Camera Lens Parts: Step Down Ring
Among the various camera lens parts, a step down ring is used to make your lens compatible with different converters, filters and other attachments. These attachments are usually sold separately and may not fit into the female thread which is in the inside of your lens tube. The thread size of your lens will be the diameter of the opening, and many attachments may not have a male thread that will fit it. To get over this problem, there are rings which act as a sort of a converter for the thread size of the attachment.
Purpose of Step Down Ring
Step down rings, as the name suggests, will bring down the diameter of your lens thread size so that an attachment of a smaller diameter can be fitted onto your lens. The male thread on the step down ring will fit into your lens's female thread, but will have a female thread that is smaller.
Usually step down rings are used more for mounting tele converters rather than filters. Step down rings come in varying thicknesses, and it is important to choose one with the least thickness. This will make sure that there is the least distance between the mounted attachment and your lens, which in turn reduces any chances of vignetting.