Trial lens kit for before and after cataract surgery
Posted , 6 users are following.
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.
2 likes, 58 replies
trilemma
Edited
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)
phil09 trilemma
Posted
Yes, it's not exactly a barrel of monkeys, but I am having some fun. Thanks for posting the suggestion!
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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.
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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.
RonAKA phil09
Posted
The angle is critical for correcting cylinder (astigmatism). I recall that an error of 5 degrees reduces the correction effect by 30%.
trilemma phil09
Posted
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.
jimluck trilemma
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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.)
RonAKA jimluck
Posted
The accuracy of an astigmatism correction also depends on how well centered the lens is on the pupil of the eye. This is pretty much assured with a toric IOL, but not so much with an eyeglass lens. That will be determined by the adjustment of the glasses height, and nose to pupil distance. This is because the correction has a polar orientation around the center of the lens.
trilemma RonAKA
Posted
Polar, as in a polar projection map? The prime meridian is arbitrary. Up is not.
RonAKA trilemma
Posted
I mean polar in that the whole surface of the cornea extends for 360 degrees around the clock, and there are 4 quadrants of increased and decreased power that have to be compensated for with an astigmatism correction lens. It is not a uniform correction applied to the whole lens.
trilemma RonAKA
Posted
Are you talking about a real IOL that is custom for high-order aberrations?
The thing that continues to sound weird to me is "quadrants" if we define a quadrant having one side on the vertical axis and the other edge horizontal.
RonAKA trilemma
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I am referring to a quadrant like a pie cut into four pieces. With a standard lens of an eyeglass or IOL that corrects astigmatism, the effective power changes as you move from quadrant to quadrant. Adjacent quadrants will differ by the cylinder diopter value. Quadrants opposite each other will be the same. And the power does not change instantly like with a bifocal lens, it changes smoothly from quadrant to quadrant. The Axis degrees determines how the quadrants are oriented.
trilemma RonAKA
Edited
So if I draw my quadrant cuts rotated 40 degrees from up and down and horizontal, are they still quadrants to you? Or are up and down and horizontal special?
My proposed lines would still be at right angles to each other.
RonAKA trilemma
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Quadrants are just 90 degree sections of a full 360 degree circle. If you wait to see the images I posted that are in moderation it may become clearer.
trilemma RonAKA
Edited
clarifying my question:
Does quadrant imply to you that one edge of a quadrant is vertical and the other horizontal?
Edit after your post came out of moderation --second question:
With the oblique example, where do you draw the quadrant lines?
I would think one would rotate the lines to maximize having the same color inside of each 90-degree wedge.
RonAKA trilemma
Edited
The 90 degree wedges stay intact and the axis is just a measure of their position on the 360 polar chart. Since the wedges are a mirror image the convention is to use angles for the axis between 0 and 180 degrees. If the edges of the wedges are horizontal or vertical, it is just a coincidence. The high slope wedges are generally vertical when the astigmatism is with the rule (hourglass shape), and horizontal when against the rule (bow tie).
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In correcting for astigmatism the assumption is made that the steeper and flatter wedges of the 4 quadrants are perfectly symmetrical. That is how only one angle can be used to specify the orientation of the wedges. If you look at that graphic in my other post, that is not always the case in real life, and the angle chosen is based on your trial and error selection. The only way I know of to correct irregular astigmatism is with custom or topography guided lasik.
trilemma RonAKA
Posted
Is that a "no"?
RonAKA trilemma
Posted
"Does quadrant imply to you that one edge of a quadrant is vertical and the other horizontal?"
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Yes in the case of perfectly oblique astigmatism. All the other possible cases, no.
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"With the oblique example, where do you draw the quadrant lines?
I would think one would rotate the lines to maximize having the same color inside of each 90-degree wedge."
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You would rotate the whole wedge so the edges would be horizontal and vertical in this specific case.