I have a Pentax K100D and I was looking at getting a 2x Converter. Pentax Rear Converter-A 2x-S is the one that I have found and I was wondering if it is a good choice or if I would be making a mistake picking it up. The price is about 60 dollars, which seems reasonable to me. Is there a better option? If there is, does it cost substantially more. What features would I not be able to use on my camera with this lens? Thanks for all the help.