The Pentax FA 645 28-45mm F4 SR HD provides users of the 645D or 645Z with a weather-resistant, ultra wide-angle zoom lens with a 35mm equivalent focal length of 22-35.5mm. This lens features the 645AF2 mount for electronic aperture control and autofocus with an internal DC motor. Also, it has a constant maximum aperture of f/4.5 throughout its zoom range and uses extra-low dispersion glass elements to limit the effects of chromatic aberrations and improve sharpness.