A camera or camcorder lens is an optical body with a single lens or a group of lenses. Some lenses are replaceable, while others are built into the structure of the camera body. Modern lenses attempt to set the angle of incidence and the angle of refraction to equal values to reduce the amount of aberration and have a focus element that allows the operator to determine which parts of the image are acceptably sharp and which parts are blurry. The aperture determines the amount of light that passes through the lens. The lens focuses the light on the image sensor (digital) or the plane of the film (analog).
What is Lens Flare?
Lens flare occurs when non-image-forming light enters the lens and then strikes the camera’s film or digital sensor. This often appears as a characteristic polygonal shape
whose sides depend on the shape of its diaphragm. Glare, which appears as polygonal shapes, is caused by light reflecting off the inner edges of the lens aperture (aperture).
This can significantly reduce the overall contrast of a photo and is often an undesirable mark, but some types of glare can actually increase the artistic meaning of a photo. Lens
elements often contain some type of anti-reflective coating aimed at minimizing glare, but no multi-element lens will completely eliminate it. Light sources will still reflect a small portion of their light, and this reflected light becomes visible as flare in regions where the intensity becomes comparable to the refracted light (created by the real image).
Surface reflection reduces the amount of light transmitted through a lens, but this is not the only negative effect. Reflection within the lens also causes problems with image
duplication and transmission of non-image light into the image, i.e. phenomena known as ghosts and flares, respectively. When light reflected from the back surface of a lens is reflected once again from the front surface, ghosts are formed, resulting in a faint secondary image that is slightly displaced from the primary image. Flares appear when
light from behind the lens rim reflects off the lens surface into the image. Ghosts and flares caused by surface reflection degrade the quality of the produced image.
While glare is technically caused by internal reflections, it usually takes very intense light sources (relative to refracted light) for this to become significant. These intense light sources that produce glare can be the sun, spotlights, other artificial lighting, or even the full moon. Even if the photo itself does not contain intense light sources, stray light can
enter the lens when it hits the front element. Normally light outside the field of view does not contribute to the final image, but if this light is reflected, it may reach the film/sensor
in an undesired way.
Thin Film Coatings
The surfaces of optical lenses are covered with thin films that play an important role in improving the performance of the lenses. Ordinary glass lenses transmit most of the light
that hits them, but still about 4% of this light is lost in surface reflection. Since the lenses have two surfaces, front and back, the light loss doubles, that is, 8%. Most camera lenses
are made up of five to 10 elements, so the total amount of light passing through is reduced by about 50%. Lens coatings have been developed to prevent surface reflection and increase light transmission. Coating lenses allow more light to pass through them.
How Do Coatings Boost Light Transmittance?
Surface reflection can be reduced by applying coatings to the lens surface. You might think that coating the lens surface will block light, but it actually increases light transmission.
This is because light is reflected first 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 thickness of the coating is one quarter of the wavelength of the 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 phenomenon of light wave interference to eliminate reflections.
Boosting Light Transmission to 99.9% with Multilayer Coatings
Magnesium fluoride (MgF2) or silicon monoxide (SiO) are used as coating materials, very thin coatings are applied evenly on the surface by techniques such as vacuum deposition
or plasma spraying. However, light consists of many different wavelengths and it is not possible for a coating to block all of the reflected light. Multiple layers of coating are required to reduce reflections of light of various wavelengths. Such multi-layer coatings are applied to high-end lenses. High technologies have been developed for the application of coatings consisting of more than 10 layers, and high-end lenses with such coatings can achieve 99.9% light transmission in a range from ultraviolet to near infrared light.
Quality Raw Material is Important
Since the light naturally passes through the lens, the glass quality and raw material of the lens is extremely important. For example, telephoto lenses can have a large amount of
chromatic aberration, and color shifts and loss of clarity 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 the “APO” (apochromatic) lens, guarantees high performance.
References
• 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%20
an%20optical%20component.
• https://av.jpn.support.panasonic.com/support/global/cs/dsc/knowhow/knowhow17.html
• Görseller / Images by: pixabay.org
Compilation and Translation by:
B. Serhat Cengiz
