simonesavo, scede, gnarlyleech, minty_ & nation_of_pomation.
Did you add this mask yourself?
Ya, I try to experiment a lot, but the Holga 135BC is quit limiting in it's composition. This I did by putting some aluminum foil along the "walls" in the space between the film and the lens. My first intent of it was to see if it would change how the light would bounce inside the camera in some way; it didn't, instead it became a nice masking effect.