Came across this Tamron lens that I was going to use with my new Pentax IST. Can anyone tell me if this is worth buying or not? http://www.bhphotovideo.com/bnh/con...ail&Q=&sku=213162&is=REG&si=acc#goto_itemInfo Being new to the film cameras, what should I look for in quality lenses? I don't understand the group and elements, but it seems that is one of the more essential characteristics when comparing lenses, right? Can someone give me a brief edification? Thanks for responses.