STM and USM are the two popular types of autofocus systems in Canon lenses. These two types of lenses are available for both Canon DSLR and Mirrorless cameras. If you are planning to buy a Canon lens for your camera body, you may get confused with these two lens variants. Don’t worry. I will help you select the best lens that suits your needs. Let us have a look at the key differences between STM vs USM lens.