In photography, there are two types of lenses: zoom lenses and prime lenses. Zoom lenses cover a range of focal lengths. If you’ve just bought a new DSLR, the kit lens that comes with your camera is almost certainly a zoom lens; most come with an 18-55mm lens, which means that lens covers every focal length between 18mm and 55mm. A prime lens, on the other hand, covers just one focal length. A 50mm lens is just a 50mm lens.
If, however, the 18-55mm lens that comes with your camera can be used as a 50mm lens, why on earth would you get a dedicated 50mm prime lens?
Prime Lenses Perform Better
Focal length is just one of the characteristic of a lens; the other important one is aperture. Just knowing the focal length of a lens doesn’t tell us a whole lot about how it will perform in the real world.
Focal length determines the field of view and apparent magnification of the lens, while aperture determines the depth of field and low light performance. All 50mm lenses will share the same field of view and magnification, but they can have vastly different apertures.
Looking back at our 18-55mm kit lens, at 50mm it probably has a maximum aperture of f/5.6. This isn’t especially wide, and won’t make pleasing portraits or perform great in low light. The 50mm prime lens, however, will have an aperture of f/1.8…