There are a multiple of ways of combining rangefinder-viewfinders, I will illustrate the ones I have seen here.
Rotating Mirror & Rotating Wedge
The rotating mirror and Rotating wedge design can made into a combined rangefinder-viewfinder by placing a galilean viewfinder behind the beamsplitter. This just has two caveats, the effective baselength is reduced by the magnification of the viewfinder, and the viewfinder has to be rather small, since the beamsplitter has to full cover the view you’re seeing through the viewfinder.
This kind of design does allow some interesting ideas though, take the Canon L2, and some other models, that offer three different magnification options. The camera does this by putting two different viewfinders on a rotating carousel. This means the user can flip between the two viewfinders, and even turn one 180 degrees to magnify the rangefinder instead.
Alternatively these designs can have the beamsplitter in between the viewfinder elements, but a third element will need to be placed along the patch’s path so that the magnifications are equal. This has the inconvenience that the front surface mirror has to be larger, as it’s size will be reduced by the viewfinder. But if you want the accuracy of the rotating wedge, a viewfinder and want to save on space, it’s the way to go. It’s the design I decided to go for with the second iteration of the Holmium, partly because I hadn’t worked out the sliding element design yet.
The sliding element design if made with a cylindrical element will inherently be designed for a rangefinder- viewfinder, one just has to use the same focal length as the front PCV element, and place it at the same distance from the beamsplitter as the former.
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