The E-185B Lens Cap for EF 600mm F/4 Lens is a very useful and a good quality lens by Canon. It is a padded lens cover that slides on around the front end of the lens barrel with or without the lens hood in place. It is specifically designed for EF 600mm F/4 Lens. It offers protection from impact, dust, moisture and fingerprints when the lens is not in use. The E-185B tightens to stay in place with a touch fastener strap.