Introduction: DIY 2.5x to 5x Opera/Concert Glasses
In July, I am going to see a concert at an outdoor amphitheater. I knew I was sitting at a fair distance from the stage, so I wanted to have a set of opera glasses so I could see the show up close. I have a very nice set of zoom binoculars, but they are heavy and are difficult to hold steady for an extended period of time.
This Instructable shows the process I went through in order to build my own set of Opera Glasses!
Supplies
- 3D Printer
- Two convex lenses
- Two concave lenses
Step 1: What You Can Buy (and Why I Chose to Make My Own)
On Amazon, they sell several hands-free opera glasses. I decided to buy one that was advertised at 3x magnification to try out. They were well built, nice and sturdy and fit me well, but I am severely nearsighted (-6 diopters) and the focus adjustment would bottom out before I could get a clear image and I couldn't wear them over my glasses. For anyone who does not wear glasses, or has limited vision correction, I would recommend them, but they did not work for me.
I ordered a similar style of hands-free opera glasses from a seller that claimed theirs had 10x magnification and a focus range of -12 to 5 diopters. If these specifications were correct, they would solve my problem. As it turned out, the second specs had the exact same magnification and range as the first one. The second seller was obviously lying, which isn't uncommon on Amazon. In retrospect, knowing what I now learned about optics, 3x magnification is the best that you can hope for opera glasses small enough to fit on your head. For 10x magnification, you would need an extremely long telescope tube!
At this point, having rejected some of the commercial offerings, it was fairly clear that I would need to build my own!
Having these gave me a good baseline on what to expect for my build. From the glasses I bought, I measured the following characteristics:
- 30 mm objective lenses
- 12 mm ocular (eyepiece) lenses
- 40 mm of length
The focal length of the lenses isn't something I could measure, but later on we will see that I came up with some fairly good approximations and found out why 3x is the magnification limit for these glasses.
Step 2: Researching How Binoculars and Opera Glasses Work
The first step, is to understand how binoculars and opera glasses work. Both of these devices allow you to see things at a distance, but they are designed differently and for different purposes:
Binoculars:
- Multiple lenses and prisms per eye
- Complex construction
- High magnification, narrow field of view
Opera Glasses:
- Two lenses per eye
- Simple Construction
- Low magnification, wide field of view
As it turns out, building opera glasses is very simple. They consist of a convex and a concave lens separated by a distance. Their construction dates back to Galileo and and opera glasses are a kind of Galilean telescope. The following page gives an explanation, and most importantly, provides formulas for computing the magnification and lens separation:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/...
Figure Credits:
Attachments
Step 3: Choosing the Right Type of Lens
Our concert glasses, being based on a Galilean telescope, will make use of two lenses per eye, a convex lens and a concave lens. The difference between these two lenses is that the convex lens bulges out towards the center while the concave is narrower in the center (it "caves in" towards the center).
To build a Galilean telescope, you need to know that lenses are described by a focal length, which is a distance, usually expressed in millimeters. The convex lens, which points towards the object we want to see, is known as the objective lens and its focal length is designated fo (this number is positive). The concave lens, which sits close to our eye, is known as the ocular (eyepiece) lens and its focal length is designated by fe (this number is negative).
The two formulas that are relevant give us the magnification and the necessary separation of the lenses:
- Magnification = -fo / fe
- Lens separation = fo + fe
There is another parameter you will see when buying lenses and that is the diameter of the lens; this is also expressed in millimeters, but won't affect either the magnification nor the lens separation. Try not to get these confused with the focal length!
In the next step, I will show you how to buy lenses relatively cheaply.
Sources:
Attachments
Step 4: Buying Some Lenses (Cheaply!)
New lenses can cost a lot of money. Fortunately, I found that Edmund Optics has a large selection of factory seconds that they sell inexpensively. Simply head over to their website, click on "Optics", then "Clearance" and "Clearance Optics". You will see many categories, but in general you are looking for:
- Experimental Quality Plano-Convex (PCX) Lenses
- Experimental Quality Double-Convex (DCX) Lenses
- Experimental Quality Plano-Concave (PCV) Lenses
- Experimental Quality Double-Concave (DCV) Lenses
Plano designates a lens which is curved on only one side, while double means it is curved on both sides. Either type works for our opera glasses. Some lenses are coated, some are not, and there are commercial and experimental grade lenses. I found that the uncoated, experimental grade lenses were the least expensive.
I made a spreadsheet where I could list the lens diameters, the focal length (listed as EPL) and prices for the lenses. Note that there is often a discount for buying two lenses, so use those values (as highlighted in yellow).
This spreadsheet would allow me to mix and match lenses and see what kind of magnification I would get for each combination. My spreadsheet would also tell me the separation of the lenses and the total price of my opera glasses (minus shipping and taxes, which adds about $8.00)
After trying out a few combinations, I came up with the following strategy:
- You want to pick concave lenses with the shortest focal length (Fe), something like -15 to -20
- Fe is in the denominator of the magnification equation, so you want it as small as possible
- You want to pick convex lenses with a focal length (Fo) between 45mm and 85mm
- Fo is in the numerator of the magnification equation, so you want it larger for more magnification
- But larger values will increase the lens separation, which will make the instrument longer
- You should aim for a magnification between 3x and 5x
- You should aim for a lens separation between 40 mm and 65 mm
- The objective lenses should have a diameter between 30 mm and 50 mm
- The ocular (eyepiece) lenses should have a diameter between 15 and 20 mm
Use your spreadsheet to pick among the choices available to you, which likely will be different than the choices I had.
