Normal extension tubes with 12, 20 and 36mm length are a good tool for many lenses with longer focal lengths. But as soon as one want to go macro with shorter focal length lenses, those 12mm are way too long. One can use wideangle lenses with the 12mm tubes, but with that one has to go very close. And neither Canon, Kenko, Zörk or Novoflex have shorter extension tubes.
For example the Zeiss 35mm f/1.4 lens (ZF,ZE and Contax verssion) have a minimum object distance (MOD) about 300 mm measured from sensor.
These lenses have no internal focussing system (IF) but are normal focussed with the distance between lens and sensor - with a helicoid. For the nearest setting they have ~6.3mm focus travel. The maginification is 1:5.55 there.
Ideal would be a 6mm thick extension tube for this lens! And in this magnification area the image quality would be better than with the bigger tubes.
With the Canon EF 35mm f/1.4 L lens one has 30cm near distance too. This lens has internal focussing (IF), not ideal for calculation, because one does not know which focal length the lens has at close focus distance setting.
The old Canon FD 24mm f/1.4L has a focus travel about 6.8mm - a 6mm thick or thin macro / extension tube would be good too.
The Olympus Zuiko 50mm f/1.4 has 6 mm foxus helicoid travel too. The Canon EF 50mm f/1.4 has 8mm focus travel.
For all these lenses extension tubes with less than 12mm width would be better than those avialable products.
Even with longer focal length with floating elements short extension rings are helpful to maintain better optical correction in close up photography!
Answer of those problems: A shorter extension ring!
Like I has shown two years ago, it is possible to DIY produce a 8 mm thin extension tube, as long was one has a lathe.
Now I started to think about a project to build a even shorter extension tube, as close as possible to the ideal 6mm thickness. Other tinkerer said it is not possible to buidl such a thin extension tube - but I suppose they have not gone through all the correct measurements, or lack imagination how to solve the lens mount lock.
As before I used cheap asian manual extension tube set, and I worked with very close tolerances as well.
The locking pin for the lens is at the same time the groove for the camera locking pin! This ide makes it possible to maintain the correct alingment of lens and camera.
One has to mount the extension tube to the lens, and correct adjustment is checked with a finger tip on the camera sided locking groove. After that the combination lens + extension tube is mounted on the camera.
Afterwards the combination is dismunted from the camera, and after this the extension tube can be un-mounted from the lens.
It is not possible to take the lens from the extension tube as long as the tube is mounted on the camera.
With this concept I get 6.0 mm thickness!
Here an image of my 6mm tube with Canon EOS 5D DSLR and Zeiss Contax 35mm f/1.4 Distagon lens - and between the short extension tube!
I have used this combination with this 600 mm heavy lens hanging on the not even glued extension tube - the small tolerances and thight fit are good enough to hold this!
After I have glued the extension tube parts correctly I have mounted a converted Canon FD 400mm f/2.8 lens on the tube and have it hanging. The tube helds this static 5.5 kg weight ~55 Newton force. Additional I tested it with a 1.1 kg russian mirror lens LZOS 3M5A - with that I firecely shaked the camera with the lens on 6mm tube. And I mounted the lens on tube on the camera, and mounted the camera (not lens) on a tripod, with lens horizontal. after 7 hours I stopped this test without harmed extension tube.
On the left side you see an image with this lens and 5mm short extension tube, on the right side an image at closes focusing ditance without the extension tube:
As a next steps I have planned to make my extension tube with full Canon EF protocol compatibility to have autofocus (AF), automatic iris operation and full EXIF data.
I have made a first working contact block with contacs to get this. It is working, but the tolerances for the holes in the POM block are still to high, but I have ideas to get things better.
AF and automatic iris control and EXIF data is working, but not with every lens-camera combination at the moment.
Unfortunately I glued the extension glue with only 5.9mm thickness together - this avert the use of some EF lenses :-/
Please visit : Heaven and Hell for photo tinkerers :-)