This Instructable will show you how to build your own adjustable focal length lens using only a few simple supplies. This lens will be able to change shape and therefore magnification based on the amount of fluid inside of it. It will also have a squishy surface which we can press on to distort whatever we see through the lens!

Disclaimer: it took me a number of tries to make a lens which didn't leak. I hope this guide will work on the first try, but if not, please post a question in the comments! Also, photos throughout this guide are from a few different attempts, so may not resemble your lens exactly. Finally, this guide uses laser cutting and acrylic solvent welding, so always ensure you are taking proper safety precautions when dealing with dangerous or unknown substances and tools!

Step 1: Background Info

Before we get started, lets first we need a bit of background about optics!

Light changes direction as it moves through different mediums (a phenomenon called Refraction). In lenses, we carefully control the angle at which light passes from one medium (air) into another medium (the glass or plastic material that makes up the lens) and back to focus or otherwise alter the light. Most camera lenses are composed of a number of individual lens elements which work together in groups to correct for various types of aberrations (optical errors) and produce a flat, focused image on the image plane (the camera's sensor). But if all of the lens elements are of a fixed size and shape, how do camera lenses focus or zoom the image? When a camera lens changes zoom or focus, it moves individual lens elements (or element groups) forwards and backwards with respect to each other inside of the lens.

In this instructable, we are going to produce only a single lens element, but unlike the lens elements in a typical camera lens, ours will be able to change shape (which will result in a change in its optical properties). This is because one side of our lens will have a transparent silicone membrane, which will change shape according to the amount of fluid inside of the lens. The other side of our lens will be flat (a plane). Our lens will be able to change shape from a plano-concave lens (one side flat, one side concave) to a plano-convex lens (one side flat, one side convex) lens and everything in between!

Step 2: Gather Materials

Below are the materials and tools necessary for this project:

Materials:

1/8" thick transparent acrylic
Syringe and tube (I used this one)
Silicone sealant / caulking of some sort
Acrylic solvent & applicator bottle (Alternatively: you could use an acrylic glue)
Transparent Silicone Sheet (I used this one)
Propylene glycol (I used this one).
Machine screws & nuts (I used 4-40 machine screws, but feel free to buy any similarly sized screws of at least 1" length)

Step 3: Gather Tools

Tools:

Laser Cutter*
Sandpaper (any grit should be OK)
Clamps to hold pieces while solvent-welding (gluing) them together

*Note: it is not strictly necessary to use a laser cutter for this project and I know accessing a laser cutter can be difficult! That said, instructions given are for a laser cutter because that is what I used. If you find another way to make a liquid lens, please post it in the comments (or better yet, make another instructable!)

Step 4: Design / Prepare File for Laser Cutter

Some design dimensions you will want to check and possibly adjust for your particular setup include:

Width of tube opening: this should be around the same width as the diameter of the tube you've sourced. The remaining space will be filled with silicone sealant.
Size of screw holes: the machine screw holes should fit whatever screws you have available. I would avoid putting them near the edges or the O-Ring.
O-Ring: this design cuts an acrylic o-ring from the same piece of acrylic that creates the o-ring groove. To make sure there is enough clearance for the silicone sheet to pass around the o-ring, I've added extra cuts to the inside and outside of the o-ring. This means that the o-ring and groove are formed from 4 concentric circles in the design. It may well be necessary to cut several o-rings and adjust their thickness until you find one that fits but doesn't leak!

fluid-lens.dwg
Download View in 3D

Step 5: Laser Cut Acrylic Parts

Cut the parts out of your sheet of acrylic using settings for the specific machine you are using!

Step 6: Solvent-Weld Bottom Half of Lens

Remove paper from Layers #1-3 and lay them on on top of the other, making sure their screw holes line up (you could use the screws for this). In a well-ventilated area, carefully solvent-weld or glue the layers together, making sure not to get solvent on the exposed center of Layer #1.

Step 7: Add Tube to Bottom Half

Next, we will add the tube which changes the amount of fluid inside the lens.

Scuff up the end of the tube and the inside of the area where the tube sits in the acrylic using sandpaper to make sure that the silicone sealant has a surface to which it can adhere,
Add enough silicone sealant to the area that the tube squishes some out,
Be careful not to seal the end of the tube.

Step 8: Continue Assembly With Layer #4

Add layer #4 on top of the bottom half and solvent-weld in place. Make sure there are no gaps in the area near the tube. You may need to use clamps to ensure a good seal. At this point, the tube should be sealed in place.

Step 9: Continue Assembly With Layer #5 (O-Ring Layer)

Layer 5 has three parts: the outer part, the o-ring, and the innermost part. Glue the outer and innermost parts to Layer 4. Leave the o-ring aside for now.

After the glue is dry, give all of the parts around the o-ring groove and the o-ring itself a good sanding to avoid sharp edges which might cut the silicone sheet.

Step 10: Add Bead of Silicone Sealant to Bottom of O-Ring Groove

One of the first things this project taught me is that it is very difficult to make things air or water-tight. "Life finds a way" and so does Propylene Glycol! Adding a bead of silicone sealant to the bottom of the o-ring groove definitely helps keep the propylene glycol in place inside of your lens!

Step 11: Continue Assembly With Silicone Sheet and Layer #6

Fair warning, this is one of the most difficult steps in this Instructable, so now would be a great time for a stretch break... Alright, that done, it definitely helps to find a friend to help you with this step!

In this step, we are trying to seal the silicone sheet onto the top of the lens and attach it all together with the machine screws and nuts. It should go a bit like this:

Hold silicone sheet relatively taut over the top of the lens,
Have a friend press the o-ring in place with layer #6,
Add clamps all around the edge so it holds together,
One by one, replace the clamps with machine screws, making sure the silicone sheet is still pressed in place by the o-ring.

Step 12: Test for Leaks!

If you've managed to get through the previous step without the silicone sheet tearing, congratulations! Now, test for air-tightness by blowing into the tube. In the video, the lens is leaking air: as I blow in, the silicone sheet expands, but as I stop blowing the air leaks out. :( Hopefully, yours will hold its shape at this point!

Step 13: Fill It Up!

At this point, if your lens is sealed, you should be able to fill it up with propylene glycol and use it. To do this,

Fill syringe with propylene glycol,
Attach the hose to the lens,
Progressively add some propylene glycol and remove the air from the lens,
Make sure not to over-fill the lens!

Step 14: Use Your Lens

At this point, you should have a working, adjustable focal length fluid lens! Very cool! Now what?

Well, there are lots of projects that could incorporate this sort of lens. You could use it in front of a projector or camera for unusual analog effects, or use it as a magnifying glass, or spend hours upon hours pressing your fingers into its gooey goodness (just watch those nails!). That is up to you. The video above has examples of how I tested / played with my lens to give you a start.

Please let me know in the comments how it goes, whether you find any way to improve this process or exciting uses for this lens.