I don't have any lens filters now, but I understand that I can buy large ones and use $5 step-up rings on my smaller-threaded cameras. I would like to do this for my present cameras, but perhaps I should go even bigger in case I get other cameras or lenses in the future. What do you think the best size filters to get so that I'm covered for the most common lenses? I'm most likely to stick with Pentax in the near future. Both my current lenses are 49mm threads; are all Pentax lenses 49mm? Right now I have two Pentax-A, one Nikon AF 50mm/1.4, and a Canon FD 50mm/1.8.