How to attach filters to a Rokinon 500mm/ 1000mm Mirror Lens for Canon EOS
For the Rokinon 500mm F/6.3 lens, you can either attach 95mm filters to the front or 34mm filters to the rear of the lens.
For the Rokinon 500mm F/8 lens, you can only attach 30.5mm filters to the rear of the lens.
To attach a filter to the rear of the lens, remove the lens from the camera (or take the rear lens cap off). There's a threaded screw mount just inside of where the lens fits onto the camera. Take your filter and screw it into place. Hand tighten until it's snug and doesn't project in the way---remember, you're going to want to take it off again later.