http://www.nikon.com/news/2015/0106_lens_02.htm
" The world's lightest fixed focal length (prime) telephoto lens with a focal length of 300mm and maximum aperture of f/4, achieved with the adoption of a PF (Phase Fresnel) lens element, a first for NIKKOR lenses. "
AF-S NIKKOR 300mm f/4E PF ED VR Primary Features
-- A fixed focal length telephoto lens with a focal length of 300mm and a maximum aperture of f/4 compatible with Nikon FX-format digital SLR cameras
-- The first NIKKOR lens for which a PF (Phase Fresnel) lens element has been adopted, making it the world's lightest*1 lens of its type
-- Equipped with a SPORT VR mode for effective photography of athletes and other subjects that are moving rapidly and unpredictably in addition to a vibration reduction (VR; camera shake compensation equivalent to a 4.5-stop*2 increase in shutter speed) function
-- Adoption of a PF (Phase Fresnel) lens element, an ED lens element, and Nano Crystal Coat for superior optical performance with which chromatic aberration and ghost are suppressed
-- Capable of expression of the beautiful blur characteristics possible only with a maximum aperture of f/4
-- Equipped with an electro-magnetic aperture mechanism that enables stable auto-exposure with high-speed continuous shooting
-- Adoption of Nikon's exclusive and highly durable fluorine coat, which effectively repels water and oils, and makes cleaning the lens easier even when it does get dirty, on the surface of the front lens element.
PF (Phase Fresnel) Lens Elements
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The PF (Phase Fresnel) lens, developed by Nikon, effectively compensates chromatic aberration utilizing the photo diffraction phenomenon*. It provides superior chromatic aberration compensation performance when combined with a normal glass lens. Compared to many general camera lenses that employ an optical system using the photorefractive phenomenon, a remarkably compact and lightweight body can be attained with less number of lens elements.