Introduction: How to Make an Adjustable Helicoid Adapter for a 85mm Projector Lens, From a Polypropylene Tube Connector

About: Photography, astronomy, arduino

I recently bought an old slide projector for about 10 euros. The projector is equipped with a 85mm f/2.8 lens, easily detachable from the projector itself (no parts need to be disassembled). So I decided to trasform it into an 85mm lens for my Pentax K5 camera, using a hydraulic polypropylene connection tube (which allows helical focusing) and a K-M42 adapter (to connect the lens directly to the camera).


This is what you need:

  • a slide projector lens (I bought a Heidosmat 2.8/85, whose back diameter is 42 mm, but any other projector lens of that diameter fits well - (find it on ebay or any flea market for about 5-20 €, it depends on brand/quality)
  • a polypropylene connection tube Ø 50mm x M 2'' , thickness 4.5 mm (in Italy: Bricoman 3.20 €)
  • M42 Lens To Pentax PK/K Adapter ring (find it on ebay for about 2-9 €, it depends on the country where you buy it);

  • hacksaw

  • chloroprene glue

  • hot glue

Step 1: Step 1: First of All the Lens

With the advent of digital photography, many slide projectors are sold cheaply on the web and in flea markets. I noticed that most projector lenses are 85mm f/2, with an outer diameter (of the lens back side) of about 42mm, so this tutorial should be adaptable to most projector lenses with that diameter. The one I bought is a Heidosmat.

Step 2: Step 2: Modding the Tube

First, buy a hydraulic connection tube, that will house the lens. In Italy DIY store: check the item here.

To modify the tube, You need to cut both edges of the tube (see red signs of picture three). Referring to the first/third picture:

  1. DO THIS FIRST! - On the right side of the tube, you have to totally remove the screw part. I used a hacksaw (picture two) to do the job, and then I finished off the edges by burning the imperfections with a lighter;
  2. the left side of the tube (the smallest in diameter) is shaped like a series of rings. If you have the Heidosmat, you should cut the last TWO rings, but if your Lens is shorter/longer, you should take some measures before, and cut the tube accordingly.

Last picture shows the final result.

Step 3: Step 3: Glue the Adapter Ring

To connect the tube to the camera I glued a "M42 to K adapter ring" to the tube itself. Of course, this is suitable to Pentax K mount, but if you have another kind of camera you should read this article from Wikipedia first (as not all cameras can be easily adaped) and then try to get the right adapter for your camera.

As much as the K mount is concearned, note that there are different M42 to K adapters (see first picture). Buy an adapter with a large base, which normally is painted black. You can find it on eBay by entering the following search: "m42 k adapter ring". Then:

  1. Glue the adapter ring on the right-end of the tube (the largest in diameter), using a chloroprene glue. Chloroprene glue needs to be put on both surfaces (ring and tube) and after 10-20 minutes the two surfaces should be pressed together... the more you press, the more the welding will be effective, so press!!!
  2. Once the ring is in place, warm the hot-glue-gun up and drop a thin layer of glue around the ring, letting the glue filter through the gap between the ring and the tube

The final result is shown in the last two pictures.

Step 4: Step 4: Insert the Lens Into the Tube

Now take the 85mm Projector Lens and litterally, but gently, screw it in the smallest end of the tube. That's all. Now you have a 85mm f/2.8 lens with a Pentax K mount.

Some comments:

  • the shape of the tube fits perfectly to the lens. Besides, the spiral grooves etched on the body of the lens, together with the shape of the tube, allows manual focusing;
  • being a Projector Lens, it has no aperture control and one has to work always at maximum aperture;
  • this projector lenses to K mount adapter can also focus at infinity;
  • this lens on a camera equipped with an APS-C sensor (as in my case), is equivalent to a 135mm lens on Full Frame;
  • there is some chromatic aberration especially at the borders of the picture when shooting at infinity, so one gets the best performances when shooting portraits.

Step 5: Step 5: Some Sample Pictures

On Sunday I took some pictures at Monza Park with the Heidosmat mounted on Pentax K5, via the same adapter I used for this tutorial. Here they are. As it was late afternoon the light was not so good, but considering the whole price this 85mm lens is not so bad.

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