This modular hood simply snaps onto the filter holder by pressing on the upper and lower faces. For telephoto lenses you can add additional Cokin Modular Hoods by clipping them into the first one. This hood helps to elimante lens flare. The hood (255) is necessary in order to use certain Cokin filters and accessories : The Prism (219), Mirage (220), Coupling Ring (308) and Colored Diffusers (082). It is a good ideal to have at lease two of the hoods. If two is too many just snap it off