A lens cap is a protective cover designed to fit securely over the front or rear of a photographic lens when it is not in active use, shielding the exposed optical glass surfaces from dust, moisture, fingerprints, scratches, and other physical damage that could degrade image quality or permanently harm the lens elements or their delicate anti-reflective coatings. It is one of the simplest, least expensive, and yet most practically important accessories in any photographer's kit, providing essential protection for what is often the most optically critical and financially significant component of the camera system.
Front lens caps are the most commonly used type, fitting over the front element of the lens to protect its outer optical surface and any filters attached to it. They are most commonly made from lightweight plastic and attach to the lens either by pressing over the outer rim of the lens barrel in a friction fit, or by engaging with the inner lip of the filter thread via a centre pinch mechanism that allows the cap to be attached and removed with a single handed pinching and pulling action - a particularly convenient design when a lens hood is fitted, as the centre pinch mechanism can be operated through the opening of the hood without removing it. Some lens caps use a spring loaded clip mechanism on either side of the cap that must be squeezed to release the cap from the lens, while others simply push on and pull off with a firm friction fit.
Rear lens caps are fitted over the rear element of a lens when it is removed from the camera body and stored separately, protecting the rear optical surface and the delicate rear element coatings from dust and physical damage during handling, transport, and storage. Rear lens caps are typically specific to the lens mount of the camera system, as the geometry and diameter of the rear mount interface varies between different manufacturers and mount standards. Body caps - similar devices that cover the camera's lens mount opening when no lens is attached - serve the complementary function of protecting the interior of the camera body, the mirror box, and the image sensor or film plane from dust and debris when a lens is not mounted.
Despite their simplicity, lens caps are among the most frequently misplaced items in a photographer's kit, and their loss can leave valuable lens surfaces exposed to damage during storage and transport. Many photographers carry spare lens caps for their most frequently used lenses, and a variety of lens cap retention accessories - including leashes and holders that keep the cap tethered to the lens or camera strap when removed during shooting - are available to reduce the risk of caps being set down and forgotten in the field.