The three "discs" can screw into either end of the "tube" (M49 threads). In the other end the discs are male M52, M37 and M25 (no internal threads). On one side of the tube the female threads are longer (making room for two stacked discs).
I don't know what the original purpose of this adapter is, but having played around with it, it seems like a very useful thing.
Most of you can probably guess why, just from looking at it. But I thought I'd show some examples anyway.
Left: In one end of the tube you'll probably often want to use the M52 disc. Right: Nikon K2 ring attached (female M52 in one end, male F-bayonet in the other, often useful if you're somewhat F-mount based).
Left:Example of lens mounting – the JML hiding behind the M37 disc. The disc has been reversed and acts like a guide to align the JML in the right angle and more importantly: as a light mask. Right: Another way of mounting the JML – the M28 disc used as a mask behind the JML and the M52 is stacked behind it.
Rear view of the second JML mounting option.
Nikon BD Plan 10/0.25. I've had some trouble with this one since the "good" image circle seem slightly off center – with this solution I'll be able to precisely fine tune the centering using the three locking screws.
Two lenses I've never been able to mount properly: Left: Summar 8cm (has some obscure thread). Right: Reversed Focotar 50/4,5. This lens lacks a filter thread so taping it to something has been my only option so far. This is much more solid (the aperture can be changed by turning the barrel – the screws locks onto the aperture ring at the front).
Another lens with similar construction mounted by its aperture ring – the Componon28/4.
(Still the Componon 28/4) Left: Add the reversed M28 disc and the masking is taken care of. Right: the M52 in place as well.
Still not convinced? Infinity corrected Nikon CF objective (25mm threads).
The padded tip of a locking screw – grabs on gently but securely.
This was only a few examples I came up having just unpacked it – I'm sure you can think of tons of other ways of using it.
To conclude: While it is probably not the ideal way of mounting most of these lenses/objectives it's a very versatile/flexible way! Considering the countless hours I've spend looking for the right step ring or adapter for some specific task, I predict this will be a very nice thing to have around.
Not sure if this is an off the shelves item (?) but it could serve as inspiration for a DIY project. Or, if enough people are interested, perhaps one of the chinese manufacturers might be interested in making more (after all they are already making some fairly obscure adapters). And I'm sure someone on this forum can come up with some ingenious way of improving the design so that it becomes even more versatile!
