What can possibly go wrong with an adapter ring?
An adapter ring is the simplest possible photographic accessory. Usually, it is just a ring-shaped piece of metal with different threads or bayonets on either side, and is used to connect mechanically two pieces of equipment together. It contains no optical elements, has no optical properties, and basically it is no more complex than a screw or a nut. So, what can possibly go wrong with an adapter ring? I am not talking of the possibility that a ring may get stuck onto a lens (like filters sometimes do) and require a spanner wrench or comparable method to come off. I am talking about things going really wrong during its design or manufacturing, so that a newly-purchased ring may be impossible to use on the equipment it is intended for, or worse, damage expensive equipment.
The idea of writing this page occurred to me after being exposed to two such incidents, on two consecutive days. I described the first one on a page about T2 adapters. In addition to a serious manufacturing fault in the Nikon F male bayonet of this ring, its female M42 thread is too tight, and M42 lenses do not screw in completely. This particular T2 ring has no brand markings, but you can see its picture here, so you may know what to avoid buying.
The subject of this page is a step-down ring for connecting filter threads of different sizes. I needed this adapter for attaching a Nikon BR2A reversing ring, which has a 52 mm male thread, to the front of a Micro Nikkor 60 mm f/2.8, which accepts 62 mm filters. What could be simpler? I mail-ordered the ring, received it after a few days, screwed it onto the Micro Nikkor. Odd - it went in less than a turn, then stuck. I tried several times, to make sure I was not screwing it in the wrong way. Same result, it did go in a short distance, then stopped with a couple of mm left between the lens and the flange of the ring. The filter mount of the lens is in perfect condition, which I could verify with other filters and rings. The thread of the ring is likewise faultless - only too big in diameter. There is no possibility of me having ordered the wrong ring, because its is marked 62mm-52mm, and the next sizes up and down from 62 mm are 67 mm and 57 mm, respectively, so there is quite a big difference between these sizes.
"This is not good", I thought, but I tried anyway to screw the BR2A ring into the other side of the adapter. Oops - the BR2A fell right into the female threaded attachment of the adapter. The female thread on this side of the adapter is over 1 mm too wide, so the BR2A goes right in and out without screwing. In the picture below, I placed a small coin under one side of the BR2A to show this.
There is only one possible conclusion to draw from this experience. Don't buy adapter rings marked "S&W Photo Accessory, Co.", unless you can do it on sight and test that they fit your equipment before leaving the shop, because their machinists apparently don't use calipers or rulers to check sizes. Incidentally, I have been unable to find a company with the above name by making a web search. The plastic pouch the ring came in is marked "S&W" only (which may be a trademark of Smith & Wesson, and they don't make camera equipment).
The seller promised me a full refund, and the price was low to start with, so it is not a matter of money. It is a matter of wasting my time, and of frustration caused by buying merchandise that does not satisfy minimal requirements of usability. This faulty ring is not an instance of a different product getting accidentally mis-labelled (the next filter sizes up and down are very different). A company that lets this kind of manufacturing error - two similar errors in the same, very simple product - slip through undetected is not worth making business with. Having seen the same type of shoddy manufacturing twice in the span of two days and in two different products, I feel it is something to start worrying about, and spreading word about.
Incidentally, immediately afterwards I bought a Kood 62-52 mm step-down ring, which works perfectly for this application.