Originally Posted by rvaubel
I use the 25mm full frame finder with the 15mm heliar on the R-D1. Its too wide but I've learned to compansate. The 21mm might be a little tight but I don't have one and am not going to buy one to find out.
Does anyone know what the difference is with the finders that are dedicated to the R-D1? Are they just re-marked existing finders or are they actually manufacted to account for the 1.5 crop factor of the R-D1?
All I can do is speculate, since I own only one of these Voigtlander "D" finders (for the 21mm lens) and don't have any other C-V finders against which to compare it.
But it looks to me as if they use the same optics as their nearest corresponding 35mm finder, with the frameline set for the R-D 1's coverage area.
For example, applying the R-D 1's 1.53x "crop factor" to a 21mm lens gives 32.13mm; so, I suspect my 21D finder has the same optics as a finder made for using a 28mm lens on a 35mm-format camera, but with a slightly smaller frameline to match it to the R-D 1.
Remember that you can't expect an auxiliary finder to match the lens' view field exactly anyway; finder designers have to provide a safety factor by making the finder so it always "sees" a bit less than the lens. That way, when framing a shot, you can be sure that what you see through the finder will show up in the picture (plus a little extra around the edges, which you can crop out.)
You certainly wouldn't want it the other way around, with the finder seeing more
than the lens. If that were the case, you might compose a picture with an important element all the way out at the edge, then find it was completely missing from the final image!
Just to make the designer's task a bit more difficult, it's an optical fact that any conventional lens' effective focal length increases as you focus closer. So, if the finder were designed to exactly match the lens' field at infinity, it would show too much at close distances (where the lens' focal length increases a bit, tightening its angle of view.)
Instead, the designer has to set the finder so that the lens view shows slightly more than the finder view at closest
distance; as a result, when you focus at infinity (where the lens' effective focal length becomes shorter) then lens will show quite a bit
more than you saw through the finder. This may make it appear that the finder is designed very inaccurately, but that isn't necessarily the case; it's just that you need a large safety factor at infinity to make sure there's still enough when you focus close. (This is why the R-D 1's built-in finder frames show only 85% of the picture area at infinity.)
In my very crude tests of the 21D finder, I'd say its frameline is quite accurately matched to the R-D 1. When I use a 21mm lens at a close distance, such as 1m, the final picture shows just slightly more than I saw through the finder. At longer distances, the final picture will show more "extra" area, because of the factors mentioned above, but that's par for the course with any accessory finder.
[Yes, in case there are any punctilious camera historians out there, I know that a few rangefinder cameras incorporated "field size correction" in their finder framelines, so that the finder framed more tightly at close distances to compensate for the increase in the lens' effective focal length. The Konica Pearl IV and the Zeiss-Ikon rangefinder on the Polaroid 180 are two examples. But I don't think this feature has ever been designed into an accessory viewfinder -- although many variable finders, such as the Tewe, have a second index mark for use at close distances that "zooms in" the finder a bit tighter.)