I bought the 40mm STM pancake lens for the 5D mkIII the other day. I am having so much fun with this lens. I am really amazed at how sharp this lens is. The cropping potential with this lens is very good. Although the lens is not a USM lens it is still fairly fast. STM (Stepper motor) was designed with video in mind as it is very quiet and focusing is very smooth. I love the pancake design for it’s size. It is very compact and makes the camera a real lightweight. The camera is a real pleasure to carry around as you lose the top heaviness that occurs with the larger lenses. Another great feature is the price. At around $1,500hk this is a very affordable lens. Mix that with a compact size for easy storage and fantastic picture quality it is a no brainer to pick one of these up for your lens collection.