Trial lens kit for before and after cataract surgery

" I measured my brand new LAL reading eye at -2.00 sphere, 0.50 cylinder (positive? negative? I don't know)"

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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.

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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.

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Does that sound right? Thanks for the tip!

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.

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."

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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).

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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.

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.

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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.

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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.

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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.

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A toric IOL, or eyeglass with cylinder is much less effective in correcting irregular astigmatism.

as you know, Lasik could address those high-order aberrations.

Toric IOLs or eyeglasses can do the same correction that you get when testing with a phoropter.

Regarding quadrant, I expect that the topography image of one eye vs anther could look similar if the images were rotated. I am saying that the 90 and 180 degree lines are not significant in describing the condition of the eye, other than to show the axis of the correction. As convoluted as that sentence was, I don't think that quadrants are significant.

The significance of quadrants is that the power of the lens changes based on polar angular position. This is not a change in power vs radius of the lens, but polar angular position. The assumption made by astigmatism corrections is that the pattern is regular in four quadrants. The steep slope of the cornea and flatter slope are assumed to be 90 degrees apart. That is why there is only one axis angle in the prescription. The rest of the quadrant correction amounts are assumed to be in 90 degree intervals around the eye.

"Simulating?" Trial lenses are the real deal, not simulations. They are glasses lenses without glasses frames.

Then, why do I not see nearly the same degree of improvement at the very far target that I see at the moderately far target?

Accurate astigmatism correction should improve vision at all distances. However if sphere is off it may be difficult to tell the difference as vision will be poor due to the incorrect sphere. I have noticed from my optometrist refraction tests that I tend to trade off sphere for cylinder. If I pick more sphere then I pick less cylinder, and vice versa.

"Accurate astigmatism correction should improve vision at all distances."

That's what I thought, but I believe trilemma is right about this not being true for near distances in presbyopes (like us). The concavity of the cylinder lens produces a hyperopic effect that outweighs the corrective effect at near distances. This does not seem to happen with a convex cylinder lens, which would make sense because a convex lens would have a generally myopic effect, even in cylinder format I suppose, which would tend to add to the improvement produced by the corrective effect rather than canceling it out.

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"However if sphere is off it may be difficult to tell the difference as vision will be poor due to the incorrect sphere."

Even after correcting sphere, the astigmatism correction effect is much less striking at 10 feet than it is at 2.5 feet. I don't understand why. Could be some sloppiness in my amateur testing - e.g., maybe the lighting on my paper 10' Snellen chart is inferior to the lighting of my computer screen.

I found this interesting article on the basics of doing a phoropter refraction. One point that surprised me is that when they add say 0.5 D of cylinder they reduce the sphere by 0.25 D to keep the spherical equivalent the same. If you did not do that it may skew the results.

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American Academy of Ophthalmology JUN 12, 2019

Refraction 101: Go Forth and Refract

By Evan Silverstein, MD

Then, why do I not see nearly the same degree of improvement at the very far target that I see at the moderately far target?

What is your test setup? Are you looking thru your prescription glasses in addition to the test lens?

How about stacking a minus sph with the minus cyl lens.

The cheap test lens kits are not made to the finest standards. So stacking lenses will degrade things.

My kit includes a plano lens, and that degrades vision very slightly. The kit is still very useful.

That's what I thought, but I believe trilemma is right about this not being true for near distances in presbyopes (like us).

I did not intend to say anything like that.

I think the fact that you see farther with negative cyl lenses is because your effective sph number is moving more negative at the same time that you are compensating for the cyl.

Compensating for the cyl with a positive lens makes your effective sph more positive, and thus you see better closer.

That was a good hint. I think this is related to the different prescription notations of optometrists vs ophthalmologists. And apparently that is not just a notation difference, but a difference in the phoropters themselves - some of them are manufactured with positive cylinder lenses and others are manufactured with negative cylinder lenses (that was news to me). It might have been less confusing if my trial lens kit contained only negative cylinder lenses, instead of including both concave and convex forms.

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In any event, I think I can refine my home refraction now and get a decent preview of my prescription before my upcoming LAL adjustment. Thanks

I have no prescription glasses, I just stack a minus cylinder lens on top of the appropriate sphere lens.

Have not really thought it through in detail, but I wonder if these cylinder lenses have some positive or negative sphere built into them to keep the spherical equivalent at zero when added to a pure sphere lens. This would eliminate the need to adjust the sphere each time you switch the cylinder lens. I also wonder if this is the reason there are positive and negative cylinder lenses. One is used in combination with positive sphere lenses, while the other is to be used in combination with negative sphere lenses.

That appears not to be the case. For one thing, a negative cylinder lens forces me to move my phone farther from my eye to read clearly. For another, I am able to use the lenses to replicate the conversion from optometrist to ophthalmologist format - for example, -1.75 sphere/-1.00 cylinder (using a negative cylinder lens) gives me the same vision as -2.75 sphere/+1.00 cylinder using a positive cylinder lens.

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With some helpful input here, my amateur refraction after one light adjustment in my new LAL eye is now: -1.75 sphere/-1.25 cylinder. This refraction is a combination of three lenses in my cheap lens kit, and it gives me 20/20 vision at 10 feet. It takes some time and repetition to get the lenses right, because my visual acuity on the Snellen chart fluctuates - sometimes it will look very sharp when I put on a lens and then quickly fade to a fuzzier view, sometimes the reverse. Is that normal?

