Initial Prototyping - Day 5

Day 5 came along and there was still a lot to do. The main priority was to get the lens mount completed so that my optics could be fitted to the camera securely. With the outer barrel finished and the mounting ring trimmed, all that was left to do was turn the inner barrel. It was made in exactly the same way as the barrel, turned from a roughly cut cylinder form a block of nylon and bored out to fit the optics. I bored the centre out so that it was 0.5mm to small for the optic to fit. I then hand cut 3 slots down the thin section of the barrel so that it could distort to securely friction fit the slightly wider diameter optic.

The inner barrel fits into outter barrel okay; it's a little loose but that shouldn't be a problem. The expansion of the barrel when the optic is inserted pushes it out to be nearly the correct size, so it slides in and out okay, but does have lateral wobble by a few degrees. It needs to be wiggled a touch to ensure the front of the optic is parallel in all directions to the image plane. A small shift in angle can make a big difference in image quality, so it's important to take note and compensate when shooting.

I decided to run a couple more processes on the housing before I decided it was finished, some purely for aesthetical reasons. For example, I trimmed 0.25mm off the mounting ring to remove the top layer of paint and expose a nice silver ring making it look a bit more professional ;). I also added a chamfer very quickly using the lathe to the front edge and the facing edges between the inner and outter barrel. The large chamfer on the inside front edge of the barrel is an effort to reduce any possible vignetting that might occur, as the optic sits deep within the barrel and I don't want light from the 50mm field of view to be blocked from hitting it. To fix it all together, the mounting ring was stuck to the outter barrel using industrial double sided tape right around the diameter, providing a more than adequate fixing. The lens at this point, was then finished enough to mount, as you can see above on my dummy EOS film camera.

With the lens as good as complete, I had one final job to do in the afternoon; making the stop down rings. To help test my optics properly I decided to calculate the theoretical diameters of the holes needed to create range of f/stop down rings. These rings could be interchanged, pushed into the inner barrel behind the optic so the hole would be between the optic and the image plane. To ensure it would fit, and that the holes would be the right size and in the right place (essential as the rings effectively become a part of the optical system, and their quality will influence overall image quality) I decided to laser cut them. I quickly did the calculations for stop down diameters of a 50mm singlet (very easy, f/1 is 50mm, f/2 is 25mm, f/4 is 12.5mm and so on), drew them on SolidWorks, exported as a DXF and ran them on the laser, cutting the rings out of 2mm special purpose laser cutting acrylic. I also programmed it to etch the f/n# as well so I could easily keep track of which is which.

As you can see I did a mixed range from f/2 through to f/16 to give me optimum opportunities to test the optics performance. Having finished the rings, the weeks work was finally completed! One lens housing with push pull focus, stop down rings, and 2 optical elements to test all manufactured within a working week. Exactly what I wanted!