How to Adapt a Door Viewer "Fish-eye Lens" on to a Full-size Camera

Step 4 Further Modifying Your Reducer/Adapter (Continued)...

With the rubber band in place, turn the reducer until the straightest section you marked (ideally, the three points closest together) are at the top and start sawing down through the reducer, keeping the blade at a 90-degree angle (as best as possible, especially if you don't have the assistance of a vise). You may end up cutting through some of the rubber band on the way down, but the end result should be a fairly straight edge (Picture2). Keep back the other piece of the reducer for later.

Now you will will to smooth off all the rough areas and scrap plastic, as well as level the edge on the wide end of reducer (Picture 3). Use whatever materials you have on hand for this; sandpaper by itself is quite effective. (I cheated and used a bench grinder.) You may want to use a utility knife to trim around the interior of each end of the reducer; just be careful, as the type of plastic used is quite soft and can be carved into (a little too) easily.

Don't try melting off any persistent rough plastic pieces with heat from a lighter, though. I found out that the plastic absorbs the heat and you get black marks on the surface.