The Schneider Center FilterF38XL is used in correcting natural darkness that appears on the sides of the images.The Schneider Center Filter for 90/5.6 is used to correct illumination falloff in Super-Angulon XL 38mm extreme wide angle lensThe filter factor of the Schneider center filter is 4x at the center and clear at the edgesIn 120/8.0 Camera Center Filter, the mounting thread is 72mm x 0.75 to fit the 38mm Super-Angulon XL lensUsing the 82mm x 0.75 thread, additional filters can be attached in front of it