There's one more thing to understand about lenses for full frame and APS-C sensors and that's the concept of the image circle. Basically all lenses produce a circular image field and the diameter of that circle has to be larger than the diagonal of the frame, otherwise the corners of the image will be dark. For full frame 35mm a lens must have an image circle larger than 43.27mm, and for a 15 x 22.5 mm APS-C frame, an image circle of at least 27.04mm. This is shown schematically below: