Because I am an idiot when it comes to optics, I can not seem to figure out the appropiate focal lenght and diameter needed to cover 11x14 and 12x20. My plan is to build a very simple one element lens, using a plano convex element. I have a plastic lens from surplus shed that is 600mm that covers the 12x20, yet this lens is no longer available. Would someone be able to let me know fl and diameter for covering these to formats. I have lens building info via alan greene, I just need to order the right sized element. Any help would be very much appreciated.