The main difference between a 50mm lens and an 18-55mm lens is their focal lengths. A 50mm lens has a fixed focal length of 50mm, which is ideal for portraits and low-light Photography. On the other hand, an 18-55mm lens has a variable focal length range from 18mm to 55mm, making it versatile for capturing a variety of scenes from wide-angle to mid-range. The 50mm lens typically produces sharper images with a shallower depth of field, while the 18-55mm lens offers more flexibility for different shooting situations.