Hello, I have a question regarding the 2x Komura extender. Is there a general consensus regarding the quality of the extender? The good and the bad of the extender? I was thinking that it would be useful on my 80mm Planar and 120 Planar, giving me a 160mm and 240mm. Is it worth spending the money? I believe I would have to compensate with 1 f stop????? Thank you!