Accessories for MidOpt Filters - Step-Up / Step-Down Rings
Step-up rings allow large filters to be used on lenses with smaller diameter
filter threads; the filter will be much larger than the diameter of the lens.
Use of step-up rings is often mandatory and routine with shorter focal length
or wide angle lenses in order to prevent vignetting.
Step-down adapters allow smaller filters to be used on lenses with filter
threads of larger diameter. This may result in vignetting so their use is
normally not recommended.