The Celestron Omni 4mm Eyepiece is a 4 -element premium symmetrical Plossl optical design, which determines your telescopes magnification and can give you a wider field of view. The features of this eyepiece include lens edges to minimise internal reflection and maximise contrast, and fold-down rubber eye cups for comfort of use. Rubberised grips come as standard as well. The eyepiece has multi-layer coating group on each lens surface to maximise light transmission and contrast, and each Omni goes through a two-step anodizing process to prevent reflection from the top eyepiece barrel, unlike most other manufacturers that use reflective surfaces. Specifications Focal Length (mm): 4 mm (0.16 in) Angular Field of View (deg): 52 deg Eye Relief (mm) : 6 mm (0.24 in) Size : 1.25 in (32 mm)