We want to try something at the school I manage and I could use some help with it. We are considering turning our studio into a camera obscura either with a very large old lens (read cheap) or with a pinhole in the wall. We have already started playing around with the pinhole and it is clear that we will need to make a relatively large hole in the wall and insert a pinhole plate in it. One issue is, we don't know what size the hole should be. We would like to be able to project something on a board or screen at least 5 or 10 feet from the wall, and something very large. Any ideas about how large the hole should be and how precise it needs to be? Also, we have considered just installing a giant old lens in the wall to project a more focused image. Is there such a beast that would not cost an arm and a leg? I keep seeing old barrel lenses in the range of 12" or so for really low prices on eBay, but I am not seeing anything bigger. I am guessing that they are out there, but I have no idea. One other idea, could we use a projection lens? In either case, image quality is a secondary consideration, although we may try to make some images on paper negatives, mostly this would be to demonstrate basic optics in a really fun way to our students. Thanks! Paul.