So, you’re wondering about optical viewfinders for a Raspberry Pi camera? I’ve give you the final verdict on this. Start out with the cardboard viewfinder trick.
First of all, some discussion on camera viewfinders. What does Wikipedia mean by “reverse telescope” when discussing camera viewfinders? Basically, they mean a refracting telescope that uses a convex objective lens and a concave eyepiece lens so that you see the image rightside up, also known as a “Galilean telescope.” By contrast, the much more popular Keplerian telescope uses a convex lens for the eyepiece, so the image appears inverted, i.e. upside down and left side on the right.
The cheapest of camera viewfinders, “direct optical viewfinders,” are constructed exactly as an “inverse telescope.” The main problem with using an inverse telescope for a camera viewfinder is that you do not see a sharp line at the edge of the camera’s viewable surface. Rather, a blurred line is seen due to the edge that defines the image boundary to be out-of-focus.