Linear focusing simply means that the movement of the focus ring is consistent with the change to the focusing distance, regardless of how quickly the ring is rotated. This is common to all mechanical focusing systems. Non-linear focusing, which is found on Sigma’s mirrorless DN line of lenses, is an electronically controlled focus system where the sensitivity of the ring changes depending on how fast it is rotated. On some of Sigma’s DN lenses, users can switch between linear and non-linear focusing using the USB Dock, and also change the focus throw of the lens.