A lens includes a casing, a first lens group, a second lens group and a heat dissipating member. The first lens group is disposed in the casing and close to a first side of the casing. The second lens group is disposed in the casing and close to a second side of the casing, wherein the first side is opposite to the second side. The heat dissipating member is disposed at the second side of the casing and contacts the casing.