LightOmatic II repair (2 of 5)

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An explanation of the viewfinder optics

The rangefinder

is a simple tunnel that collects an image for B. The aperture in viewfinder mask determines the shape of the rangefinder patch.

is the rangefinder mirror, which reflects the image 90 degrees, and sends it through a porthole to C1.

C1 is part-mirrored with a plain glass centre, through which the image passes to D1.

D1 is a tube with a lens at the entrance, set within another lens (D2 in figure 14), which focuses the image to produce a defined patch, and projects this on to E.

is a beam-splitter, which reflects the image 90 degrees, and sends it to F.

is the viewfinder lens.

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Figure 13. The rangefinder optics. Note that the rangefinder mirror (B) is set on a pivoting turntable. The lens holder (D1) is threaded into the tube, and possibly adjustable. However, this component is very tight, and probably something it's better not to fiddle with if there isn't a problem with the sharpness of the rangefinder patch image. 

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The bright-frame

is a an opaque window, which prevents an image forming, and merely provides illumination for H.

is a mask which generates the bright-frame image for C2.

C2 is plain glass with mirrored edges, and reflects the bright-frame 90 degrees to D2.

D2 is a lens which focuses the bright-frame, and projects it on to E.

is a beam-splitter, which reflects the image 90 degrees, and sends it to F.

is the viewfinder lens.

Figure 14. The bright-frame optics. Note that the bright-frame mask (H) rides in a housing slot (to keep it upright), and hangs from a flat-spring top hinge.

There are a few features to note on the underside of the viewfinder assembly (figure 15 - where "A" is at the front of the camera, and in contact with the lens helicoid). Changes to the focus setting move the stud (A) back and forth, which in turn rocks the hinged arm (B) to shift the bright-frame mask up and down*. The diagonal limb (unmarked but below the "B") pivots around the large brass screw axis, which pushes the end-stop on the rangefinder mirror arm (C), which in turn rotates the rangefinder mirror on the other side, thus moving the rangefinder patch image synchronously with focus changes, along with the projected bright-frame lines.

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Figure 15. Viewfinder controls. The large brass screw head (bottom left) attaches the rangefinder-mirror turntable above.

* I have a bit of an issue here. The Lightomatic II instruction manual says "the trimming frame in the viewfinder moves automatically as (sic) to correct the parallax", and shows a diagram where the frame-lines correct down and right at close focus. However, the mechanism in this camera provides for an up and down movement only. I have checked my other Lightomatic II (without disassembling it). This has a slightly wobbly bright-frame, but it too only corrects up and down. 

 

Rangefinder and Bright-frame adjustments

  • It should be noted that, the various lenses and mirrors were originally cemented in place (except the bright-frame mask H which requires some movement). However, after 60 years the adhesive will have most likely dried-out, making some of the components removable (and then in need of re-gluing), which is easier than cleaning in situ. Lens cleaning fluid can be used to remove grime, but DO NOT GET ANY FLUIDS ON THE SILVERED SIDE OF THE BEAM-SPLITTER (the side facing the viewfinder eyepiece): this will remove the silvering and ruin it.

  • There is a provision for horizontal adjustment of the rangefinder. The arm that rotates the rangefinder-mirror has an adjustable infinity end-stop at its far end (figure 16). The bolt is threaded, and has a screwdriver slot at the opposite end to the stop, which faces the back of the camera. I would suggest temporarily refitting the lid of the viewfinder assembly (figure 8), before embarking on adjustments to the rangefinder's infinity focus off the camera. Correctly setting the rangefinder to infinity - where  necessary - requires tiny adjustments and is an iterative process.

  • The rangefinder can also be adjusted without any disassembly. Screw (a), referenced in step 4 of the top-plate removal section (i.e. the one on the rear face, in line with the accessory shoe) is a cover for the rangefinder adjuster. All you need is a 2mm slot screwdriver (with a shaft not exceeding 2mm). I've mocked-up figure 17 to show this "no-disassembly" adjustment.

  • The most likely problems with the bright-frame is it appears (when seen through the viewfinder) to be wobbly, doesn't sit level, or move smoothly. The most likely cause may lie elsewhere; a loose beam-splitter. The flat spring screw can be tightened (if required), or the spring gently reshaped, but there isn't scope to do very much with the bright-frame mask itself.

  • A final option is the entire viewfinder assembly, thanks to its modular nature, can be swapped with one from a donor camera if it is beyond restoration to health.

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Figure 16 - the rangefinder horizontal adjustment screw alters the infinity end-stop on the arm that turns the rangefinder mirror. Traces of old lubricant can be seen here, and a good clean-up and re-lubrication would be of benefit. 

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Figure 17 - here's the viewfinder assembly fitted back in the top-plate. I've placed a needle through the (a) screw hole to show its alignment with the rangefinder adjustment screw. Note that a hair-spring pulls against a post on the underside of the adjusting arm, and this must be re-instated if the arm is removed to lubricate the pivot point.