A Stereo Viewer for Medium Format Slides

This stereo viewer produces excellent image quality that is consistent with the high resolution and low grain images that can be produced with medium format film. Medium format stereo images can be truly striking when care is taken to ensure optimum performance. In this respect, the design of this viewer leaves very little to be desired.

Summary Specification

Lens: 80mm achromatic doublet, 40mm diameter
Focus: +3 dipoter to -4 diopter nominal
Interoccular: Not required for the given lens diameter

Type of illumination: 2 x 12 Volt, 10 Watt halogen lamps, colour corrected
Diffuser: Flashed opal glass

Materials: 3mm and 6mm MDF board glued, and finished in hammer tone enamel.
Formats supported: External slide dimensions 132mm x 80mm; Aperture dimensions up to 53 x 53 mm
Other: Not for sale. Plans located at
http://werple.net.au/~kiewavly/stereo.html

Lens System

The design of the viewer started with the choice of a suitable lens, which should be an achromatic design so that colour fringing effects in the image will be minimised (generally this means that the lens will be a cemented doublet, although air spaced doublets are also possible). The focal length required for the lens should be close to the focal length of the camera. For medium format, the camera and viewer lenses will be in the range of 75mm to 85mm focal length. In choosing the lens, it was always in mind that a lens with a sufficiently large enough diameter would mean that inter-ocular adjustments would be come unnecessary (see later). Another desirable feature is to use lenses that have anti-reflection coatings on the lens surfaces, thus increasing the contrast in the image.

Of course, all these requirements need to be balanced against what is commercially available, and at what cost. The lenses chosen are expensive, but meet these stringent requirements very well.

 

Focussing Adjustment

The viewer has been designed to give a wide range of focussing adjustment, so that people may view the images without wearing glasses (unless astigmatism correction is also required). Eye correction generally requires glasses with a power varying from -4 dipoters to +3 diopters (or in terms of focal length, -250mm to 333mm). In the viewer, this will mean that the distance between the lens and the slide will need to be adjustable by the person viewing. The amount of movement required can be found as follows:

 

The design aim was to provide this much movement from the mechanism, however it was found that only 40mm was achieved.

Interocular Adjustments

When using a pair of lenses with insufficient diameters, the lenses need to be moved apart to cater for viewers with wider than average eye spacing, or moved closer for viewer with less eye spacing than normal. This is often referred to as interocular adjustment.

However, if the lens is of sufficient diameter then interocular adjustment is not required. A pair of lenses spaced at 63.4mm (the infinity point spacing in the slides) will always require the viewers eyes to be converged at infinity, regardless of the eye spacing of the person. It has been found in practice that the 40mm diameter of the lenses used is adequate, and can certainly cope with eye spacings of up to 70mm with the image size of the slides (50x50mm).

 

Illumination System

The illumination system uses two 12 Volt / 10 Watt halogen bulbs, spaced 72mm apart - not 62mm which is the aperture separation. (The photos show two 6Volt / 10 Watt lamps running in series, however it was later changed to the 12V /10 Watts running in parallel). The increased separation gives a slightly more even coverage of the diffuser illumination, as the area of the diffuser between the lamps is lit by the output from both lamps, where as the area of the diffuser towards the outside of the viewer is predominantly lit by the closest lamp.

The lamps have been attached to the same sub-assembly as the diffuser and slide gate. This is the best place, as the any variation in the distance between the lamps and diffuser would result in a variation in brightness when the focus is adjusted. (Note also that the diffuser is fixed 30mm behind the slide so that any dust that accumulates on the diffuser will be out of focus when the slide is in focus, thus making any dust largely invisible).

The diffuser user is "flashed opal" glass, which is probably one of the best choices in terms of giving an even diffuse light, with little loss in the diffuser. No actual reflector was used around the back of the lamps, however the inside of the lamp house was covered in white plastic or painted white.

The colour temperature of the illumination system was raised by using a blue filter gel between the lamps and the opal glass diffuser. It is designed to raise the temperature from about 3000K to 3400K, which clearly gave a better colour balance.

 

Design Plans

The considerations above led to the design plans attached, which were developed to scale on a computer before making any parts. This was especially useful in determining the design of the focussing mechanism, as the motion of the parts could be seen before anything was built. The cutout for the nose was however determined in practice when making the parts.

Construction Comments

Materials

A variety of materials could have been used for the construction of the viewer components. The material depends on type of finish required, durability, and ease of working to tight tolerances (say better than 0.5mm). This viewer was made using a mix of 6mm and 3mm "medium density fibre" board (also known as Custom Wood). It similar to chip board, but the grain is much finer - so fine that when a butt-joint is painted it is impossible to see the end-grain. In addition, it is fairly easy to work, although the dust created should not be inhaled excessively (use a mask). Most of the viewer was made using a circular saw on a Triton workbench, including rebates of corners etc. Parts were glued together with PVA glue.

Focussing System

The focussing mechanism uses a rack and pinion gear system to move an internal sub-assembly back and forth. The photos show the two black rack gears attached to the sub assembly on either side of the lamps. The pinion is attached to the external viewer casing, and the gears mesh when the subassembly and baseplate are assembled inside the upper casing. (The height of the pinion has provision for fine adjustment by moving the bearing up or down).

The sub-assembly is approximately 1mm narrower than the internal width of the viewer casing, and about 2mm shorter in height. The sub assembly sits on the bottom plate of the viewer (which is removable). Plastic pads reduce friction between the bottom plate and the sliding sub-assembly. The walls of the outer casing form the guide for the sub-assembly so that it only moves back and forth. The sub assembly cannot move upwards as it is held down by the pinion gear meshing with the rack.

The components for the slide holder are designed to slide into rebates in the walls of the sliding sub-assembly. (The photos show these components removed from their rebates.) The slide is held in position by a spring that presses the slide up against the front of the slot, thus ensuring that no light leaks around the perimeter of the slide. All components here were made from 3mm medium density fibre board. The slide gate is can accept slide up to 3mm thick, and with nominal 132mm wide by 80mm high dimensions. The nominal slide aperture is 50x50mm, however the slide gate should cope with apertures up to 53x53mm.

Lens Holders

The lens holders for the viewer were designed by John Bercovitz. A design drawing is attached, from which the components could be turned by a machinist. The holders are finished in black anodised aluminium, and have been carefully designed to allow the maximum field of view to be obtained.

Internal Baffling and Flare Reduction

A baffle is required between the lenses and the slide to stop the right hand slide being seen in the periphery of left hand lens. As stereo slide moves back and forth, this baffle is made in two parts - one part attached to the sliding sub assembly moves inside another fixed part attached to the back of the nose cut out.

The areas between the lenses and the stereo slide is painted matt black, or for larger areas it has been covered with black flocking material (as suggested by Greg Erker) to reduce flare and distractions to a minimum.

Power Supply

A dedicated external power supply has not been built for this viewer yet. Desirable features for powering lamps would be: slow-starting circuit for increased lamp life; and current source regulation to reduce likelihood of flicker and effects of voltage drops. The lamps used required 12V at 0.83A each.

Finish

The viewer has been finished with one undercoat, and two coats of "hammer-tone" enamel paint in "gun-metal" grey.


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