There's an
article about auxiliary viewfinders on CameraQuest, and if you scroll down a ways you'll find a section about variable-focal-length viewfinders.
About that requested 28-135 range: I'm not sure why you'd need this for the R-D 1, since it already has a frameline for 28mm lenses; if you want it so you can use a 28 on your CL, you'd almost certainly be happier buying a Voigtlander brightline finder for this focal length, for reasons I'll discuss in a moment.
BUT... according to the CameraQuest article, Tewe did make a zoom finder covering the 28-135 range.I don't think the 28-135 Tewe is all that common, though; the ones I see on eBay are usually the models that start at 35mm, so finding the 28mm version may take some looking. There are some other variable finders (Nikon and Canon come to mind) that go to 28mm or even wider by adding a front attachment, but the attachments tend to be super-rare and may well cost several times as much as the finder, assuming you can locate one at all!
If you decide you can live with a finder that only zooms out to 35mm, you've got more choices. I've had fairly good luck with the 35-200 Tewe, an example of which I bought on eBay for about $35.
The advantage of this longer-zoom finder on an R-D 1 is that it gives you the option to use longer-than-90mm lenses that you might acquire eventually. For example, if you happen to run across a 100mm f/2 Canon or screwmount 105mm f/2.5 Nikkor-P (both excellent lenses) you can just dial in a setting slightly past 150mm and you're good to go. Conceivably you could even get away with using a 135mm lens by setting the finder to 200mm, as long as you kept important objects away from the edges.
It's on the subject of edges, though, that these zoom finders get a little dodgy. Unlike a Zeiss turret finder or a Leitz Imarect finder, the Tewe doesn't have a sharply-defined outer border on its image. (Neither does the Nikon zoom finder.) The outer edge is a bit fuzzy, somewhat like a simple old-fashioned viewfinder camera without framelines. General composition is absolutely no problem with this type of finder, but if you're concerned about the exact placement of objects at the edges of the frame, you're likely to be a bit frustrated. (Of course, for static subjects, you can always take a test shot, view it on the R-D 1's monitor, and readjust accordingly.)
That's why I say that for using a 28mm lens on your CL, you'll probably be happier with a brightline finder -- it's easier to frame with confidence. Also, if my experience is any guide, you probably can buy a new Voigtlander 28 finder and a used 35-135 or 35-200 Tewe finder for less than you'd pay for a Nikon or Canon zoom finder with the scarce 28mm attachment!
Even with the blurry edges, though, I'd have to say that the zoom finders (Tewe, Nikon, etc.) are better than the alternatives, such as the Leitz Imarect finder. This isn't really a "zoom" finder, by the way -- the image size stays constant, and changing the focal-length adjustment just moves masks inward so the frame gets smaller and smaller. These masks are sharp-edged and make it easy to define the edges of the image -- BUT the frame size gets pretty darn small by the time you get toward the 135mm setting, rendering it difficult to see details of the subject! The zoom finders enlarge the image, so you can see more detail at longer focal lengths.
You shouldn't be trying for ultra-precise composition with ANY of these finders, though, because the parallax compensation and finder-to-lens-axis alignment are only compromises at best. When I first got the Tewe finder, I did some crude test shots with my R-D 1 to see how accurately it framed various lenses; you can see the results at
this page. What I learned is that it's best to set the finder to a somewhat longer focal length than the R-D 1's 1.5x crop factor would suggest, to give yourself a little extra safety margin for possible framing or parallax-compensation errors.
Even with all these nuisances, I found that with some experience, I'm pretty comfortable shooting with an auxiliary finder on the R-D 1. In practice, rather than trying to frame each shot precisely through the aux finder, I just look through it when picking a shooting location, to see what type of crop I'm getting at that distance: a head-and-shoulders shot, or a waist-up shot, or a full-body shot, or whatever. (Obviously I'm mostly a people shooter; you'll have to adapt this technique to your own preferred subject matter.)
Once I know this, I don't really need to look through the aux finder that often; I can concentrate on follow-focusing through the camera's range/viewfinder, and just look at the aux finder occasionally to refresh my memory as to the field of view.
Oh, one more important thing about using aux viewfinders on the R-D 1:
Carry a clean, unstarched white handkerchief in your camera bag! Why? Because when you're looking through the aux finder, the most natural place for the tip of your nose to go is right into the RF/VF eyepiece; you need the handkerchief to wipe off nose-grease marks!
