Why should I buy a 50mm prime lens?

You should buy a 50mm prime because: - Almost without exception they're really fast, so you can shoot lowlight. This one was taken with just a TV on. -They can achieve low depth of field, which can be used to isolate the focused parts of the picture, in order to give emphasis to them, or to create an optical effect for whatever artistic reasons. I believe this is the case here (emphasis on the eye, isolated from the rest of the face) - Some of the are really cheap and have incredible price/performance ratio (this one costs around 100 euros) - They can achieve very high image quality, especially regarding sharpness. This is not the case here (this is iso 1600 at f 1.8 at very low light), but generally speaking it is.