UPDATE: At this point, I stand by my choice of the eyepiece lens, but I found out using 85 mm FL for the objective lens leads to distortion around the edges; I found a 45mm FL lens from a Google Cardboard kit gave me less distortion.
Step 5: What I Ended Up Getting
In my case, I settled for the following:
- 45 mm dia x 83mm FL, Grade 1, Double-Convex Lens
- 16 mm dia x -17mm FL, Grade 2, Plano-Concave Lens
I can now plug these values into the equations:
- Magnification = -fo / fe = -83 / -17 = 4.88
- Lens separation = fo + fe = 83 + -17 = 66
This should give me a magnification of nearly 5x and cost only $13.30 plus S/H, bringing the total to $22.78, which was far less than the ones I could buy on Amazon!
At 66 mm of lens separation, it's quite a bit long, but I think it will turn out okay!
UPDATE: For my final glasses, I found that the higher magnifications led to too much distortion in the periphery. While I would still buy the concave lens I initially selected, I now recommend using a convex lens with a focal length of about 45mm. You can use readily available acrylic Google Cardboard lenses for the objective lens in your opera glasses.
Step 6: Designing a Mini Optics Test Bench
To better understand my lenses, I used Fusion 360 and a 3D printer to design a mini optical test bench. This allowed me to mount my lenses on a ruler, adjust the spacing easily and try out different lens combinations.
I am providing a link to the Fusion 360 project. This document is parameterized, so you can quickly change it to allow for different lens sizes.
Hint: To change parameters, click on the "Modify" drop-down and select Change Parameters.
Step 7: Comparing High and Low Magnifications
To better understand my lenses, I used Fusion 360 and a 3D printer to design a mini optical test bench. This allowed me to mount my lenses on a ruler, adjust the spacing easily and try out different lens combinations.
In addition to the convex lens I had bought from Edmund Optics, I also decided to try out a smaller convex lens I took out from a Google Cardboard kit. These 45mm focal length lenses are very inexpensive.
Using these two lenses gave me two set ups to compare:
- High Magnification Setup:
- Objective Lens: 45mm dia, 82.6 focal length
- Ocular Lens: 16mm dia, -17 focal length
- Theoretical Magnification: -Fo/Fe = -82.6/-17 = 4.86x
- Low Magnification Setup:
- Objective Lens: 25mm dia, 45 focal length
- Ocular Lens: 16mm dia, -17 focal length
- Theoretical Magnification: -Fo/Fe = -45/-17 = 2.65x
My first trial with the high magnification lenses was somewhat disappointing. Far from getting a clear picture and a wide field of view, I was getting significant distortion along the edges of the image. When I tried the low magnification setup, the distortion was significantly reduced.
At this point, it is unclear to me whether the effect has more to do with the lens diameter or the focal lengths.
In the next step, I will use an iPhone and an eye chart to measure the magnification of the lens pairs and compare them to the theoretical results.
Step 8: Measuring High Magnification (~5x) Using Mini Optical Test Bench
Using my optical test bench, I decided to see whether I could get some precise measurements of the magnification by taking pictures of an eye-chart through my iPhone.
The above picture was taken by placing an eye chart a few feet away, mounting my iPhone on a tripod and taking pictures with and without the lenses. Using the Measure tool in the GIMP, I was able to measure the number of pixels taken up by the letter E and use this to compare the theoretical vs measured magnification:
- Optical setup:
- Objective Lens: 45mm dia, 82.6 focal length
- Ocular Lens: 16mm dia, -17 focal length
- Theoretical Magnification: -Fo/Fe = -82.6/-17 = 4.86
- Size of E in Photographs:
- Without Magnification: 21 pixels
- With Magnification: 99 pixels
- Measured Magnification: 4.71
Sources:
Step 9: Measuring Low Magnification (~2.5x) Using Mini Optical Test Bench
I decided to try the same setup with a different objective lens, which I took from a Google Cardboard VR viewer.
- Optical setup:
- Objective Lens: 25mm dia, 45 focal length
- Ocular Lens: 16mm dia, -17 focal length
- Theoretical Magnification: -Fo/Fe = -45/-17 = 2.65
- Size of E in Photographs:
- Without Magnification: 21 pixels
- With Magnification: 47 pixels
- Measured Magnification: 2.24
Because this combination of lenses gave me significantly less distortion, I chose to use these lenses for my final opera glasses.
UPDATE: I have not investigated this distortion effect thoroughly, but I suspect it may be related to the F/ratio value.
Step 10: Designing the Opera Glasses Frame
Having a selected my lenses, I was ready to begin the design of the opera glasses themselves.
My idea was to have a solid frame for that held the ocular lenses in place with screw on cups for the objective lenses. This would not only allow for focus adjustment, but also allow the lenses to be interchangeable for multiple magnification powers.
I created a Fusion 360 project. This document is parameterized so you can easily change the inter-pupillary distance (IPD) as well as make adjustments to the diameters of the ocular and objective lenses if your lenses don't quite match mine.
Hint: To change parameters, click on the "Modify" drop-down and select Change Parameters.
To complete the assembly, 3D print the frame and two lens holders. Use a dab of glue to secure the lenses in place and assemble!
Step 11: Experimenting With Different Objective Lenses
The nice thing about these opera glasses is that the lens mounts are threaded and interchangeable, so you can experiment with different objective lenses and magnifications! While I was disappointed with the optical performance of these 45mm lenses, they certainly make for a cool looking set of steam-punk binoculars!
Question: Could you make a telescope with the techniques outlined in this Instructable?
Answer: Of course! Look for large focal length objective lenses and mount them further apart!
Step 12:
Participated in the
STEM Contest
