Objective Lens Coatings

Objective Lens Coatings A camera lens is an optical assembly consisting of a single lens or a group of lenses. Some lenses are interchangeable, while others are built into the camera body structure. Modern lenses attempt to minimize aberration by setting the angle of incidence and angle of refraction to equal values, and they incorporate a focusing element that allows the operator to determine which areas of the image are acceptably sharp and which are blurred. The aperture determines the amount of light passing through the lens. The lens focuses light onto the image sensor (digital) or film plane (analog).

What is Lens Flare?

Lens flare occurs when light that does not form an image enters the lens and subsequently strikes the camera's film or digital sensor. This typically appears as a characteristic polygonal shape, with edges dependent on the shape of the aperture. The glare appearing as polygonal shapes is caused by light reflecting from the internal edges of the lens opening (aperture). This condition can significantly reduce the overall contrast of a photograph and is generally an unwanted artifact, though certain types of flare can actually enhance the artistic quality of a photograph. Lens elements typically include a type of anti-reflective coating aimed at minimizing flare, but no multi-element lens eliminates it completely. Light sources will still reflect a small portion of their light, and this reflected light becomes visible as flare in regions where its intensity becomes comparable to the refracted light (which produces the actual image). Surface reflection reduces the amount of light transmitted from a lens, but this is not the only negative effect. Reflections within the lens also cause problems such as image duplication and transmission of non-image light into the image—phenomena known respectively as ghosts and flare. Ghosts are created when light reflected from the rear surface of a lens is reflected once more from the front surface, resulting in a faint second image slightly displaced from the primary image. Flare becomes visible when light from behind the lens frame is reflected from the lens surface into the image. Ghosts and flare resulting from surface reflection reduce the quality of the produced image. Although flare is technically caused by internal reflections, very intense light sources are usually required for it to become significant (relative to refracted light). These intense light sources causing flare can be the sun, spotlights, other artificial lighting, and even the full moon. Even if the photograph itself does not contain intense light sources, stray light can enter the lens when it strikes the front element. Light normally outside the field of view does not contribute to the final image, but if this light is reflected, it can reach the film/sensor through an undesired path.

Thin Film Coatings

The surfaces of optical lenses are coated with thin films that play an important role in enhancing lens performance. Ordinary glass lenses transmit most of the light striking them, yet approximately 4% of this light is lost to surface reflection. Since lenses have two surfaces—front and rear—the light loss doubles to approximately 8%. Most camera lenses consist of five to ten elements, and therefore the total amount of transmitted light is reduced by approximately 50%. Lens coatings have been developed to prevent surface reflection and increase light transmission. Coated lenses allow more light to pass through them.

How Coatings Increase Light Transmittance

Surface reflection can be reduced by applying coatings to the lens surface. You might think that coating the lens surface would block light, but it actually increases light transmission. This is because light is first reflected by the coating surface and then by the lens surface itself. The light reflected from the coating surface and the light reflected from the lens surface have a phase difference of twice the coating thickness. If the coating thickness is one-quarter of the wavelength of light to be suppressed, the light of that wavelength reflected from the coating surface and the light reflected from the lens surface will cancel each other out. This reduces the total amount of reflected light. In short, coatings use the light wave interference phenomenon to eliminate reflections.

Increasing Light Transmittance to 99.9% with Multi-Layer Coatings

Coating materials such as magnesium fluoride (MgF2) or silicon monoxide (SiO) are applied as extremely thin, uniform coatings to the surface using techniques such as vacuum deposition or plasma sputtering. However, light consists of many different wavelengths, and it is impossible for a single coating to block all reflected light. Many coating layers are required to reduce reflections of light at various wavelengths. Such multi-layer coatings are applied to premium lenses. Advanced technologies have been developed for applying coatings consisting of more than ten layers, and premium lenses with such coatings are capable of achieving 99.9% light transmittance across a range extending from ultraviolet to near-infrared light.

Quality Raw Materials Matter

Since light passes through the lens, the glass quality and raw materials of the lens are extremely important. For example, the degree of chromatic aberration in telephoto lenses can be substantial, and color shifts and loss of sharpness can easily occur. However, these problems can be significantly reduced with an ED (extra-low dispersion) lens. For example, this type of lens, also called an "APO" (apochromatic) lens, guarantees high performance. Sources • https://www.studiobinder.com/blog/understanding-camera-lenses-explained/ • https://www.cambridgeincolour.com/tutorials/lens-flare.htm •https://global.canon/en/technology/s_labo/light/003/03.html#:~:text=Lens%20coatings%20are%20used%20not,and%20reflect%20all%20other%20wavelengths. • https://www.shutterbug.com/content/why-lens-coatings-are-so-important-these-lovely-layers-are-more-just-pretty-face •https://www.newport.com/n/opticalcoatings#:~:text=Optical%20coatings%20typically%20consist%20of,properties%20of%20an%20optical%20component. • https://av.jpn.support.panasonic.com/support/global/cs/dsc/knowhow/knowhow17.html • Images by: pixabay.org Compilation and Translation: B. Serhat Cengiz