Lens coating refers to the application of one or more extremely thin, transparent layers of metallic oxide or other optical materials to the surfaces of individual lens elements, designed to reduce the amount of light reflected at each air to glass interface within the lens and thereby minimise flare, improve image contrast, and increase the overall light transmission of the lens system. Lens coating is one of the most important optical technologies in modern lens manufacturing, and the quality and sophistication of the coatings applied to a lens have a profound influence on its real world optical performance, particularly when shooting in challenging lighting conditions involving bright light sources within or near the frame.
When light passes from air into glass or from glass back into air, a proportion of that light is reflected at the interface rather than continuing through the optical system. For a single uncoated air to glass surface, approximately four to five percent of the incident light is reflected rather than transmitted, depending on the refractive index of the glass. In a complex modern zoom lens containing twenty or more individual elements - and therefore forty or more air to glass surfaces - the cumulative effect of these reflections would result in a significant loss of light transmission, a dramatic reduction in image contrast due to the scattered reflected light bouncing around inside the barrel, and pronounced susceptibility to flare and ghosting when bright light sources are present in or near the frame.
Lens coatings address this problem by exploiting the principle of thin film optical interference. A coating of precisely calculated thickness - typically one quarter of the wavelength of the target light - applied to a lens surface causes light waves reflected from the top and bottom surfaces of the coating to travel slightly different path lengths and arrive at a phase relationship where they interfere destructively, effectively cancelling each other out and eliminating or greatly reducing the reflection. The earliest single layer coatings, applied from the 1940s onwards, reduced surface reflections to around one to two percent and gave coated lenses a characteristic purple, blue, or amber tint when viewed in reflected light - a visual indicator of the coating's presence that remains familiar today.
Modern lens designs employ multi-coating technology, in which multiple layers of different optical materials are applied to each lens surface in a carefully engineered stack, each layer tuned to suppress reflections across a different part of the visible spectrum. Multi-layer coatings can reduce surface reflections to as little as 0.1 to 0.3 percent per surface, dramatically improving light transmission, contrast, and flare resistance compared to single layer coatings or uncoated glass. The most advanced proprietary multi-coating systems developed by major lens manufacturers - such as Nikon's Nano Crystal Coat, Canon's Sub Wavelength Structure Coat, and Zeiss's T* coating - use nanostructured surface treatments and highly sophisticated multilayer stacks to achieve exceptional performance even in the most demanding lighting situations.
The practical benefits of high quality lens coatings are most clearly apparent when shooting into the light, in scenes with strong specular highlights, or when bright artificial light sources are included within the frame. A well coated lens maintains rich shadow detail, deep blacks, and accurate colour rendition in these conditions, while a poorly coated or uncoated lens of otherwise similar optical design would exhibit significantly reduced contrast, washed out shadows, colour shifts, and distracting flare artefacts that degrade the quality and impact of the image.