Tom,

Maggot hit the nail on the head. Fresnel lenses have circles etched in them that get smaller and smaller as they go toward the center of the lens. This concentrates the light in the direct center of the lens. You can cut an 8" X 10" lens down to 2" X 3" (not the cheapest method), as long as you get the center portion. I have to cut down the Fresnel lens in my smallest kit to make it fit, but since I maintain the center, it still works great.

Semper Fi, George