I'm thinking of making a huge Fresnel lens out of a plastic sheet I found in my storage. As it's for fun, and I'm not very concerned about precision, I thought I could make it using a nail, rope and two blades positioned so as to form a sharp angle between their cutting edges (or simply form a "V" shape).
From looking at a flat Fresnel lens condensation diagram and sketching my own "expanded" lens, I figured I would need to account for groove angle changing when cutting grooves to allow the resulting lens to focus light. But as I'm not very knowledgeable in optics, I don't know if it's important.
Will the lens still focus light in one spot if the grooves are cut at the same angle? If so, which angle should I choose then?