Rangefinder EBL; let's talk effective base length. There's more to it than meets the eye.
In a manual rangefinder camera you look through the viewfinder whilst turning the lens barrel to and fro to bring the subject into focus by matching two images (using the so called rangefinder patch) of part of the subject shown in the centre of the viewfinder until they coincide perfectly. This is illustrated in figure 4 right hand picture, centre of the image.
The rangefinder mechanism which enables you to this is illustrated in figure 2 above and is housed in the top of camera as shown in figure 1 at the top of the page. The window at the right of the camera, as it faces you, is the viewfinder window and that on the far left is the so called rangefinder window which transmits the second image to the rangefinder patch into the viewfinder for you to match up when focusing. The light transmission paths for this are illustrated in figure 6.
Focusing is achieved by the method of triangulation of the two images, one from the viewfinder window and one from the rangefinder window the back of the lens moving backwards and forwards by its' focus helicoid, which can just bee seen protruding from the bottom of the lens in figure 3, engaging with the cam arm on the rangefinder mechanism and seen in figure 2 which can itself also just be seen protruding from inside of the top of the lens mount of the camera in figure 1.
There is one last point to explore which is bright lines. The camera will project into the viewfinder a set of bright lines corresponding to the focal length of the lens you have got attached to the front of the camera to show you the field of view of that lens. These are are illustrate for the Zeiss Ikon below in figure 5 and an example view is shown above in figure 4 for a 35mm lens. The opaque middle window on camera body, see figure 1, is used to illuminate these bright lines in the viewfinder, see also the light path for this in figure 7.
OK, we finally get to discussing the topic in hand EBL, well almost. We actually must start with BL which is base length i.e. the distance between the rangefinder and the viewfinder windows. The longer this distance is the more accurate your rangefinder will be, all else being equal.
There is, however one other factor to be taken into account and that is the magnification provided by your viewfinder. In general, the higher the magnification of your viewfinder the more acute your vision is going to be when matching the two images in the rangefinder patch and the better your chances of achieving critical focus. Hence we talk, at last, about EBL i.e. effective base length which is defined as BL x VM or base line x viewfinder magnification.
Thus the higher your EBL the better yes? Well yes but it is a compromise. You can build a rangefinder with a very short BL but a very high VM or one of a long BL and more moderate VM which attain a similar degree of practical accuracy but there are consequences. These are all about what you can see in the viewfinder with which lenses. Oh, by the way a very high VM would be 1.5x and there on downwards with a very low one being typically around 0.52x.
Some examples. If you take the Leica M3 of the 1950's that the Leicaphiles still rave over as the most accurate RF camera ever made. It had a BL of 68.5mm and a VM of 9.1x for an EBL of 62.3mm. Here comes the but; but the widest bright frame it could accommodate was for a 50mm lens thereafter it had frames for 90mm and 135mm; 35mm could only be achieved with a special lens which had "spectacles" attached to it which covered both the range finder and viewfinder windows to change the finder magnification, otherwise there were no finders for other 35mm nor any 28mm lenses.
The Leica M7 had an 0.72x viewfinder with frame lines for 28/35/50/75/90/135 lenses with a BL of 69.25mm and EBL of 49.86mm. However, the 28mm frame lines are too hard to see, especially is you wear spectacles and long lenses like the 135mm and fast lenses like 50mm f1.1 are hard to focus accurately. So there are two alternative versions of the M7 offered, one with a 0.58x viewfinder with frame lines for 28/35/50/75/90 lenses and one with a 0.85x viewfinder with frame lines for 35/50/75/90/135 lenses, which speaks volumes about the limitations of the design.
The Zeiss Ikon had a BL of 75mm with a VM of 0.74x. giving an EBL of 55.5mm with a big, bright viewfinder with long eye relief (see figure 6) and bright frame lines for 28/35/50/85 lenses. It was and still is for that matter fully capable of allowing all the frames to be easily viewed and all lenses in the frame range to be easily focused aided in no small part by its' EBL and by its' class leading (even to this day) large and super bright viewfinder.
In the end EBL is important but so are a number of other factors when it comes to ranger finder performance such as the aforementioned viewfinder size and brightness, the size and clarity of its' RF patch, its susceptibility to going out of alignment, the quality of its engineering and construction, plus the proper adjustment of individual lenses to individual camera bodies - especially in older classical models.
Blimey, I set out to write a simple article on a simple concept i.e. EBL = BL x VM and ended up writing a monograph as it underpins quite complex design and precision engineering criteria. Phew.