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How I built my own DSLR lens

I'm not much of a photographer. Or an engineer. But I had an old World War Two spotting scope laying around, one thing led to another, and now you can read about it.

The genesis of this project was seeing that big piece of glass sitting unloved in a dilapidated spotting scope. It was over three inches across and could be re stationed to create either a very fast or very long camera lens that would probably end up being of low optical quality. After pulling the glass lens from the front of the spotting scope I measured the focal length. It was difficult to measure accurately but it appeared that the focal length was approximately 240mm. The three and one quarter inch diameter lens would give a theoretical maximum aperture of about f:2.9. I knew quite well that the optical quality would be low but a lens with these proportions sounded interesting so I decided to move ahead.

The lens body would be made up of tin cans that previously held refried beans. A short length of three inch black PVC pipe would link the two halves and allow for rudimentary focusing. An EOS body cap would be enlisted to join the body to the camera.

The black PVC pipe was the first victim. I made two lengthwise cuts one half an inch apart to provide the gap shown in the parts photo. A hacksaw would have easily performed this task but my chop saw got the nod as it was already set up. This slot allows the pipe to compress a bit and each can to slide over the pipe. Next I sacrificed the body cap from my Canon Rebel 400D. The goal was a hole large enough to prevent vignetting while still small enough to fit inside the attachment points. My drill press and a hole saw were utilized to remove the center of the cap.

Home made lens parts
Drilling the EOS body cap
Drilling the can

The hole in the body cap was used to draw a circle on the end of the can. This circle was the approximate hole size needed to match up with the hole in the body cap. The drill press was utilized again to make a series of small holes in the shape of a large hole. Using what I learned in grade school art class I connected the dots with my tin snips to finish the job. It wasn't pretty but it was done.

The body cap was glued to the can using the last bit of a ten year old tube of two part epoxy. (By now the reader should come to the realization that I spared no expense on this project.) An appropriate weight, my empty coffee cup, was enlisted as a gravity clamp. The epoxy was given 24 hours to cure.

The next day I assembled the contraption. The glass element just barely fit into the can. It was slid down until it seated against the first row of ribs stamped into the can. A few strips of electrical tape applied to the inside of the can reduced the diameter enough to keep the lens from falling out the front. Yes, is was that snug. The black PVC pipe was simply squeezed by hand enough to allow the cans to slide over. Assembly complete.

Gluing EOS lens cap to can
The assembled home made EOS lens

I knew from Internet research that the camera would loose most of it's automation. Utilizing Aperture Priority mode instructed the camera to choose an appropriate shutter speed for the given aperture. I would be required to focus by hand using only the viewfinder. What I didn't realize until the first installation is that the lens cap does not lock onto the body. With no lock the lens is free to rotate and disconnect from the body at any time. From here forward I took the precaution to alway wear the neck strap for fear of dropping my costly xTi while focusing my $3 lens. My first snap shot showed a slightly cloudy viewfinder. The image, however, was horrible. It is shown below.

I didn't know what caused the poor image but I did know that most store bought lenses are black inside and mine was colored the equivalent of chrome. I grabbed a can of flat black barbecue paint and went crazy. The next day I reassembled and didn't see much improvement in the captured image. Here is a shot after the interior (and exterior) black out treatment.

First sample image from DIY lens
Second sample image from DIY lens
Third sample image from DIY lens
Fourth sample image from DIY lens

Next I decided to stop the lens down a bit with paper apertures cut to size and fit onto the front of the lens. The change was from wide open, about f:2.9, to f:4.0. The image improved some.

The final test was with an aperture of about f:5.0 and showed further improvement.

So now what? I can't say I am too surprised at how poor the quality is. I hoped for just a little bit better. And I certainly don't think I'll be using this lens much. But it was an interesting project none the less.