Filter threads are magic numbers. These are ones that matches a lens with its perfect filter partner. Miss it and a filter won’t match with the lens and you will have no way to use it. Right? Wrong. Introducing step-up/down rings. These are even more interesting than the little lens wrenches that for a few dollars can save you thousands of dollars worth of blushes.
So what do they do? Let’s say you have the popular NIKKOR 50mm f/1.4 lens. The filter thread specifications reads 52m. That’s okay but you have a problem. You’re out in the field and you don’t have a 52mm neutral density filter with you. What you do have is a filter for the larger Nikkor 17-35mm f/2.8D IF-ED which has a thread specification of 77mm. Luckily you also have a Tiffen 52-77mm Step-up ring. You simply screw in the step up ring to the front of the lens and Voila! Now you can screw in the 72mm ND filter on top of it. The step-up ring simply increased the lens diameter and allowed you to attach a filter of a larger specification. So that you dont have to buy or carry multiple filters for all the lenses you have. Saves money & bulk.
Step-up rings can be stacked up one on top of the other to accommodate for larger filters on a comparatively smaller thread diameter. Say one needs a 25-52mm step-up ring. That is very difficult to obtain. However two step-up rings, one 25-37mm and the other from 37-52mm can be easily obtained. They can be stacked on top of each other and then a filter of 52mm can be screwed on top. However it is never advisable to stack too many step-up rings because filters are ideally needed to be as close to lens elements as possible for best results. Another reason going against the use of multiple step-up or step-down rings is image degradation. Most users who use multiple rings stacked on top of each other suggest that they have image degradation of some sort.
Step-down rings do the exact opposite of a step-up ring. They adjust a larger lens thread to accommodate a smaller filter. There are usually used to mount accessories. When using step-down rings and filters, one can notice a lot of vignetting. Basically the corners are getting less light because of the tunnel that is created. Mounting and un-mounting multiple rings can be a hassle too. When you’re unscrewing one, the ones in the middle may start to move as well making it difficult to get them off the lens.
A few things about a step-up ring
Step-up rings are mentioned as a combination of numbers. In the above example we mentioned 52-77mm. The first number denotes the filter thread specification of the lens on which the step-up ring is to be used. The second number denotes the filter diameter specification that will be screwed on to the wider end of the ring.
When buying step-up or down filters always insist on buying the thinnest ones. These are better in terms of retaining the optical quality of the images. The further the lens is away from the filter the more image quality degradation you’re going to notice.