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.
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This Instructable shows the process I went through in order to build my own set of Opera Glasses!
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:
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.
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:
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:
http://hyperphysics.phy-astr.gsu.edu/hbase/geoopt/teles.html
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:
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.
For more information, please visit double concave lens.
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:
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.
In my case, I settled for the following:
I can now plug these values into the equations:
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.
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:
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.
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