# Microscope eyepiece modification for the near sighted



## GailInNM (Aug 16, 2009)

I have one of the little Fowler 20X pocket microscopes. They also make a 40X version. It's great for inspecting cutting tool edges, particularly when the tool is mounted in a turret and can not be removed for inspection with out recalibrating the tool position. Also good for checking the tip radius and things like that with the built in reticle scale.

Being near sighted, I can not read the scale with my glasses off as the eyepiece is set for standard distance vision for focus on the reticle. The graduated scale looks like a fuzzy millipede. Focus on the part is no problem as that is set by positioning the microscope. With my glasses on it is not a problem, but the eyepiece is spaced too far away from to have a full field of view. If your glasses have a correction of -2.5 diopter or more you may have the same problem.

Now if you a cheap old coot like me, you probably have a few pair of older glasses around that you have replaced, but are holding onto just in case. I keep my most recent pair in the car in case something happens while I am out, and the next most recent pair in the house in a convenient location so I can use them to find my regular pair when I lay them down somewhere. 

Now days most of the lens are currently made of polycarbonate or some other plastic. This means that it is machinable. I took the right lens from an old pair (I use my right eye for a microscope), and after making a simple cutter, cut a disk out of the distance vision section. The lens was gripped in 4 jaw chuck and the lathe run about 700 rpm. 

This disk was glued into an machined adapter that is a slip friction fit on the eyepiece of the microscope. The adapter has a small lip for the disk to rest on, and the edges are rounded for comfort.

The cutter is just a short length of 12L14 steel that is bored out to the diameter of the disk. The outside is turned down so there is about 0.025 wall and teeth filed in with a small triangle shaped file. No precision required. 10 minutes work max.

The adapter is made of aluminum and has a wall thickness of 0.015 and a small slit was put in one side so it could be deformed a little bit to make it grip the eyepiece with a light friction fit.

The adapter has taken up permanent residence on my microscope, but can be slipped off if someone with normal vision is visiting the shop and needs to use it.  I am planning to do the same thing to the tool makers microscope that I am rebuilding so the cross hairs will be clearer.

Full field of view and no more fuzzy millipedes. I can easily make a measurement with it now.


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## 4156df (Aug 16, 2009)

Gail,
This is a great idea. Thanks for posting it.
Dennis


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## arnoldb (Aug 16, 2009)

Great idea Gail - Thanks! :bow:
I've always specified my glasses with hardened glass lenses; guess the next one will have to be "plastic" :big: - or I'll ask my optometrist to make a spare plastic lens!
Kind Regards, Arnold


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## CrewCab (Aug 16, 2009)

What a great idea Gail, and nicely done too 

CC


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## black85vette (Aug 16, 2009)

That is just too cool. But my question is; what on earth made you think to cut out a piece of lens ??? ???  I'm not sure I would ever come up with that.


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## Maryak (Aug 16, 2009)

Good one Gail :bow:

Best Regards
Bob


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## GailInNM (Aug 16, 2009)

Thanks for the kind comments everyone.

I forgot to mention that if you have an astigmatism correction in your Rx, it may help if you rotate the new eyepiece adapter relative to the scale in the position you normally use it in. I have very little astigmatism correction (0.25) and could not notice any improvement in doing so. The improvement you would see is that the horizontal and vertical lines may be a little out of focus relative to each other. The astigmatism correction will appear on your Rx as "Cyl" or "Cylinder" and if it is less than 20 percent of the spherical main correction factor you probably won't notice any difference. 

Arnold: I used to use nothing but tempered safety glass lens, but I would wear out the frames while the lens were still in good shape. By that time the frame styles would have changed and I could not get new frames for the old lens. Besides, about ten or so years ago glass lens were all but outlawed under New Mexico regulations and I had to go through a lot of hassle to get them. So I switched to polycarbonate lens with a hard coat surface treatment. I thought I would have scratching issues with it, but I have not had. I do take a little more care cleaning them. Actually use lens tissue instead of shirt tail.

