Canon EF 8-15mm f/4L Fisheye USM EF8-15mm f/4L Fisheye USM Instruction Manual - Page 13

7. Lens Cap 8. Filter (Sold separately) Recessed Section While the lens hood (included) is attached to the lens, Lens Cap 8-15 can be attached to the lens hood by pressing the buttons located on the upper and lower sides of the lens cap and aligning its recessed parts with the convex parts found in the lens hood. There is a gelatin filter holder at the rear of the lens. Cut the gelatin filter to fit within the white frames. Then insert the gelatin filter into the filter holder. ENG-12