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Chapter 5

Focussing mount for 2" lens

Les Robotham & R. Pedro

The finished lens

Introduction

This idea came from trying to use an old 8mm cine camera with zoom lens for a Nipkow disc camera. It was noticed that the middle lens moves a different amount compared to the front one. Regrettably, the picture was always distorted when using a 10" disc. But there were some 2" diameter lenses available. By using two of these lenses a suitable lens system could be built which has a zoom function. Both lenses can be shifted independently. The front lens defines, more or less, the zoom function, while the rear lens places the picture in focus.

First requirements, two sheets of brass from your local model shop, size 10 x 4 by 0.015 inches thick, plus a length of 2 inch plastic downpipe (black or dark grey with 1/8th side walls). You will also require 3 Jubilee hose clips to take a 2 to 3 inch diameter and three ½" to ¾" length screws 2BA or 3mm depending on your stock of taps.

Making the brass tube

Now to start, mark out the brass as per Fig. A, cut out the slots as shown to give a gradual slope of about ¾". Make sure that the angle is the same for each of them, the distance apart is not as critical. Put the offset in the end for overlapping and soldering together when bent into a tube. To do this easily put a strip of metal or card either side of the line (see fig. E) and tighten in the vice. Cut the tabs (try not to cut too far in) and then put tabs in the vice and bend to 90 degrees. Before bending make certain that the long side is at right angles to the vice otherwise the view through disc will be out of true.
Now we are going to bend it into a tube. Cut a 10" or so piece of pipe as a former, cover 4" of one part in tape or plastic to increase its diameter a fraction, put the other end in the vice. This is where the Jubilee clips come in, open them out fully, bend the brass by hand until you can slide the clips on, now gradually tighten each clip in turn until the new tube is to size. It should slide on the former, albeit stiffly. Check that the ends are true and at 90 degrees to the sides also that the offset bend does not hit or over ride the other end of the brass. You can now apply solder to the parts clear of the clips, allow to cool, take off the clips and finish off soldering the edges a bit at a time. We wouldn't want it to spring open again.

Mounting on the base plate

The hole in the base plate should be at least an 1/8-inch larger than the outside of the drainpipe. Try the new brass tube with a length of pipe in it and verify that when placed on the base that the pipe and tube is at 90 degrees true. If so and centred, drill through both tab and base at the four cardinal points and fix down to a block of wood with suitable woodscrews, then you can solder all 37 (or so) tangs/tabs to the base plate, remove the screws and fill holes with solder.

Fixing of the lenses.

From the pipe cut off a 2" and a 2½" plus four pieces of ¼" inch length. Clean the edges, are they true? Try putting electrical tape round first to give a straight line to follow when cutting. Cut a 1/8th" out of a ¼" ring and try it in the 2" pipe for size. Cut further slices until it is a tight fit. Now you know the size for the other three. It should be a tight fit. Cut a spare 4" to 5" from the pipe and cut the same amount out of its side, make sure the wanted end is at 90 degrees to the sides. This is used to push one ring down the 2½" tube. You can now put a lens in and the second ring to hold it in place on the other side of the lens. The same can be done for the 2" part C. This way should ensure that the lens is not tilted.

The three cutouts are at a suitable angle to give a gradual slide in/out of the rear lens. Note the plastic pipe covers the slots at all positions to prevent light entering. Ah yes, the adjustment levers. Slide the tube C into the new brass tube until it covers the slots plus a fraction. Drill a hole through the plastic at the base plate end. The slots should have been of a size to take the screws you will use, i.e. 2 or 4BA or 3 or 3.5mm. Tap the plastic and put your screw in, with it held in its position drill and tap the other holes. The plastic should slide evenly over its 3/4" travel. If it tends to be stiff, lightly grease it with silicon or petroleum jelly. Before fixing the unit to the front panel of your camera it would be as well to try out the distance between the unit and a white card with a focused picture. Brightly light a suitable picture (black & white) with the back of the lights to the white card and unit, say 3 to 5 feet away and try for a clean focus on a 1 to 2 inch square. With the metal of the unit held you will be able to change the size of the picture shown. Note the distance from the baseplate of the unit to the white card. thiswlll be the distance of the disc to the front of the cabinet. Note also at this point that the 'D' tube can be slid into the brass tube in the reverse way. This makes a difference in the size of the image focused on the disc. In this case tube 'D' would need to be 3.5 to 4 inches long for sufficient zoom range.

Now you can fix the unit. If there is too much baseplate then cut off the extra but not before you have checked that it is accurately aligned and the centre of the lens system is aligned with the optics behind the disc.

Some experimentation may be required if lenses of sizes which differ from those given in the text are the ones you wish to use.

 

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