Originally Posted by Syb
I guess you tried to buy a conversion ring, too? I thought the 49-52 were widely available...
I already have one, so when I saw 52mm close-up set I thought "that's what I need because I have 52mm". However, 49->52 is not quite like you have 52mm thread: you need a 52mm lens cap, then if you want to move close-up filter from lens to lens you need to move 49->52 as well, or to have several step-ups, and... and it just got too complicated so I got 49mm set.
I was/am mostly using close-ups with M100, but also with M40, M50 and FA35 (never tried with my M28/3.5 though). All are with 49mm thread so 49mm close-up set came as an obvious and much simpler solution than playing with step-up ring all the time.