|Camera Anatomy - Rangefinder Cameras|
A rangefinder camera is defined by it's method of determining focus. Focus is fundamental to photography, whether it's a family group gathered around the BBQ or a close up of a flower. This fact that has determined the development of the different broad types of camera. Focus is dependent upon a number of relationships, distance of the subject from the camera being the most important. A rangefinder is just that - a device to find the range of a given subject from the camera, that the camera, in turn, can be focussed. Rangefinder cameras fall into two separate sub categories, those in which the rangefinder is coupled to the focussing mechanism of the camera (coupled rangefinder cameras) and those in which the rangefinder is used to determine the distance only - which is then transferred to the lens manually, clearly a less convenient, but cheaper, option.
To appreciate how the rangefinder works, hold up one of your index fingers in front of your face at about 20cm. No doubt you can feel your eye muscles turning both eyes in so that a single image is formed. Still with your finger in front of your face, turn you attention to the background and refocus, now allow your attention back to the finger preventing you eyes from refocusing - you will see that there is a double image of your finger. Finally, allow the muscles to turn in again and the images will merge into one. Take a rest, that can be uncomfortable after a couple of times. It is clear that the eyes are required to "angle in" significantly when something is close to you and that there is a relationship between this angle and the distance. The rangefinder fitted to a camera uses a similar action via a system of lenses and mirrors to determine the distance.
In use, the photographer peers through the rangefinder to observe a main image onto which is superimposed a secondary image (often tinted yellow covering a small circular part in the centre of the main image). The photographer chooses the part of the image required to be in the centre of focus and moves the rangefinder control until that part of the secondary image is coincident with the main image, see below.
In the left hand image the secondary image appears as a tinted area, within is a double image - because the sheep is not in focus. In the right hand image the rangefinder controls have been moved to bring the two images together, at this setting the rangefinder will indicate the distance to the sheep. Note that the trestles in the background are not coincident, because they are further away. If the camera is the coupled rangefinder type, the action of bringing the two images together will automatically transfer the distance to the lens.
The yellow tinted secondary image is the most common method employed in rangefinder cameras, but certainly not the only one. The principle difficulty with this method is the need to maintain absolute accuracy in both horizontal and vertical planes, thus some designs simply place the secondary image beneath the first (A common fault in old rangefinder cameras is the non vertical alignment of the secondary image).The rangefinder is not always the main composition window either, as there are number of design considerations. Firstly, for accuracy, the two windows that act as the rangefinders' "eyes" should be as far apart as is practical as this gives a larger closing angle, the distance between the rangefinder windows is called the 'rangefinder base'. The main viewing window really ought to be as close to the lens centre axis as possible, especially when close to the subject - otherwise the subject might be cropped from the final photograph. If the two functions are using a common window there is a conflict of interests and a compromise needs to be reached. The alternative is to have a central composition window and two independent rangefinder windows, this is a compromise in it's own right as the photographer now has to focus through the rangefinder and compose through the viewfinder. Ultimately the user makes a choice between the various compromises.
An excellent example of these considerations influencing
a camera design can be seen in the tiny Bolsey B2 below. This camera uses
an ideally positioned central viewfinder to compose the picture. However,
because the camera is small, two separate rangefinder windows are used
- to maintain a reasonable rangefinder base. Further to this, a tinted
inset secondary image was not used, this allows the use of fully silvered
mirrors, rather than the more difficult to produce half silvered type
needed in the superimposed types, as well as to avoid the manufacturing
difficulties to ensure vertical accuracy. Looking at the picture of the
camera below, the left hand window on the camera is the source of the
main rangefinder image whilst the much longer right hand window projects
the secondary (lower in this case) rangefinder image. The reason this
window is twice the length as the main window is that the beams of light
passing through it have to travel twice as far, bounced by internal mirrors,
to reach the users eye and are half the brightness when they get there,
internally the optics sort out the size issues. In the Bolsey B2 the rangefinder
image is a considerably magnified sample of the total image to allow close
inspection. The photograph at right shows the view of the sheep as seen
through the Bolsey B2 rangefinder, when the camera is not quite focussed.
The blurred line delineates the two images, the lower one being supplied
by the longer window.
At one time rangefinder cameras were commonplace but the Single Lens Reflex (SLR) has largely taken over these days. However they have a loyal following amongst users despite the main disadvantage of not actually seeing through the lens and the principle endures in some of the high quality makers, the Leica M7, Contax G2, Fuji 645 and Voigtlander Bessa-R being recently introduced models. The rangefinder camera has a significant weight and size advantage over an SLR in medium format particularly, a fact that should ensure it's survival in higher quality photography for a while yet and from the lens design viewpoint the lack of a reflex mirror allows the rear element to be closer to the focal plane, which is of academic interest to the user.
Some other rangefinder cameras
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