Above is an example of a separate Watameter Rangefinder. There are two windows at the front and one at the back, the right hand window, from this side, is a through viewfinder the left window has an adjusting wheel behind. This adjuster moves a mirror which projects a superimposed image into the main through window when the two images coincide the object is in focus. What has happened here is that the distance between the 2 windows is a constant and the angle of the reflecting mirror is known. By mathematical triangulation this can now be transposed onto a scale which, in this case is marked on the wheel. The value of this is then, in turn, transposed onto the distance ring on the lens of the camera. Incidentally, it is a known mathematical fact that the wider the spacing between the 2 windows the greater the accuracy because this allows for a finer granularity of measurement. Through evolution these rangefinders were gradually introduced into the top housing of higher end cameras, initially as separate units and later coupled to operate through a single viewfinder window. Early examples of rangefinder cameras had 2 windows in the back, one for setting focus via the rangefinder and the other as the viewfinder for composition. With these built in rangefinders the adjustment was now moved to the taking lens. By using a series of levers inside the camera body as the lens was turned the angle of the mirror was adjusted thereby allowing the focus to be set directly by the lens. So there you have the rangefinder camera which is effectively a compact camera with a rangefinder built in to the top housing. 

Although they appear different in design they have one common aspect in that they all have two windows at the front. The Graflex has a large viewfinder window with a small second window for the rangefinder. The Agfa Super Silette, however has two windows which appear to be the same size but if you look closely you will see that there is a small round window within the left side so this is only cosmetic. The Braun has it's rangefinder window set into the light meter housing. The Zorki also has a large viewfinder window with a small rangefinder window but this has a wider base than the others in the group allowing a finer degree of accuracy. The Zorki and the Braun do have one thing in common which is different to the other units because these both have interchangeable lenses. This means that they have a slightly different mechanism for adjustment. Where the others are fixed lens they can operate through a simple lever connected to the mirror. The Zorki and Braun both have to allow for different lenses to be fitted. These lenses have a tube built in to the lens unit which extends rearwards and as the lens focus ring is turned this moves in and out and moves an adjusting lever which is coupled to the mirror housing base plate. In the case of the Zorki this lever is inside the body whereas the Braun is built into the lens mounting plate but the principle is the same. The Minolta is the odd one out in the group. This has both windows set within a single window and the rangefinder window is placed directly above the lens. The others all have the lens roughly half way between the two windows. Theoretically this arrangement should be slightly more accurate in that the measurement is being taken from the same point as the taking lens. The downside is that any attachment like an extension lens or hood etc. will obstruct the rangefinder.

Finally, one thing they all have in common is that the accuracy is only as good as their calibration. Over time the mirror plate and associated levers can wear and cause a small movement in the angle of the mirror which is crucial to accurate calibration. One degree of movement can cause an inaccuracy in the the measurement. This calibration is a highly skilled and painstaking job requiring very specialised equipment.

Graflex Century 35 

Agfa Super Silette 

Braun Super Paxette

Zorki 4 

Minolta Himatic