The design includes aspherical lenses which compensate for distortion and an ED element which suppresses chromatic aberration, for excellent corner-to-corner sharpness (even with the aperture wide open). Direct Manual Focus (DMF) allows you to manually focus after autofocus has locked on, so you can easily fine-tune your focusing without switching modes. This is especially useful when shooting with very shallow depth-of-field. A linear motor used for autofocus drive helps make this lens ideal for video--a very low-noise focus operation, with a stepping motor built into the lens for smooth, quiet aperture actuation. This lens is a wonderful addition to any photographer′s camera bag.