Chatting with Jonny G about a potential collaboration putting lenses in one of his and Dale H's sculptures I find myself looking for examples on the image search engines and stumble upon this, which looks like an Abelardo Morell, but isn't.
It's an image made with a Bonfoton lens, made by a small Finnish company and sold for €27. I've immediately ordered one, of course.
It's a plastic lens with a clip-on frame, intended to fit on a hole cut in a pull-down blind or similar. The view outside the window is then projected onto the walls of the room, upside down because light travels in straight lines.
What's cool about Bonfoto is these kind of lenses are hard to get hold of at reasonable prices. A lens like this that will project a sharp image on the wall opposite a window (4-7 metres away) is effectively a very weak magnifying glass, and there's not much market for those.
The weakest we've been able to buy at consumer prices are +1 closeup filters for under a tenner. They max out at 78mm diameter and project at around metre, which is good for small chambers but not for rooms. Nice and cheap though.
If you want a longer throw, you're looking at bespoke lenses, either getting a batch of cheap ones made of plastic or a single expensive one made of glass. For the Birmingham Obscura we went for the latter, paying £500 for an semi-retired man to hand grind an inch-thick slab of glass. It's gorgeous, but it's definitely a one-off, given it took him 6 months and he's the only person doing this kind of non-scientific work in the UK. (Lenses for science cost insane times more, as you'd expect.)
The Bonfoton lens seems to be a relatively mass-produced plastic lens which won't have the optical sharpness of our glass lens but that won't be a problem. When mine arrives I'll let you know how well it works. And then I may order a dozen of the things!