This Butler Creek flip-up Eyepiece lens cover opens at the touch of your thumb and features a watertight friction mount and semi O-ring that securely anchors the cover, sealing out dust and moisture.  The ocular lens covers feature a large push button for quick spring loaded opening when you want to keep your optics protected until the perfect shot presents itself. To select the correct cover, refer to the scope's owners manual or measure the outside diameter with calipers and select the closest measurement from the table below that is  .01" under to .025" ( .25mm- .64mm) over the measurement you get .