The sigma 17 50mm f28 ex dc hsm lens is an image stabilised fast standard zoom for digital slr cameras with an aps c sized sensor the lens covers a focal length from 17mm wide angle and offers a large aperture of f28 throughout the entire zoom range making it ideal for many types of photography such as portraiture and landscapes the 17 50mm lens includes a hypersonic hsm autofocus motor to ensure quick and quiet focusing it uses two elements made from sigmas new fld glass plus two gl