The Kaiser 37mm Plastic Lens Cap is a push-on style cap. It is designed to fit any camera or enlarger lens, binocular, tele-, micro-, or spotting scope lens. To use one, measure the outside diameter of your lens barrel. Generally speaking, order the cap size that is 1 or 2 millimeters larger than the size measured. The actual size needed depends on the lens in question. The cap should fit snuggly but not so tight that it is hard to remove.