The 'E' is for 'Extreme.'. The EXPS3 offers true 2-eyes-open shooting and a 7 mm raised base that still allows for iron sight access. It features easy-to-adjust side buttons and an adjustable locking quick-detach lever for easy attachment and removal. EXPS3 was chosen by the U.S. Special Operations Command (USSOCOM) as their primary optic for close-quarter engagements. It is also the optic of choice for the technologically advanced modern rifle hunter. Its night vision-compatibility makes it a versatile choice for day or night hunting. Product Details Night vision compatible - Length/Width/Height: 3.5 x 2 x 2.4 - Weight: 8.0 oz - Waterproof: Submersible to 33 ft. depth - Mount: 1 Weaver or MIL-STD-1913 rail - Return to Zero: Repeatable to within 1 MOA after re-mounting. Brightness: 20 daylight settings 10 additional settings for Gen I through III+ night vision devices - Power Source: One 123 lithium battery - Battery Life: 600 continuous hours at nominal setting 12 - Brightness: 20 daylight settings 10 additional settings for Gen I through III+ night vision devices