Lens cap fits! This is a great filter, worth every penny. I am using it on a Nikon lens, but the threads are such that there is room for a lens cap, so no need to constantly remove the filter. In fact, I get great results indoors and out with this filter, so if you forget to remove it indoors, your images should be fine. This is just another one of the many additional features that justify the price of the B + W filters. It also attaches onto my B+W UV filter and then the lens cap attaches on to the polarizing filter without popping off. Layering filters is not something you want to do with an inexpensive filter as the optics are not refined like expensive filters such as the B+W brand, which would cause image quality to suffer. I have had no problem with both my B+W UV AND B+W Polarizer on the lens at the same time. I frequently use the lens with this set up for commercial projects and the images are tac sharp. When buying filters I now only look at the selections by B+W since there quality meets or exceeds their specifications.
EXTRA NOTE: I see comments below about getting a slim filter for a 10-22, which needs a 77mm. This is CORRECT. I have the B+W 77mm polarizing filter for my Nikon 12-24mm lens. I purchased the SLIM version which is: B+W 77mm XS-Pro Digital KSM C-POL MRC nano. It is very expensive, BUT some of the features making it worth it are that it is slim, no vignetting, specs state that it can be used INDOORS or OUTDOORS and I can attest that I use it indoors with no problems. Lens cap fits onto filter, and the KSM model tolerates extreme temperatures. The model that does not have that feature is significantly less expensive. The Adorama specs are very thorough, so check them out when trying to decide between expensive and less expensive models even within the B+W line. They have something for everyone's needs unless you want to go cheap and sacrifice quality. Enjoy! B+W's are great filters.