There are test kits available that have a bunch of glass lenses. You can hold one in front of an eye, or you can use a “trial lens frame” that may even be available in the kit.
The best place to see and get those is via ebay, but you can buy the same things elsewhere at a premium.
In my use, I made a target by printing a “360 degree protractor” that has radial lines converging to the center. Those are widely available for printing free. Tape that to a suitable distance of maybe 20 ft, or 12 ft… whatever works for you, but farther is better if you can still see the target well enough.
While I bought a trial lens frame and one came with my kit, I most often just held the lens in front of an eye. In my case, it was cyl that interested me most, and was my axis very repeatable day to day.
I bought a much bigger kit than needed, because it had 1/8 D lenses for each of SPH and CYL. And for each of those it has positive and negative. 0.125 D is very subtle.
Right now a -0.5D test lens by itself sees best for me, That would be equiv to -0.25D SPH +0.25D CYL.
The 68 piece kits would be more than enough for most, but the 104 piece kits are not that much more, and some of those include a trial frame.
So is this a toy that solves curiosity? Is this a good sanity check? Is this a needless expense? Some pay that for a meal out.
I find the method advocated by Dr. Graham Barrett the author of the Barrett IOL power calculation formula to be very good. It is basically what I followed. The key points are:
.
1. Do the distance eye first
2. He advocates use a trial lens/frame at -1.25 D in this eye to simulate what IOL vision of that myopia will be like. He claims that 50% go for monovision, and 50% do not, and instead opt for distance vision in both eyes.
.
My first eye turned out to be 0.0 D sphere, and -0.50 D cylinder for a spherical equivalent of -0.25 D. Instead of using trial lenses I just bought a few pairs of dollar store readers. In my testing I found a combined -1.25 D was not good enough for my reading needs and that +1.25 D readers plus my -0.25 D myopia for a combined -1.50 D was best. So that is what I targeted. This method using spherical equivalent may not be the best however if significant astigmatism is involved though.
.
Interestingly I still have the +1.25 D readers and those are the ones I reach for in difficult reading situations of small print or dim light. They give me around -1.50 D in my distance eye and -2.75 or so in my near eye. I can read very well and manage near vision tasks with the glasses. I originally thought I would need to get some custom prescription readers to balance the eyes, but have found that the dollar store readers work just fine. I also have prescription progressive which correct sphere and cylinder in both eyes for distance with a +2.5 D add. But, I virtually never use them.
.
Google this for the Barrett article.
.
CRSTG CATARACT SURGERY | OCT 2009
My Standpoint on Monovision as a Cataract Surgeon
The success of monovision depends on the level of targeted myopia for near vision.
Graham D. Barrett, MD, FRACO
Wouldn't it be less expensive to just visit an optometrist? It would be cool to own a trial frame and a kit full of lenses but I don't imagine they are cheap?
Costs similar to 2 refractions. The advantage is that you can play around and check over time.
I would not do this instead of a refraction, although much of the world does use trial lenses rather than a "which is better, one or two" machine (phoropter).
I expect their opticians tend to use a higher grade of test frames, and maybe lenses.
I did get an extra test frame with more adjustability . Not comfortable. My test set came with a plastic test frame with a fixed pupil distance of 62 mm.
"So is this a toy that solves curiosity? Is this a good sanity check? Is this a needless expense?"
.
I'm gonna say "toy". But I like toys and I have a birthday coming up, so I got one. Looks like you can get them for around a hundred bucks, plus thirty for a decent frame if you want one.
.
Knowing next to nothing and just fooling around with the lenses for a few minutes, I measured my brand new LAL reading eye at -2.00 sphere, 0.50 cylinder (positive? negative? I don't know), which is somewhat off the target of -1.50, 0.00, suggesting potential for improvement. If my measurement turns out to be accurate, I would ask the doctor to use my second light adjustment to target maybe -1.75/0.00 next week, hoping to improve far vision while maintaining my near vision. I don't really expect my self-refraction to be very precise, but it was fun to try it and start thinking about the possibilities for my next lens adjustment.
.
I'm not sure what else I would do with the lenses. Maybe I could use them to check my remaining natural eye from time to time - it may well develop a cataract and cataract-induced myopia as my LAL eye did over the past few years, and home refractions may help me know when it's time for eyeglasses and/or surgery for the other eye. What else could you use them for?
Glad it is at least fun. And it may help things. And your observations and results will be interesting reading.
The astigmatism part is the most interesting I think. Try comparing the axis over time if you are so inclined. Using the dial on the test frame would make the measurement more objective vs holding the lens.
With astigmatism, a minus lens also makes the effective focus farther . You know the conversion from one type of prescription to the other.
Also note that the measured axis should be 90 degrees different when you use positive and negative cyl.
Try finding a 360 degree protractor to print where many radial lines meet at the middle (ideal, but I did not find an ideal one). So the next best is one that has a smallish circle in the middle. Tape that to something as close to 20 ft away as practical, Let us know what you think the best target is.
