Back in the days of film and darkrooms I used to lug around an SLR camera. I never really called photography a hobby, so I gave it away when digital point-and-shoot cameras came out. These days I can usually take all the pictures I want with my smartphone.

The only thing I miss about SLR cameras is the ability to take photos with shallow depth of field, the ones where the background or foreground is blurred. You need a macro or zoom lens to do that, and smartphones don't have them. My Samsung S5 camera has a feature called Selective Focus that tries to achieve the effect using multiple shots and software tricks, but it's very finicky. I can also reproduce the effect by processing the digital image with an app, but it takes a lot of work for the result not to look fake. The other option is fit my phone with a Sony QX lens, but that costs as much as a professional camera.

Some time ago I was told that it was possible to use zoom lenses on a smartphone by putting a telescope eyepiece between them. This week I finally got the chance to test the idea when I found a used nonworking DSLR lens for $10. As I try to demonstrate in this project, for the fraction of the price of a point-and-shoot you can vastly improve the power of your smartphone camera and rescue a fully functioning but obsolete piece of precision technology from destruction.

Comments and tips from the more knowledgeable will be appreciated!

Step 1: Gather Parts and Tools

Parts:

- Zoom lens with rear cover. The one I found was a Canon EFS 18-55. The rear cover is where the eyepiece will attach to.

- "Telescope Camera Lens." It's really just cheap zoom lens for smartphones. The quotation marks are there because that's what it's called on ebay. I bought one a long time ago because I thought it might be useful, but it was a waste of $9. This project gives me a chance to get something out of it.

Mine came with a case specifically for my Samsung S5. Its eyepiece attaches to the phone case using mounting slots. This was a big plus, because I didn't have to figure out how to connect the two together.

- Plastic tube with inner diameter exactly equal to the barrel of the cheap zoom lens. The fit should be tight. This part is actually optional but is useful if the barrel is too short, and it creates another way to get focus by letting you adjust the distance between zoom lens and the eyepiece. You will then be able to use the full range of focal lengths of the zoom lens.

I found the perfect tube in my pile of random junk. I believe it's the tube that dispensed those clear plastic bags in the vegetable aisle of the supermarket. I have a habit of picking up trash like this all the time, and this time it paid off.

Tools:

Hacksaw or similar tool for cutting the plastic tubes
Glue gun
Drill and small drill bits, x-Acto knife, or any other tool you prefer for cutting a big hole on plastic
Fine sandpaper.

Update: I finally found the perfect Android app to use with the lens. The app is called Open Camera, which has a mode that rotates the image on the camera viewfinder, correcting the rotated image seen through the lens.

Step 2: Trim the Eyepiece

That cheap telescope camera lens has four main parts: the eyepiece you look into, the roof prism, the objective lens at the end, and the barrel holding everything together. I unscrewed the cap at the end to remove the focusing knob and everything inside, then I cut off the end to get a straight cylinder. I ended up with a cylinder about 2 inches long. On hindsight I should have done the cutting only after I finished with the step below.

I also unscrewed the eyepiece so I can clean the barrel. I found out that the eyepiece itself is actually two lenses inside a housing, which I shouldn't have taken apart because I had to clean it afterward. After I sanded off the trimmings from the barrel I screwed the eyepiece back on.

I determined how far I needed to place the eyepiece lens relative to the zoom lens by looking through the eyepiece into the zoom lens. The correct distance is when you are able to see an image clearly through the zoom lens like a telescope (although the image is upside down). You may need another person to measure this distance as you hold focus, but it does not have to be exact. As it happened, the eyepiece barrel was very close to the correct length if I set the zoom lens to its maximum focal length (to get the shallowest depth of field).

If I used a different focal length and the subject is at a different distance, I needed to move the eyepiece farther out to get proper focus. I could also take a picture of an object 1 mm in front of the zoom lens with a very different placement of the eyepiece. In the end I decided on an eyepiece distance that gives me the effect that I'm much more likely to use.

Step 3: Cut a Hole on the Rear Lens Cover

I traced a circle on the rear cover with exact the same inside diameter as the eyepiece barrel. I drilled small holes along the inside of this circle, then I cut between the holes using an x-acto knife. After that I cleaned up the hole with sandpaper.

You can use any method you want to cut this hole. Just remember the plastic rear cover might crack if you use a big drill bit.

