A lens hood, also commonly referred to as a lens shade, is an accessory fitted to the front of a photographic lens that extends forward from the lens barrel to shade and shield the front element from stray light sources - such as the sun, artificial lighting, or any other bright source - that fall outside the angle of view of the lens but whose light would otherwise strike the front element at an oblique angle and cause flare, reduced contrast, and ghosting in the image. It is one of the most practically valuable and optically beneficial accessories a photographer can use, and many photographers consider it an essential piece of equipment that should be in place whenever the lens is in use.
The fundamental principle behind the lens hood is straightforward. Flare is caused by non-image-forming light - light that does not originate from the scene being photographed - entering the lens at an angle and reflecting internally between the lens elements and the barrel walls, scattering across the image and reducing contrast or creating bright artefacts. By extending a physical barrier forward from the front of the lens, the hood intercepts these oblique light rays before they reach the front element, preventing them from entering the optical system and causing internal reflections without obstructing any of the light coming from the subject within the actual field of view.
Lens hoods are available in several designs, the most appropriate of which depends on the focal length and angle of view of the lens it is intended to shade. A cylindrical or straight sided hood is the simplest design, suitable for telephoto lenses with a narrow angle of view where a deep, straight sided hood can shade the front element very effectively without risk of the hood itself intruding into the image area. For wide angle and standard lenses with a broader angle of view, a petal or tulip shaped hood - with a scalloped profile featuring alternating deeper and shallower sections corresponding to the corners and sides of the rectangular image frame - allows the hood to extend as far as possible into the corners of the frame while cutting back at the sides to avoid vignetting the wider horizontal or vertical extent of the image.
The depth of the hood is critically important to its effectiveness and must be carefully matched to the angle of view of the specific lens it is designed for. A hood that is too shallow provides insufficient shading and leaves the front element vulnerable to oblique light. A hood that is too deep for the lens's angle of view will intrude into the image area and cause vignetting - darkening of the corners - particularly at wider focal length settings on zoom lenses. For this reason, lens manufacturers design and supply hoods specifically matched to each individual lens model, and using a hood intended for a different focal length or angle of view can compromise either its effectiveness or the integrity of the image.
In addition to their primary optical function of reducing flare, lens hoods provide the secondary practical benefit of offering a degree of physical protection to the front element of the lens, acting as a bumper that absorbs minor impacts and helps prevent direct contact with the glass surface during shooting. Lens hoods are made from a variety of materials including rigid plastic, metal, and flexible rubber - the rubber collapsible type being particularly convenient for storage and travel as it can be folded back against the lens barrel when not needed without requiring removal.