How do auxiliary viewfinders work? I'm just thinking...

I was thinking of how to build a waist level finder for a Hasselblad SWC, given that the ones Voigtlander made are (A) unavailable and (B) terribly expensive if found on the used market.

So I started thinking about how to make one, maybe by adapting a waist level eyepiece-adapted finder made by Canon/Nikon etc, which led to wonder about how the eye focuses through a standard external finder as used primarily by rangefinder users. For example, any of a number of Japanese or Leica finders with projected framelines -- we look through them and see the angle of view as designed, and we see the subject clearly even at infinity, and we see the projected lines clearly even though they're within about 1/4 inch from my eyeball.

How do they engineer a finder that allows the eye to see infinity and the framelines at the same time? If I understand that, maybe I can understand better how to make a $30 WLF for my SWC.

You can get an adapter plate for the SWC which will take a WLF, 45º prism, 90º prism, magnifying hood, a horizontal or vertical RMFX finder. See Dead Link Removed for examples.

Optical trickery.

What you are focused at looking through the eyepiece is a plane in which both the image of the frame lines and the image produced by the (front) lens of the finder appear.

Just rip the viewfinder off an old folder. It won't have the same angle of view though.

I'm aware of the option of putting a viewfinder on the back, but prefer to work faster than that allows. I could remove a viewer from an older camera, or disassemble a P/S -- though the latter would not give me a waist level viewer unless I took apart a Yashica T4. As for the idea of focusing on a plane -- what plane would that be? In an SLR, the eye is actually looking at an image that is on the ground glass, i.e., on a physical surface that exists. The lens focuses the image on that plane (ground glass) and the job of the viewfinder optics is to permit us to have our eye about two inches from the GG yet make it appear to be much further and permit us to focus our eye upon it.

In a "straight" optical viewer, there is no "artificial" plane as provided by a GG -- the light rays are bouncing off that mountain ten miles away and entering the front lens of the viewer, through the optics and hence to my eye. I can easily understand how that works and how the lenses in the viewer alter the angle of view -- what I don't get is how they can also include framelines etc that literally are 1/2 inch from my eyeball but I see them in focus AND I see the mountain in focus.

Any lens produces an image in a single plane (no need for a ground glass - a ground glass does not provide that plane, just makes it more easy to see), and if you manage to make something else - like (the image of) a frame - appear in that same plane, you will see both.

The only thing that a reflex type viewer will do is help for fast framing. I don't believe there's anything in after market accessories that would give reflex viewing and the ability to focus. I would be just as easy to use a wire sports finder for speed in composing but not much else.
The image with a mirror viewer also means the image will be reversed.