Constructed to work at only one focal point lenght. They are distinctive by high quality photography, simplest construction and high light flow (e.g. F/1.4). Only disadvantage is that they work on only one focal point lenght so they don't have usableness of zoom objective. They are available in range from very wide 15mm to unbelievable 12000mm of whose prices is obscene to talk.
Canon 24mm prime lens