This is a projector lens my Dad got some time ago, well before I had a 3D printer, with the intent of turning it into a camera lens. If you knew my Dad, you'd know Hell would freeze over before he got around to adapting it, so when I got a 3D printer, the responsibility (or should I say opportunity) fell on me.

It's made in East Germany/GDR/DDR, so you know it's not exactly new. Even if it's not the latest, the sharpest, or the fastest, these lenses are prized for their "bubble" bokeh (out-of-focus object blur). Focal length is 80mm, aperture f/2.8.

This cutaway of the CAD model shows the lens (transparent green), the body (yellow), lens hood (pink), and a filter holder (blue). The lens screws in to the body using the helical channel cut into it, and is held in place simply by friction. It can be screwed in or out to put it at the correct position for focusing at infinity (i.e. when the helicoid is fully collapsed).

The filter holder is a separate piece so that installing, removing, and adjusting the lens within the body is easy to do. The screw threads on the body are M42, which screws into a focusing helicoid, which then screws into a M42 to E mount (Sony). The last two were purchased instead of being printed, as he wanted a metal mount, and it would be very difficult to get a nice smooth helicoid out of printed parts.

I tried printing the filter holder twice, but since I didn't know the tolerance for plastic to plastic threading, neither fit. He's not terribly interested in putting a filter on it, so although I could print one now that would fit, I'll wait until there's a need.

Finally, this is actually the second iteration of my design for this. The 'mk I' had no lens or filter mount, and was mostly to test for fit and feasibility. This 'mk II' version is also significantly stronger, as I used thicker walls for the body.

Here it is fully assembled and packed for transport, with a 20mm calibration cube for scale. As you can see, the lens hood is reversible, which makes transport and storage easier, but did limit the length to slightly shorter than would be ideal optically. It attaches to the body using a bayonet mount, similar to commercial lens hoods. However, it does not have a "spring lock" to keep it in place like many commercial lens hoods, as this would have make it larger in diameter, making it less compact and even worse optically. Instead, it's held in place simply by friction, which was a challenge to get just right.

The lens cap at the top, just below the cube, came with the lens and was not made by me. The E mount cap at the bottom, however, is printed, and used to keep dust from sneaking in to the back of the lens. The red mark, showing how to align the cap properly, was added with nail polish.

The lens hood can be quickly removed, flipped around, and remounted thanks to the bayonet mount. Here you can see that I sanded the body that holds the lens, but didn't sand the hood, as that would cause it to reflect more light. It still reflects more than I would like, so I might add some flocking material or ultra-black paint (e.g. Black 3.0) in the future to further improve things.

Also, you can see the helicoid is labeled as a "variable close-up ring," as these are frequently used instead of extension tubes for macro photography (instead of being used for primary focus as it is in this application).

The bayonet mount on the body, seen here, as well as the portion on the inside of the lens hood had to be carefully sanded to provide just the right amount of friction. It needs to be held in place when it's not being handled, but not be too difficult to move by hand when you want to change the configuration.

This also shows how much the front of the lens protrudes beyond the body, in order to provide an easy grip surface for adjusting the infinity point, installation, and removal. The filter holder would cover this to allow mounting a filter in front of the lens, but still be removable when you need to adjust the lens.

Here's what the lens looks like with the hood installed. Note the difference between the sanded and unsanded finish on the plastic.

With all this talk about lens hoods, is it really that big of a deal? Well, here's a shot through the lens without the lens hood being used. As you can see, the light is coming from the left (off frame), and the flare on this lens is particularly bad.

The exact same shot in terms of camera position and lighting conditions, only this time with the lens hood in place. Yes, there's still flare there, which can probably be reduced with proper flocking material. It's on my 'to-do' list, but as you can see it still provides a big improvement over nothing.

And last but not least, a shot through the lens that's not just about flare. This is my parents' cat, who was kind enough to pose for this shot. You can see the beautiful bokeh this lens produces, even on a difficult material like this upholstery fabric. Depth of field is quite shallow at this distance, and the lens has no aperture control (fixed at f/2.8), so it will never be the sharpest. But hey, with bokeh like this, would you really want to stop it down?

Around the time I finished up the 'mk II' mount for his projector lens, I found that my workplace was getting rid of a projector. While it was made well after East Germany ceased to exist, it was still quite old, but optically it seemed decent to me. What's more, it wasn't a prime, but a zoom! At the time, I had no idea what its focal length range was, but I wanted to find out, and the challenge alone was far too tempting.

Here the two are naked and side-by-side for comparison, with the zoom lens facing down as the rear-most element protrudes and is convex. However, you can see technology (or perhaps economics?) has changed over the years, as the reflections off the zoom carry a green or magenta hue, characteristic of coated lenses. These coatings are anti-reflective, meaning more of the light that hits the lens goes through it instead of bouncing off, leading to brighter images for a given design of a lens, and usually with more contrast as well.

On the bottom (in this case front) of the zoom, you can see the grape-colored, somewhat rubbery ring used to adjust the focal length (AKA "zoom") of the lens. When twisted, this moves the front of the lens forward and the rear of the lens backward, while the ring in the middle stays stationary. You can see the angled grooves in the lens, and the pins that slide in the slot to perform this complex movement.

This also presents a challenge when mounting the lens though, as it cannot be secured too far back or too far forward. Mounting either of the moving elements would make zooming the lens impossible, and I consider that unacceptable. However, mounting the lens via the threads (as it was done in the old projector) wasn't practical with plastic, as these threads have a very fine pitch and large diameter.