Defocused Light Source ("Flood" Position). The light rays have more spread, and become less contrasty.
Fresnel Lens with Light Source at Focal Point ("Spot" Position). The light rays become collimated, with more contrast and less spread.
Fresnel Lens with Light in Spot (Focused) and Flooded (Defocused) Positions.
Examples of Fresnel Lenses:
Fresnel Lights in Use:
Acclaimed Hollywood photographer George Hurrell using Fresnel lighting on set. Perhaps the most famous Hollywood portraitist of his time, Hurrell was known for the dramatic lighting he used to capture the glamorous movie stars of the 1930's.
John Ford, John Wayne, and a large 24-inch Fresnel lighting the set.
Mario Testino shooting Gisele Bundchen for Vogue UK December 2011, using a combination of natural light, a 10" Fresnel, and small strobe held close to camera. Interesting to note also that the ambient daylight here is being shaped or "focused" by the use of black V-flats on the sides.
Parabolic Reflectors and the Fresnel Lens
A Parabolic Reflector has the same effect on light rays as a Fresnel Lens. They both have a focal point, and they both take the scattered rays of a point light source and shape them into parallel light rays with even spread. However, instead of shooting light directly through a Fresnel lens, the light is shot indirectly into the surface of the parabolic reflector, reflecting back onto the subject in a smooth, collimated beam of light. That's why parabolic reflectors are best used with an indirect light source - so your subject only receives the even light rays from the reflector, not the harsh scattered rays from the bulb itself - and a method for focusing the light in and out of the focal point. When the light is closest to the focal point, you get parallel light rays, harder light with sharper contrast. As you move the light source away from the focal point, the light spreads (diffuses) more and appears softer. As a result of this focusing ability, a parabolic reflector with a light focusing system allows you to get a variety of light qualities from a single reflector. This is the basis of the Parabolix Light-Focusing System™.
Schematic diagram showing a parabolic reflector with the light at the focal point. Collimated light, more contrast.
Schematic diagram showing the light placed in a defocused position. More spread, less contrast.
Schematic diagram showing a parabolic reflector with the light at the focal point. Collimated light, more contrast.
Size Matters
A Fresnel-based light uses a glass lens and a metal housing to hold and support the lamp and electronics, therefore size, cost and portability play important factors in the ability to use this light-shaping tool. The largest Fresnel-based lights used for photography and cinema have a diameter of 24 inches. These units are very large and heavy, and can cost upwards of $30,000. A parabolic reflector can be produced for a fraction of that cost, and are light-weight, collapsable, portable and durable. Because of these benefits, parabolic reflectors are not limited in how large - or small - they can be made.
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