I have a great collection of Canon L lenses.  All of them are the standard 77mm diameter which allows me to use the same set of filters on all of the lenses.  When a pro-version circular polarizer costs $170, that’s a great thing.

Well, I just bought the iconic Canon 85mm F/1.2 II L lens.  For some reason, Canon designed this massive lens with a 72mm diameter instead of the standard 77mm.  Now I could spend another $500 on filters for this size or I could spend $20 and buy a Step-up Ring.

A Step-Up Ring looks just like a filter.  There is no lens in it, so it really is just a ring.  There are two different sized threads, one on each side of the ring.  In my case, I purchased a 72-to-77mm Step-up Ring.  They are offered in most standard sizes.

One important note — you should only consider a step-UP ring to go from a smaller lens thread to a larger filter thread.  If you were to Step-DOWN, you would create vignetting around the edges of your image.