Camera Lens Parts: Lens Cap
Among camera lens parts, a lens cap is the simplest. It is a hard plastic lid that fits over the front of a lens. They are measured by diameter in millimeters. It is important to note that this is lens diameter, not focal length. Sometimes, lens diameter is referred to as, "Filter size." These two numbers can be quite similar and easily confused. For example, a common Canon 50mm f1.8 lens has a filter size of 52mm. As long as they are the right size, most lens caps can be interchanged.
Although inexpensive, lens caps serve two important purposes.
A lens cap protects a camera lens from damage. The glass elements of a lens can easily be scratched or broken by incoming objects and repairs can be very expensive. Lens caps are not easily broken and cheap to replace if needed.
Dust, fingerprints and moisture can dirty a lens, resulting in blurred images. A lens cap keeps these off the glass.
Problems with lens caps
Being small and round, lens caps have a tendency to be misplaced or to roll away. Some photographers use elastic bands to connect their lens caps to their lenses, thus preventing loss. Many camera stores sell used lens caps, which usually work fine.