I have the setup that is shown in this figure. The view is from the top. I have a beam splitter which splits the light that is collected from a SLR lens. To be able to focus the light I have a diverging lens in front of the slr lens. I lose a lot of light with this setup. Are there any other options I have for focusing the light? Maybe a relay lens or some type of transfer optics? I don't want to reinvent the wheel. Thanks
The problem is that placing the beam splitter between the lens and the cameras increases the distance between the lens and the sensor to the point where you can focus only at very close distances.
One solution is to buy a lens for a large-format or viewfinder camera: these are designed for greater lens-to-focal plane distances and many of them have largish maximum apertures. There's a good market in secondhand lenses. See Ken Rockwell's article for a start. The Ebony site has a good discussion of lens extension and points out that retrotelephoto lenses can be used to get greater extension (than their nominal focal lengths indicate).