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Spherical equivalent is -2.375 D based on my self-refraction. This would explain why I have been so pleased with my reading vision. But it is disturbing to see so much astigmatism remaining - actually, the -1.25 D I am measuring now is slightly greater than the -1.00 D the optometrist measured last week! I am certain she told me she was targeting correction of the -1.00 D to 0.00 D cylinder with last week's treatment, so either:

• she didn't do what she said, or
• the computer badly missed the target, or
• maybe a secondary astigmatism is rearing its head, now that the primary astigmatism has been treated?

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  So, I think I have significant astigmatism remaining, and I hope that it is correctible by the LAL and that I have enough adjustment diopters left. I could live with the result I have now - I think it is pretty fair monovision - but would certainly like to improve. If my refraction is correct, I think I would want to target a final outcome of -2.00 D sphere/0.00 D cylinder, which was my expected preference at the outset. I used the trial lenses to correct my eye to that result just now, and it gives me a small improvement in intermediate and distance vision, with little or no degradation of near vision (I do need to read from a little farther away, however). I would be less happy with a spherical equivalent any smaller than -2.00 D.

It is hard to guess what is going on. Higher order astigmatism will not be corrected with basic cylinder lenses though. It would seem that you will be prepared to make a decision when you get the phoropter results before your next adjustment. Reducing the cylinder to zero while still having good near vision seems like a good outcome. Reducing cylinder can reduce near vision. My best indicator of reduced cylinder error is how clean white letters on a black background appear. With uncorrected astigmatism, especially in the one eye that has irregular astigmatism results in a drop shadow on the white letters. I can still read them, but the drop shadow slows me down.

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With your vision not being steady during testing, you may want to try using dry eye drops 30 minutes or so before testing. You need a big of time for the blur from the drops to go away, but after that vision may be more consistent.

"Higher order astigmatism will not be corrected with basic cylinder lenses though."

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That is certainly true of a basic cylinder lens. But with an adjustable lens, a cylinder could be added at one axis, and in a subsequent light treatment, a second cylinder could be added at a different axis. I don't know whether the current LAL and software do this or not.

Yes, that might work.

My docs say two cyl lenses at diff axes can be replaced by one cyl lens at one axis . just have to calculate the resultant of the two to determine power and axis of the single lens that is equivalent to the combination . I don't find that to be true, but both an ophthalmologist and an optometrist told me it is

"My docs say two cyl lenses at diff axes can be replaced by one cyl lens at one axis."

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That sounds reasonable to me, and presumably that's what you must do in order to use a basic cylinder lens. But I would expect better results if you are able to adjust both cylinders individually, which I imagine might be done using an adjustable IOL or wavefront LASIK.

My docs say two cyl lenses at diff axes can be replaced by one cyl lens at one axis . just have to calculate the resultant of the two to determine power and axis of the single lens that is equivalent to the combination .

I suspec 2 cyl lenses in series at different axis might be able to behave as a single cyl lens. The test set has more granularity in choice, and if your combo results in a net -1.375, a -1.375lens could be ordered for eyeglasses. And that could be combined with sph too, of course.

I wonder what the effect of two -1D cyl lenses in series but 90 degrees off on the axis would be. I suspect they would cancel, but I certainly am not sure.

A +1 cyl and a -1 cyl lens in series with the same axis should cancel... neglecting the imperfect optics.

My docs say two cyl lenses at diff axes can be replaced by one cyl lens at one axis . just have to calculate the resultant of the two to determine power and axis of the single lens that is equivalent to the combination .

I had failed to mark the above as a quote by jimluck

From my fading knowledge of advanced math, I recall that when you add and subtract sine waves of a single frequency the outcome is still a sine wave of the same frequency. So, it would make sense that you could combine two different sign waves of the same frequency and the result would be a single sine wave. This is the case as I understand the technology you can only program in a four quadrant correction (two peaks in slope and two flatter points 90 degrees apart). And to correct a higher order aberration you would need a higher peak and valley correction, like 8 times instead of four.

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But I think it is important to remember that the LAL is a trial and error exercise. Whatever you get you get. Then you retest with a phoropter and come up with the best compromise correction using a four quadrant correction. You will guide the correction by what you see in the phoropter test. The real limit may come in when there is no correction in cylinder that improves things.

"I wonder what the effect of two -1D cyl lenses in series but 90 degrees off on the axis would be. I suspect they would cancel, but I certainly am not sure."

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No, those two lenses both add negative spherical equivalent. They can be canceled out by a single +1.00 D sphere lens.

I used to know some math... It sounds right that a higher-order astigmatism would not be correctible with a simple cylinder lens. I was thinking of multiple first-order astigmatisms, e.g., one with the rule and one oblique. I had a corneal wavefront scan that indicated that result. Would the answer be the same?

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Yes, trial and error iteration. I'll be very satisfied if the LAL can correct me as well as the home lens kit.

Unless they can program in the results from a topography scan like the Pentacam I don't think they can apply anything other than a four quadrant correction. From discussion in another thread I don't believe the LAL has any capability at the present time to do that. From what I understand they can only program in a cylinder and axis, like an eyeglass correction.