BAK4, or Barium Crown glass, is considered the best type of prism material. It has a high refractive index and lower critical angle than other materials, which means it transmits light better with less peripheral light being lost due to internal reflection. Field of view is expressed in feet at a distance of 1,000 yards or meters at 1,000 meters, and is the width of the visible area that can be seen without moving the binoculars. The 27.4mm eye relief is the optimal distance from the eyepiece to your eye, or the focal point where the light passes through the ocular lens. Great to use even with eyeglasses. Waterproof structure allows the product to be submersible to a depth of 0.5m (time- 20mins). The product is nitrogen filled for fog-proofing. Additional Features- • Built-in tripod mount. • Neck strap, eye-cup protectors, objective lens protectors, carrying case, lens cloth and instructions included.