There are a couple of different types of lens adaptors available. Some of them are threaded and attach to the end of your lens and will allow you to attach filters to them. Other adapters can attach to your camera's body or lens and they will allow you to attach other lenses to them. These can come in handy as most lens mounts are either bayonet or screw models. Adapters will let you attach a different type of lens to your camera. You may attach a screw lens to your bayonet camera with an adapter. They also allow you to attach other lenses such as wide angle or zoom lenses. Because most camera manufacturers make lenses only for their brand of cameras these adapters will help you fit a different brand lens onto your make of camera. Adapters usually start at about $20, but can go up to almost any price depending on the brand and the quality.