Marathon Focus Free Binocular 10 x 50. 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. This binocular has a unique focus free optical design. This feature means that you do not have to worry about adjusting any focus knobs or diopter rings, just pick up this binocular and look. It allows you to stay in focus at most viewing distances. The default setting (focus distance) is 70 feet depending on the user's eyesight. Due to the fine quality of our fixed focus optics, you can view clean and crisp images of fast moving objects from a relatively close distance of 45 feet (15 metres) to infinity.