Just thinking. Maybe printing the same target-- but zoomed out-- could be interesting. Perhaps the big protractor target on one part of a sheet would allow room for a shrunken protractor
Different levels of light will affect things. I think that minimum light should give the most sensitive results (more affected by small differences in the lenses)
" I measured my brand new LAL reading eye at -2.00 sphere, 0.50 cylinder (positive? negative? I don't know)"
.
It makes a difference. If it is positive then it converts to -1.50 D sphere and -0.50 D cylinder or a spherical equivalent of -1.75 D. If it is negative then it converts to a spherical equivalent of -2.25 D.
Well, the lenses that work best so far for my LAL eye are both concave, sphere and cylinder. That means negative cylinder, so my spherical equivalent would be -2.25 D. I have low confidence in my self-measurements, but that number seems consistent with my highly satisfactory near vision right now.
.
I think you're suggesting that moving from -2.25 D to -1.75 D would not maintain my current near vision, but instead would noticeably degrade it. So, maybe a better second round target for me would be -2.00 D sphere, 0.00 D cylinder - perhaps producing a slight improvement in distance vision, and no perceptible loss of near vision.
.
Does that sound right? Thanks for the tip!
Yes, it's not exactly a barrel of monkeys, but I am having some fun. Thanks for posting the suggestion!
.
I have not yet gotten around to the radial lines thing, but will look into it when time permits. For now, I've been using a 10-foot Snellen chart, my computer screen, and a wall clock. I am finding that the Snellen chart works best for differentiating sphere, while my computer display at about 31" distance works much better for differentiating cylinder - even after distance correction.
.
I've always figured that while axis is crucial for making a lens, it is totally irrelevant for my layman's purposes, like understanding my current vision and potential correction results. Am I missing something? Also, why would it move, especially with an artificial lens in my eye? Change in the shape of the cornea? I would have thought cornea shape is very stable in adults.
I don't know how the kit works for measuring cylinder. Is there just a selection of cylinder powers and you just do trial and error with them to see which works best and then rotate them to get the different axial positions? The positive and negative cylinder is a just a convention. Your cornea will have an angle where the slope is steepest, and another one 90 degrees away where it is flattest. It is where you start counting degrees from that is the difference.
The angle is critical for correcting cylinder (astigmatism). I recall that an error of 5 degrees reduces the correction effect by 30%.
I've always figured that while axis is crucial for making a lens, it is totally irrelevant for my layman's purposes, like understanding my current vision and potential correction results. Am I missing something? Also, why would it move, especially with an artificial lens in my eye? Change in the shape of the cornea? I would have thought cornea shape is very stable in adults.
When I tried making note with my trials, I thought my axis numbers were not as stable as I had hoped. Measurement noise, or eye drift? I think I was doing that before getting locked in, and likely before my first adjustment.
If you started logging it, the results would be interesting. Try to make this a blind test, to the best of your ability.
I don't know how the kit works for measuring cylinder. Is there just a selection of cylinder powers and you just do trial and error with them to see which works best and then rotate them to get the different axial positions?
You get both positive and negative cyl lenses. Yes, you rotate them. If you use the test frames, they have a little knob that will do the rotation.
The positive and negative cylinder is a just a convention.
Not really. Positive cyl lenses are convex, and negative are concave. So if you are wanting to use a single lens, the difference is significant. Stacking lenses degrades things a tad.
Yes, that confirms I have no idea how this test kit works. It is true that positive SPHERE lenses are convex, and negative SPHERE lenses are concave. However, cylinder is more complex. It is when the power of the lens is a function of the angular position in the lens. The power varies based on the quadrant of the lens. In an eyeglass lens this angular effect is hard baked into the lens based on the specified axis angle of the cylinder. In an IOL, the power only of the cylinder effect is baked in, but the angular position is not. The surgeon simply rotates the lens into the required angular position in the eye.
"It is true that positive SPHERE lenses are convex, and negative SPHERE lenses are concave."
.
It appears the same is true of cylinder lenses. I see this explained on optical websites, and I see the lenses in my kit are labeled convex/positive power and concave/negative power. Trying again today, I have updated my astigmatism estimate to -0.75 D. The +0.75 lens and the -0.75 lens are clearly different, no matter how you flip or rotate them. Only one of the two improves my vision, and only when rotated to a specific axis (or close to that axis).
.
The -0.75 D cylinder lens, properly rotated, greatly sharpens my intermediate distance vision, although the improvement is small at far and near distances. I wonder why that happens. Regardless, I hope that the optometrist and the LAL can make the adjustment and improve my results next week.
The -0.75 D cylinder lens, properly rotated, greatly sharpens my intermediate distance vision, although the improvement is small at far and near distances.
That is because you have a moderately far target.
If you were to pick a target 10 inches away, the convex (positive) lens would be better. But the axis marks would be at 90 degrees to the best focus for the negative (far) lens.
I buy into your first 3 sentences, but not many of the rest. I especially do not have a clue regarding quadrant. But yes, getting the axis right is a big deal.
If I look at a -10D concave lens, it is flat on one side, and the other side has a smooth valley down the middle. I suspect the thickness at the bottom of the valley is the same from rim to rim.
