If you are lucky enough to own a Rokinon 8mm lens for Canon cameras then you have thought about using filters at some point. I have had this one for over a year an think they are something special, but there are no rear threads for adding filters. One day I was looking at the rear mount and happened to notice a small groove inside the mount. On a hunch I measured and found the groove to be 40.5mm across. So I went on Ebay and picked up some cheap 40.5mm filters, placed it facing forward and it fit perfectly in the groove!
Caution: This is for APS-C DSLR cameras. It has ONLY been tested on Canon Rebel XT and T3i models. Be careful testing, I am not responsible for any damage caused. If the mirror jams against the filter you will need to unmount the lens to free the mechanism.
Step 1: Adding the filter to the lens
My cameras are all EF-S mount bodies. EF-S lenses stick out a bit into the mount compared to EF style lenses, so the mirror is smaller than a full frame and is the main reason you can't use EF-S lenses on a full frame camera. Looking at the Rokinon on the left and a kit lens 18-55 on the right, you can see the filter makes the fish eye lenes mount look just like the EF-S mount lens. So good so far.
Remember to face the filter in the same orientation as if you were mounting it to the front of a lens.
Step 2: Testing the camera body fit (will the mirror jam against the filter?)
The easiest way to test your camera boy is to remove the lens and place the filter inside the mount. On my Rebel bodies there is a groove it sits in rather comfortably. Then simply take a photo and see if the mirror makes contact with the filter. If you use a clear or UV filter like I did you can watch the whole process go off without a hitch. Remember to face the filter in the same orientation as if you were mounting it to the front of a lens.
Step 3: Mounting the lens with filter to camera
Now for the tricky part. With the filter sitting inside the groove of the lens (Orientated correctly. It will wiggle a little, that's ok) hold the lens facing down so the filter doesn't fall out, mount the lens to the camera body as normal. The lens should click in place and if you shake the camera the filter will have a slight rattle but not sound like it is moving freely inside the mount.
I used a macro extension tube to simulate the camera mount and check for fit. In the image you can see it sits in place and does not move but a half millimeter side to side (since it only sits in the groove and is not threaded into place).
Take a test frame and make sure the mirror isn't hitting the filter. If it does then dismount the lens from the body gently and let the mechanism return, then without the filter, make sure nothing was damaged, and do not try again with the filter added.
If everything fits then you are good to go, have fun using neutral density, IR, or colored filters.
Caution: Do not use circular polarized filters as the filter ring is too thick and will not allow the lens to mount to the camera without damage.