Hi! It's a great diy projector. I'm in progress to build one with OHP

lenses too, But i have some question, How to cut an OHP fresnel lens?

It's looks like two different focal fresnel lens glued in the edges to become like one piece of lens glass, is it right? if yes, do i have to split them before?. from your picture i saw, you just cut it from the egde to the center? what kind tools that u use? it's look like a cutter/saw. Please answer as soon as possible, and sorry if my english is so bad. thanks for your nice instructables.