Originally Posted by
zhorton
Bryan, thanks for your support, and your excellent questions! Let me give them a quick shot:
For ultra wide lenses, the normal Mercury Focus Unit is swapped out for a special "Rotating Wide Helical." For the 23 format, this is everything below 65mm. For the 45 format, that's everything below 90mm (as you correctly guessed). This special helical allows lenses down to about 35mm work on 23. I'm not sure about the limit on 45 (this helical is designed to allow recessing as well as extension). I've only tested down to 65mm, which works fine. I would love to know if your ultimate extreme example would work. If you or anyone else wants to loan me the lens, I'd be thrilled to try to make it work (seriously)! For now, though, I'd say that shorter than 65mm on large format is unknown territory. With the 23 back that 38mm lens would work with no problem. I've tested it with the Zeiss Biogon 38mm and that certainly works. I don't think the Schneider has a shorter flange distance...
The 45 back adapter adds 30mm of rear extension to the camera.
The downside of the rotating helical is just that: the lens rotates as you adjust focus. That's the price to pay on the Mercury for ultra wide angles. But we have a full focus scale system for this helical just as we do with the standard one, so focusing is very precise.
In general, each lens has its own focus scale calibrated to the Mercury. That slips on the Focus Unit when you change lenses. In general, to change a lens you unscrew its barrel from the helical. If you're changing to a lens that is radically different in focal length, you also have to swap out a front spacer or a rear spacer, or the helical itself. Front spacers and helicals are both swapped by loosening the four front bolts (like a technical camera).
I don't want to get too technical, but basically there are multiple possible ways to adapt any given lens (longer or shorter barrel, longer or shorter front spacer, longer or shorter rear spacer, etc. There are basic kits to keep things simple, and more advanced options for users with specific needs and who, for instance, want to make it as easy as possible to switch between their exact lenses. But I hope my basic description here has cleared things up a bit. Different lenses have different requirements, so the solution depends on the lens (and what other lenses you want to swap easily).
But the bottom line is that almost all super wides work well, as long as you are okay with the lens rotating (this is fine for most people, but not for some who uses CPL and/or circular gradient filters all the time).
As for cheap rangefinders, they do work with all lenses. They just measure distance; you then enter that distance on the helical. It's one more little step than a coupled rangefinder, but the advantage is that it then becomes possible to adapt any lens (universal). The problem you describe, of a rangefinder only working with one lens, only applies to couple rangefinders. This is why only a handful of lenses will work with a Graflex XL, etc. (Some coupled rangefinders have removable cams, but these just push the problem to the cams... but I digress.) For the Mercury, the burden of getting the focus to work falls on the focus scale, which has to be created by someone at some point; it will then work with any rangefinder. One does have to have an accurate one, though, and really old ones may need to be calibrated for accuracy. Another option is to use a cheap laser distant finder for absolute accuracy (but limited range). We'll make a simple cold shoe mount for one that is readily available on ebay for under $20.
I hope this helps clear things up and allow you to decide which setup will work best for you! (But really, now I want to test that 38mm on 4x5; I think it would work with some significant recessing.)
Cheers,
Zach
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