Clarification for my own education

thanks ron. good info

but that makes no sense to me. if one has a long eyeball and far vision is falling short of the retina due to myopia, u want light dispersed, not focused. unless the light from each crosses before the iol. please explain..thx

You might want the light dispersed further, but in myopia that is not what happens. Myopia is a defect in vision, and in your example it is caused by the long eyeball. The lens of the eye remains biconvex regardless.

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To correct myopia, we use eyeglass lenses that are concave, not convex. I imagine something similar is done with intraocular lenses, involving a lesser degree of convexity than would be appropriate for eyeballs of shorter length.

Yes, IOLs are optically convex down to 0.0 D power, and then become concave as the power goes negative below zero. These are only used in extreme cases of myopia. And, I say optically because the front and back surfaces can be concave or convex but the sum of them is what counts. In very low powers the lens switches to a meniscus design which can be concave on front and back.

so when someone is -6d myopia from too long axial length, the concave eyeglass lens is considered a negative power to correct for distant vision? is this correct?

and instead of saying the cornea and iol combined are "too powerful" for the vision structure, would it not be more accurate to say they are just right for a normal axial length eye?

thx

You have to keep in mind that eyeglass or contact lens refraction numbers are on a different scale than IOL powers. That is because with a normal length eye that requires no eyeglass or contact correction the required IOL power to maintain that balance is about 18-19 D in power. Myopic eyes with the cornea and lens combined are too powerful to focus on the retina without correction. They focus short of the retina (short sighted). For that reason a power of less than 18 D is used for the IOL to correct this short focus error. An approximate estimate of what IOL power is needed can be made by multiplying your eye refraction times 1.25 and adding 18.0 to the result. For a -6.0 eye this would be:

(-6.0x1.25) + 18.0 = 10.5 D

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The IOL powers do not start to become negative until the myopia is much worse than that. Your eye refraction would have to be up into the -14 to -15 range before you get to the point of needing a negative power IOL.

what is the reason for multiplying the refraction times 1.25? is that spherical equivalent? if so, isnt spherical equivalent sphere plus 50% of cylinder?

and back to the eyeglass questions. i understand the difference in scale between glasses/contacts and iol, but lets say i still have my natural lens with the requisite +18-19 D in power but due to axial length, im still short sited. is my -6D concave eyeglass lens REDUCING the power to get me down near the +10.5 or so i need to reach plano? just trying to understand. thx

It has nothing to do with spherical equivalent. The reason is due to the difference between the distance from the retina to the lens, and the retina to the cornea. The lens is closer to the retina than the cornea. It takes a higher power effect at the lens plane to have the same effect as a lower power at the cornea. Standard eyeglass refraction is based on the cornea plane. So if you are 6 D off at the cornea you need 1.25 x 6.0 D at the lens plane. IOLs powers are measured at the lens plane. That is why you don't get a 0.5 D step in reference refraction with a 0.5 D step in the lens power. It is more like a 0.375 D step at the cornea plane.

A -6.0 D eyeglass lens actually opens up the convergence of light due to the combination of the cornea and the natural lens bending the light too much.

ok, that makes sense.