I found that the lens cap that came with the 17/1.8 fell off easily. I was prepared to replace it with the exact same cap before I saw this option available. Earlier internet research suggested that it wouldn't fit with any filters on, but I've left a UV filter on in the past and it fit just fine. Note that there is no screw or discernible clicking sound to indicate that the cap is on securely, but it still feels like it's on pretty securely.