lukus. You just wasted 8 or 9 dollars. Buy the size you need. Cutting up those full sized lenses only ruins them. The invention of the fesnel lens was because it was found tha only the curve of a convex lens is what causes the magnification. The idea is that the inside of the lens is removed. The first were glass this removed most of the weight. Then the lens is seperated ad pushed flat into concentric rings. Actually the cast the glass lenses flat on one side with a series of rings of curved glass.

If you look closely at the plastic lenses they appear a series of ringgs converging at the center. If you could look closely enough you would find that the inside of each rinng is fllat while the outside is curved toward the inside. If you could seperate all those rings you could stack them up to form a convex lens.

If you cut one of the large sheets the only one that would work woul;d be the one made from the center. Sorry to disalusion you.