A lens hood is a baffle attached to the front of a lens that blocks stray light from striking the front element, reducing lens flare and veiling glare while improving contrast. It also serves as a physical bumper, taking the impact of an accidental brush against a doorframe or a wall and protecting the front element from fingerprints, raindrops, and snowflakes. Most lens manufacturers include a dedicated hood with each lens and shape it to the lens’s specific angle of view.
The shape varies by focal length. Wide-angle and short telephoto zooms use petal-shaped (also called tulip) hoods, with cutouts at the corners that match the rectangular image and a flange at the top and bottom that blocks light without intruding on the diagonals. Long telephotos use cylindrical hoods, since the narrower angle of view allows a deeper baffle without vignetting the corners. Fisheye lenses usually have only a small integrated petal because any larger hood would appear in the 180-degree frame.
Flare reduction is the optical reason hoods exist. Light striking the front element at a steep oblique angle bounces between glass surfaces and the aperture blades, producing colored ghosts, polygonal artifacts, and a wash of veiling that lowers contrast. A properly designed hood intercepts that oblique light before it reaches the glass while leaving the actual imaging angle untouched. The same principle underlies the matte boxes used on cinema lenses, which are square hoods with adjustable flags and filter slots.
Physical protection is the practical reason most photographers leave hoods attached at all times. Wedding shooters elbow through crowds with $2,000 zooms forward, and the hood absorbs every nudge that would otherwise crack the front element or knock the filter ring out of round. A hood is also cheaper to replace than a UV filter and adds no optical element to the path. Many photographers skip protective filters entirely on lenses that carry a substantial hood.
Mounting is usually bayonet-style, with a quarter turn and a click stop. Most hoods can be reversed for transport, sliding over the lens barrel so the whole package occupies the same space as the lens alone. The reversed hood obscures focus and zoom rings, which is the trade for compactness. Some hoods include a sliding window so a polarizing filter can be rotated without removing the hood, a convenience that disappears on cheaper third-party copies.
Common mistakes include using a hood designed for a longer lens (which causes vignetting in the corners) and leaving the hood reversed during shooting. The hood also interferes with built-in flashes on small bodies, casting a curved shadow across the bottom of the frame at the widest focal lengths, which is why pop-up flashes and wide zooms are a notoriously bad combination. The cheapest improvement most beginners can make to their image quality is to attach the supplied hood and leave it on.