A +10D convex lens looks like it has a smooth hill . I suspect the thickness at the top of the hill is the same from rim to rim.
The axis marks correspond to the top of the hill and the bottom of the vally.
If you used a micrometer, the thickness at the middle would be much smaller for a concave cyl lens then for a convex cyl lens.
This is the first time I did anything with my 10D lenses.
Love my trial lens set. It helps me to become a better-informed patient.
Yes, the negative cyl lenses are quite, quite different from the positive cyl lenses. It is very confusing that there is also that difference in how cyl can be written on a prescription that uses positive in one convention and negative in another and they are equivalent. I don't know how the industry keeps that straight -- when does sign mean the difference between concave and convex cyl and when is it just a notation convention?
Here is an article that explains the optical difference in the two https://www.clzoptics.com/news/difference-between-positive-and-negative-cylindrical-lenses.html . This article also has an interesting but inscrutable paragraph about combining a positive cyl lens with a negative cyl lens in order to get zero cyl on one meridian and it says this is done in ophthalmology. That's interesting to me because I have very high cyl in my right lens (-8.25) and I still don't get perfect sharpness. Using my trial lens set I found a +2 cyl lens held in front of my right lens gives me greater sharpness than any glasses ever prescribed for me, but I have to get the axis just right. Taking this info to my docs (two optometrists and one ophthalmologist) , most of them say that combination just boils down to a different negative cyl lens at a different axis. One doc even did the math and made up a trial frame with what was supposed to be the mathematical equivalent to my two-lens combination. But it wasn't at all equivalent in the vision it gave me. One doc said "you're correcting a higher-order aberration and no lab will make that in a single lens. So your experiment is interesting but of no practical value." So I don't know who to believe.
I used my trial lens set to prescribe glasses for my wife. We ordered them from Zenni . She just needed a little bit of negative sphere to sharpen up her distance vision.
Using trial lenses I discovered that under correction of -0.75 sphere gives me wonderful versatility -- computer plus far plus a useful amount of near. The comfort of my livingroom, working alone, with no time pressure makes it far easier to decide what I like. I ordered this prescription and these are the glasses I now wear 99.9% of the time. No doc ever suggested this. I had to figure it out on my own with trial lenses. I was very unhappy with my vision until I discovered this. Their answer was get cataract surgery. Now I can put that off and give science more time to come up with better IOLs. I'm a DIY type person in many realms, so this is just another one where I'm happier doing it at least partially myself. (I won't be getting my own scalpels to do my own cataract surgery, however.)
My understanding of cylinder is quite different. I won't say it is right as I have zero training in optometry. I am an engineer and have reverse engineered my understanding of the field. Let me try and to see if I can explain it. Say for example you are -4.0 D Sphere, or medium myopic. For sure your eyeglass lens will be concave to correct that. And now lets say you have 1.0 D of astigmatism or cylinder expressed as a plus value. My understanding is that this is effectively a "wave" of increased power that various around the angular position of the lens, like different positions on a clock. The convention is to use degrees. So lets say the cylinder effect required is maximum at 0 degrees. At that position once you overlay the +1.0 D cylinder on top of the sphere, the power of the lens is reduced to -3.0 D at 0 degrees Then as you go to 90 degrees, the effect goes to zero, and the net power goes back up to -4.0 D again. At 180 degrees the net power goes to -3.0 D again, and at 270 degrees is goes to -4.0 D, then as you go fully around the clock back to 0 degrees the power is of course -3.0 D. The effect of cylinder is like a wave as you go around the clock. The thickness of the lens will change thicker and thinner depending on the angular position to get the wave effect in power. The trick in getting a cylinder correction is to get the correct power amplitude of the wave (cylinder D), and having the wave start at the correct angular position (axis) to reverse the error in the eye. That is why a prescription which includes Sphere D, Cylinder D, and Axis.
.
And I think this example explains the arbitrary convention difference between the optometrist negative cylinder vs the ophthalmologist positive cylinder. The optometrist would describe the lens in my example as -3.0 D sphere and -1.0 D cylinder, to give the wave in power between -3.0 D and -4.0 D. The ophthalmologist would consider the base sphere power to be -4.0 D Sphere, and the Cylinder +1.0 D. As you go around the clock you get the same wave as you add the 0 to +1.0 D to get the wave in power from 3.0 D to 4.0 D. And axis starting points would be 90 degrees different.
.
That said, I have no idea how a lens test kit works for simulating a cylinder correction. The bottom like is that it would have to create a wave effect in power based on angular position.
When I refer to the quadrant I am talking about a 1/4 section of a polar graph. That is how your eye is looked at. See the image below of the topography of an eye that has regular astigmatism. The colours represent the different slope (power) of the cornea. You can see how the power changes in a wave shape as you go around the angular position of the polar graph. That is what you are trying to correct with the cylinder in a lens.
.
.
And for reference here is irregular astigmatism. The change in power does not go in a uniform wave pattern as you go from quadrant to quadrant. A toric IOL, or eyeglass with cylinder is much less effective in correcting irregular astigmatism.
.