After I cut the hole, I sanded away the logos to get a completely flat surface to glue on.

Step 4: Cut the Tube and Glue to Rear Cover

Based on my tests of the correct distance between the eyepiece and the zoom lens, I needed about 2" of the plastic tube. I chose to err on the short side, since I can just move the eyepiece further out to get focus. I used the hacksaw to cut the tube.

With the glue gun, I fixed the tube to the rear cover. I tried my best to place the tube at the exact center of the cover.

This piece is now an adapter that holds the eyepiece to the zoom lens.

Step 5: Assemble the Parts

I attached the adapter to the zoom lens, inserted the eyepiece barrel into the adapter, mounted the eyepiece to the phone case, and placed the phone on the case. Time to take pictures.

To get coarse focus I adjust the position of the eyepiece in the tube, and for fine adjustment I use the focusing ring on the zoom lens. I can hold the setup with the lens resting on my palm, and I can also use my thumb and forefinger to turn the focusing ring -- just like a film camera with no autofocus. The whole thing stays intact when I move around, adjust the lens, and operate the phone to take pictures.

If your eyepiece is not snug in the tube, you can drill a couple of holes on the tube to which you can attach screws or bolts that will hold the eyepiece in place. You can also duct tape the eyepiece to the adapter. Better yet, you can 3D print the whole adapter by downloading a model of your rear lens cover, then adding the tube. Adding a 1/4" tripod mounting nut at the bottom might also be a good idea.

Step 6: Take Pictures

Here are some pictures I took. You can see the shallow depth of field I was going for. It may not be as good as a professional camera, but it certainly expands the possibilities of what you can do with your smartphone.

You will notice a lot of vignetting, which is the degradation of the image at the edges. This is partly due to the fact that smartphone cameras have wide angle lenses, so they capture the edges of the eyepiece. Moving the eyepiece closer to the smartphone lens won't solve this problem because the smartphone will instead capture the inside of the lens. The correct distance (the "relief") is a carefully designed value in all optical instruments, so it's asking too much to get a perfect image from this crude setup.

When you consider that vignetting is a commonly used filter to add character to an image, this imperfection becomes easier to accept.

Step 7: Notes

1. The image on the camera screen is upside down (rotated 180 degrees). The roof prism on the telescope is supposed to fix this, but when I put it back on the eyepiece the resulting image was too small. Fortunately, there's a free app called Open Camera that rotates the image on the camera viewfinder. Install it, then go to Settings->On Screen GUI ->Rotate preview (at the very bottom of the menu).

2. Before I remembered that I had those telescope camera lens, I tried my telescope eyepieces which are of better quality. The problem was that the diameters of their eyepiece lenses were too small, which meant that the resulting photo was also small and circular. They will also need a much more precise placement of the eyepiece on the phone camera. I decided bigger was better.

3. I also have one of those universal smartphone holders that let you attach use phone camera to a telescope or microscope (see picture). They're not very expensive and they work well enough on something stationary like the telescope, but they are difficult to use on the zoom lens. There are three bolts to adjust, and they won't stay in place when I operate the camera.

4. Update: I have since tried using eyepieces from binoculars. I found that the best to use are those from wide-angle binoculars, which have bigger diameters. The biggest problem I encountered is building the system that connects the phone, eyepiece, and lens. A fixed system is easy, but one that I can take apart for travel is too much work. I hope someone comes up with a good design.

5. I have also experimented with film camera lenses. What I learned is that a lens with a fixed 50 mm focal length is preferable to one with variable focal length. A long zoom lens sounds like a good idea, but it's a bad choice for a couple of reasons. First those things are heavy, so you'll need a pretty sturdy setup. Second, taking a picture using long zoom lens means that small movements of your hand will translate to a lot of movement of the image. This problem is minimized when using conventional cameras because the camera is held close to the body as the photographer looks through the viewfinder. On a smartphone you hold the camera farther away, so you can't expect to take a stable picture with very long lens unless you have a way to set things up on a tripod.

There are of course shorter variable lens, typically starting at 28 mm. But remember that the goal of this project is to achieve a shallow depth of field effect. My impression is that I don't get what I want at that focal length, so I only take pictures at 50 mm anyway. A fixed-length lens has the added advantage of being lighter and smaller than zoom lenses.

Introduction: Attach Zoom Lens to a Smartphone