[Leica] Rom lenses & the 934 Depth of Focus Patent

["Robert Rose" <rjr@usip.com>]

David wrote:
I couldn't find anything on the Leica website explaining what the ROM
lenses are for the R system. Can anyone tell me the difference, and how
one can tell if a lens is ROM or not? I understand it's for the R system
but not more than that.

Bob's reply:
I have often wondered about this myself.  It is easy to tell, since it is apparent from the electrical contacts on the back of the lens.  The bigger question is what does it do?

It obviously sends information about the lens focal length to the camera, because when I zoom a ROM lens, the correct focal length gets passed up to a Metz flash.  With a non-ROM lens, I have to set the focal length myself on the flash.  Other than that, a non-ROM lens seems to work just fine on the R8, and a ROM lens works fine on my R4sP.

Oh, yes, on the older style non-ROM 19mm, the P flashes all the time, but otherwise works fine on the R8.

So, I went off hunting through the patent archives.  I found what might be the genesis of the ROM in the R lens, in an invention for a Leitz auto-focus system, a 1988 patent to Leitz (the '934 Patent).

The invention uses existing art for auto-focus; what was new were two things, both related to depth of focus, and use of the ROM.

According to the patent, the ROM stores the lens focal length, the largest and smallest aperture, and (in a future AF lens) the gear transmission ratio.

The invention would allow you to focus on a "back" plane, hit a button, and the camera would refocus to a "front" plane, showing you the extent of your depth of focus.  Or, you could focus on the front and back planes, and the ROM would store the needed aperture to give you that depth of focus.

So, my hunch is the ROM is a legacy of an auto-focus system that didn't develop (pun) and is intended to give us some functionality now (for flash) while keeping the ROM lenses functional as legacy lenses on a possible future AF system.

Bob