? about a .25x fish eye conversion mount

[BoblyBill]

does anyone know anything about the .25x part of the lens. For excample, I have a brother inlaw that I bought a .42x fish eye lens that you screw onto the end of his lens. Now I'm assuming that these types of lens just decrease the lens focus lenth (ie. on a 35mm 28mm-70mm lens (converted to digital of 44.8mm-112mm) you would multiply the 44.8 by .42 to get the lens to be a 18.8mm-47.04mm). I don't like to assume because there is one that I would like to buy click here. I'd rather buy a dedicated fish eye but I don't have the money to spend $400.00 on a lens right now. What do you guys think?