A diffractive optical lens, otherwise called a Phase Fresnel or Focal Fresnel lens appears totally alien to most quality stills photographers, right up to the point where they start explaining the benefits of diffraction. A diffractive lens has two important advantages over conventional lenses. The first is its ability to shrink the size and weight of an optical lens. The second is its ability to focus light into a very small spot. If you want to take pictures of stars at night, you need to understand all the benefits of diffraction.
Important details regarding diffractive optics
Most people think that diffractive optics only works if the object is in a coma state, because it bends the light in such a way that the incident light has two different wavelengths. However, many objects actually have only one wave of light coming through, which is just what is called a monochromatic lens. This is because the single wave of the single optical power is the same all the way through. For example, a standard full-aperture camera with a magnesium thiocyanate objective lens produces just f/3.5 on the long end of the scale and f/10 on the short end.
Standard spotlights with traditional diffractive optics can be used to create a scene with very large amounts of background blur because the single focal length lens is so small. In the case of this situation, the spotlights are used along with a computer software program that produces a corrective chromatic aberration effect on the lens at various exposures. Chromatic aberration occurs when light with different wavelengths comes into contact with the lens, creating an uneven tint on the object. Although the source of the chromatic aberration will always be the same, different wavelengths of light coming into contact with the lens cause the spotlights to appear differently in the final image. These types of aberration errors can be eliminated by using an objective lens with diffractive optics.
Lenses with diffractive optical elements can be made out of two types of materials: optical glass and metallic lenses. Both of these types of lenses can work well for photography and many other applications because they are relatively inexpensive. Optical glass is most commonly made by pouring together a combination of two high-precision glasses known as refractory (fiberglass) and a substrate with a high index of crystalline crystals. These two materials are combined to form a thin layer on the inside of the lens where the light can pass through to complete the objective lens.