The BaK-4 prisms have a high refractive index for complete edge-to-edge clarity. The center knob is in perfect placement for your fingers to comfortably adjust the focus. The built-in dioptric correction adjusts for individual eyesight so everyone can use the binocular to its full potential. The Tracker also has eyecups that twist up and down, allowing each user to personalize the binocular to their comfort, while the long eye relief is a necessary feature for eyeglass wearers.