- I did not have an M42 body cap and
- the extension tube is made of metal, whereas pretty much all body caps are plastic -- this is a plus!
If you know the 'theory' behind a reverse ring, or just want to see how you can make one, then you can skip to the section below. If you are interested in why you might need a reversal ring, then read on :-). Lenses are designed to operate in a specific 'focus regime'. Usually from the order of a meter up to infinity. Within this regime, the lens reaches its optimal sharpness and minimizes its distortions and aberrations. Extension tubes, macro bellows or any other means of placing the lens further away from the body, allow one to focus on objects much closer to the lens. This results in magnifications that are far greater than could be achieved otherwise. However, this means that the light follows an optical path that the lens was not designed for! The results are often a loss of sharpness and an exaggeration of optical distortions and aberrations of the lens. We need to keep the light path 'within specifications' of the lens to reach optimum performance. An easy trick to achieve this while still maintaining the possibility to focus on objects close to the lens, is simply to reverse the lens!
Nevertheless, it's still worth it to see if reversing your lens won't improve the optical quality! In general, if you want to achieve a magnification greater then 1:1, you want to reverse your lens. OK! Just build the damn thing already!
As I said in the introduction, I used an old 49mm UV filter and a small M42 extension tube. Of course, if you want to build this yourself, you need to pick a filter size that will fit on the lens you want to reverse mount. I will be using an old M42 Helios 44-2 58mm f/2 lens. It will be mounted in reverse on a set of M42 extension tubes which attach to my Canon 500D through an EOS to M42 adapter. Below is a picture of the filter and extension tube that will get 'sacrificed'. Preliminary results
I attached the reversing ring to my Helios 44-2 58mm f/2 and mounted it on a modest set of extension tubes (total extension of 40mm), as can be seen below: Epilogue Some very quick tests of reversed versus normal orientation of the lens at modest magnification have not really shown major changes in sharpness yet. I'm planning to do some proper in-depth tests on the effect of reversing the lens at various magnifications soon. I expect the reversed lens to score better, especially at higher magnifications.