Not everybody has a camera with interchangeable lenses, but if you do than you should have a 50mm f/1.8. We’ll start with the simplest reason, it’s cheap. You can usually find it for around $100 dollars used. With it you get a lens with a great shallow depth of field (that fancy blurry effect in great photos). Also, it gives you a fantastic lens for portraits as well as a lens for low light photos. This is not to say that this is the only lens that you should use, but certainly one of the most versatile lenses that you should own.