Black85vette: When I first had the idea of adding a corrective lens, I started by looking online for a lens that would be suitable. Very few negative diopter lens are available, and I could not find anything suitable. Next I thought about making one out of acrylic (typical home shop machinist reaction). Even calculated out the radius of curvature needed for a plano-concave lens with the index of refraction for the acrylic. Figured I could just make it if I improvised a little bit with a temporary radius cutter on the lathe, but my natural lazy streak prevailed. It was a week or two later that I realized that I already had the correct lens in my "I paid good money for it so I am not going to throw it away even if I never need it" junk drawer, where I had an old pair of glasses. Just needed to shape the lens a little bit. An hour later I had my adapter. 

Gail in NM


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## Tin Falcon (Aug 16, 2009)

Gail would using a pair of inexpensive reading or "close up work" welding lenses or safety glasses work ? 
I know glass lenses are virtulay imposable to get any more. I think I was one of the last hold outs. I do not own any Old glasses of my perscription in plastic.the only poycarb pair are my good ones I am bout due for a new pair but will keep these for spares. 
They have been trying to push plastic on me for years. the lat pair I got before my current pair they did no want to give me glass. I told the tech I work in a fiberglass shop if I can not wash my lenses in acetone I do not want them. I got glass. The set I have now are polycarb fortunatly the Shop I deal with give me a good price otherwise I might have to deal with the cheapy dept store glasses LOL.


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## black85vette (Aug 16, 2009)

GailInNM  said:
			
		

> Even calculated out the radius of curvature needed for a plano-concave lens with the index of refraction for the acrylic.



Oh yeah, no wonder I didn't think of your approach. I simply forgot how to calculate the radius of curvature for a plano-concave lens.  :big:  I will just have to be humbly impressed since I have no clue.  ???

I gave up on glass a long time ago. My left eye is 20/400 and right is 20/380 so glass lens are pretty heavy and thick. Then I went to a smaller polycarb lens that was "high index" and they were the thinest and lightest I have ever had. More expensive but I love them. Now I don't look like I wear coke bottle bottoms.


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## GailInNM (Aug 16, 2009)

Tin,
Reading or close up glasses will not work. They are all a positive diopter focal length so you can focus closer. 

To see the reticule in the microscope you have to be able to focus at a distance. If one is nearsighted, then they can not focus at a long distance, and to correct this, a negative diopter is required. Diopter is a way of measuring the focal length of a lens. If you look through a negative diopter lens, then objects appear smaller, the opposite of the far more common magnifying lens.

A lot of the problem comes as we age, the focal range that the eye can cover diminishes as the eye and the associated muscles used to focus start to harden. In my case, my unaided focal range is from about 10 to 15 inches away from my nose. I have to put my glasses on to see if I still have all my toes in the morning. ;D

black85vette,
Yes, the polycarbonate lens are a lot lighter. The weight never bothered me with glass lens except they would try to fall off when looking down on a hot sweaty day. BTY, simple optics is one of the things I had to play with early in my working life so I have had a little exposure to it on a practical level as well as a couple of college courses a LONG time ago. 

Gail in NM


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## GailInNM (Aug 22, 2009)

Shortly after my last post I received an email from a shy member who asked how I would calculate and cut a lens from scratch. He thought that this would not be a good place for such a discussion. I thought that there might be someone else interested so am replying to his email here. I doubt that many people will be interested, but it's nice to know that we can make unusual things if necessary.

Before calculating anything, we need to know two things. First the power of the lens required, and that can be gotten from your current Rx for your glasses. Second we need to know the index of refraction of the material that the lens will be made from. Most plastics are going to be around 1.5 to 1.6. Acrylic is nominally 1.5, with some small variation between manufacturing process. Polycarbonate is about 1.6 or a little less. For this discussion I am going to assume acrylic as it is very common.

