And, alternate perspective, I wouldn't. This why I've ended up with both 58 and 72 mm ND64s.I'd go with the larger one.
One of the early points made in that thread is there's some practical upper bound on how much each of us likes to step up.
I disagree about that being the only point of a step up ring.I'm not sure that the OP (@grahng ) understands the point of a step-up ring. You have an EXISTING lens (like your 12-45), and you also have an EXISTING filter. You want to screw that filter onto the front of your lens. So you buy a step-up ring for that filter size (assuming it isn't the correct size already. So, for example, if your lens is 58mm and you have a filter that is say 62mm, you'd but a 58-62mm step-up ring.
That was what prompted me to start the other thread, typical uses for circular polarizers being wide to tele and NDs ultrawide to normal. In NDs I suspect what usually motivates the larger sizes (at least in a format normalized sense) is the ultrawide end though, unless I've missed a leak, at the moment we know only the 8-18's thread size and not the 8-25's.What they often fail to take into account, for a given filter type how likely are you to use it on certain lenses?
I agree with everything you say, but I didn't say "only." I just meant that to consider using a 72 or 77 step-up ring on a 58mm lens (a pretty large boost up) to avoid vignetting, and no mention of the filter size that was going to be used, seemed odd to me.I disagree about that being the only point of a step up ring.
The OP was concerned about vignetting caused by the filter and there are a few cases where that can be a problem & using a step up will solve it.
They can also be useful if you're buying a new lens that takes smaller filters than your current filters, and for lenses that take non standard filters. I have quite a few lenses that have filter threads you rarely get filters of any sort in. If I wanted to fit a filter I didn't have to a fujinon 35mm/1.8 c-mount lens I'd get a stepping ring to bring it to 37mm & buy the filter in that size...
Much of the time a ring will be to link lens & filter you already have, but there are other circumstances that are just as legitimate.
Some of my lenses give a choice of sizes a small one at the back (which must be the exact size) or a BIG one at the front - one of them would want a 95mm filter at the front.This is what happens when you get sundry sizes of filters and lenses. Gets worse in time when one finds you need two or three rings to get there plus the filter, then find the stack of rings vignette so you buy another single-step ring to get to the filter. Get one large filter and work up to it. I thought 77mm as on the 300mm f/4 Pro lens would be my end-all filter size, but Nikon decided to jump up to 82mm now. Ugh!