The Naiad is part of the “Neptune Convertible Art Lens System” that’s made of a rear module and an interchangeable front optical group. In perfect Lomography style, no real-meaning technical info is given, and the “plus” you are buying is the “unexpected effect”. Despite this, the price is far away from a “toy lens”. Even the optical scheme ( 9 elements in 7 groups ) it’s not clear if it’s regarding the front element itself or the whole (front + rear) working lens. But I strongly suspect it has to be added to the 3 /3 rear group making it a 12 / 10 lens.
Declared Field of View is 135°, not specifying if horizontal or diagonal, from the examples it seem to be a fisheye projection, more than a “heavy barrel distortion rectilinear lens”, a very large front filter holder is available. Online tests say it’s giving good sharpness on the whole frame from 3.8 and that it’s low in contrast, like many vintage lenses.