Next we need to calculate the radius of curvature for our lens. We can start out with the lens makers equation, but it is a little more complicated than necessary for our purpose. If you are interested You can look up the complete equation at 
http://en.wikipedia.org/wiki/Lens_(optics)

The first simplification will be to assume that our lens is going to be thin compared to the radius of curvature. This is valid if the thickness of the lens is less than about 5 percent of the radius.  Then we can discard about half of the complete equation as it will be refining the calculations to a closer limit that we will be working anyway. The thin lens version is also illustrated in the above reference.

Next, by making one side of the lens flat, one more variable can be thrown out and we are left with a very simple equation that would normally be expressed as  1/F = (N-1)/R  where F is the focal length of the lens, N is the index of refraction and R is the radius of curvature. Now F is equal to the 1/Diopter (in meters) as expressed on the RX, so by juggling the terms around we are left with R = (N-1)/Diopter 

Using acrylic, N = 1.5 so R= (1.5 -1)/ Diopter or R= 0.5/Diopter (in meters).

For illustration, lets assume that your Rx calls for -3.0 diopter. Then the radius of curvature (R) = 0.5/3.0 or 0.16 meters or 6-1/2 inches for us imperial measurement types. 

If you have a radius cutter set up on you lathe that will handle this large a concave radius you are home free. I don't, so I had to improvise a bit. For my pivot I used a wiggler with a pointed tip that I use for milling machine set up. For my cutting tool I used a piece of3/32 music wire. I cut away half of it for a flat top tool at the tip and added lots of side and and front relief and then stoned a more or less to a half round shape. I did the cutting with a file while the wire was held in a vice. I cut it off so it was about an inch long.

I drilled one end of a 1/4 diameter alum rod about 1/2 inch deep to accept the major diameter of the wiggler point and then continued the hole with a drill a little smaller so the point would jam in the hole. I cut the rod off so from the center of the wiggler ball to the end was 6 inches. This left 1/2 inch for the cutting tool to stick out at the far end.

I drilled a 3/32 hole in the end of a 1/4 alum rod about 1/2 inch deep and inserted the cutting tool in with stud mount Loctite, leaving 1/2 inch sticking out so from the ball center to the cutting edge is our desired radius.

With a piece of acrylic rod chucked in the headstock the wiggler was chucked int eh tail stock. The rod should be greater than 2 times the diameter of your finished lens. After facing off the acrylic, I put a flat topped tool in the tool holder and adjusted it close to the acrylic and set the height so the radius cutting tool would be on center when resting on the flat topped tool. It is just acting like a steady rest. The flat top of the cutting tool is kept facing up with finger pressure. 

With the lathe running at about 500 rpm I advanced the tailstock and slid the cutting tool along the steady rest from the center of the acrylic to the outer edge to cut the radius. Advance and repeat in small increments. You won't be cutting more than about 0.01 inch deep at the center. Get as smooth a cut as possible. 

After getting the tools out of the way, I polished the cut with 1000 grit abrasive paper and water. Then I polished with two fine grades of plastic polish. The end should be perfectly clear. Cut off the end so it is about 3/32 inch thick. Use low RPM and a slow feed. If you melt the plastic any it will be hard to polish out the cutoff end. Clip or file off the cut off tip and then polish the end the same way you did the other side. At this point you can look through the lens to see if it gives you a correction close to what your eyeglasses do. You will probably notice that the center of the lens gives less correction than the outer part as it will not have as much cut away when polishing due to the slow speed in the center of the lens. Thats why we made the lens so big. Now cut a section out of the good section of the finished lens the same way as I did when cutting out a section of the eyeglass lens. 

For reference, it took be about an hour to type this up, but only about 45 minutes to actually make the part and take photos. 

Gail in NM


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## ttrikalin (Jul 5, 2012)

nice topic, and thanks for the tip. 
really nice.


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