I'm looking for some information on the actual lens formula of the plastic lens that comes on a Holga. If anyone knows of a diagram of this, it would be extremely helpful. I'm adapting the lens from a Holga to a Mamiya 645 and need to add an element to essentially make the lens retrofocus so I can clear the mirror box. This would be much easier if I had some info on the formula of the lens to start with. Thanks!