jlw
Rangefinder camera pedant
RML said:
VF/RF brightness relates partly to the quality of the components, and also to the design of the rangefinder optics.
In a combined VF/RF, the rangefinder image has to be "inserted" into the viewfinder image path. To make this work in the usual coincident-image design, there needs to be a semi-reflecting mirror (aka beamsplitter) in the path, so that both the viewfinder and rangefinder images can be combined together.
The inexpensive way to do this is to have the semi-reflecting mirror cover the entire finder area; that's how they do it on a Fed (and most other RF cameras that have a rangefinder patch with blurry edges.) This reduces overall finder brightness, because the part of the viewfinder outside the RF patch is also only semi-transparent, and doesn't pass all the light.
A more expensive and elaborate way of doing this is seen on some Canon and Nikon RF cameras. In these, the beamsplitter was masked before the reflecting material was coated onto it, so that it's ONLY semi-reflecting in roughly the area of the RF patch. (Looking at the beamsplitter from the front, often you can see vertical and horizontal 'gunsight' marks left by the wires that supported the mask.)
This allows the rest of the finder area to be brighter. The downside is that since the beamsplitter, RF image, and viewfinder image are focused at different distances, the RF patch not only has a blurry edge, but often a dark halo around it (from the unsilvered area of the beamsplitter.)
The best but really expensive way to do it, as used on Leicas starting with the M3 as well as on the Minolta CLE and the Bessas, is to use a complex optical system inside the RF/VF module. This incorporates a system of lenses and prisms that allow the designer to achieve focus of the viewfinder image, the framelines, the rangefinder image, and the edges of the rangefinder mask, all at the same point. This gives a sharp-edged rangefinder patch and also allows the semi-transparent area of the beamsplitter to affect ONLY the patch. This allows full brightness for the rest of the focusing area. (Actually the rest of the area must still be somewhat semi-transparent, so that the framellines can be reflected into it, but the brightness loss is not as great.)
Take a look at a diagram of a Leica or Bessa RF/VF module, and you'll see how complex it is. That's one reason a modern RF camera is so much more expensive than a similarly-featured SLR!
As to trying to make the finder image brighter in a Fed or similar camera: You probably don't want to do it! You could certainly brighten the image by polishing off some of the semi-reflecting material from the beamsplitter. But that would make the rangefinder patch harder to see, because there would be less material to reflect it into the finder view.
Your best bet probably is simply to make sure that the optical surfaces inside the finder are as clean as possible. Use great care if you decide to clean the beamsplitter itself, because the semi-reflecting coating rests on the glass surface and is easy to remove by accident.
