How do diopters determine broadness of range of vision when choosing a target?
Posted , 5 users are following.
I'm not exactly sure the right way to ask this question, but when i look at a chart of refraction, showing the various -.25 (D) and -.5 (D), etc and the corresponding focal range 4m, 2m, etc it seems the range between each step gets increasingly smaller, such that if i target -1.75 or 57cm (22"), the defocus curve shifts accordingly and the range of 20/20 vision is smaller than compared to only shifting by, say -.25, right?
so it seems the more myopic the target, the smaller the range of useful vision will result (given a monofocal lens). such that choosing a reading distance target of say -2.75 would only give me a range of inches on either side of the target before dropping off to 20/40 vision. is this right?
if so, it seems much more functional to choose a minimal undercorrection resulting in several feet of range on either side of the target (understanding there is no benefit beyond plano) and then achieving closer vision with glasses/contact lenses.
am i understanding this correctly?
0 likes, 13 replies
RonAKA jay3210
Posted
The issue with the defocus curve is that the 0.0 D position translates to a distance of infinity, and as you point out the difference between a single diopter on the right side is just inches. So if your objective is to see the widest range of distances it is best to get a distance set IOL. But, the problem is that you may want to read your iPhone, or a book, as well as see far distances sharp. The natural lens can do that as the lens accommodates kind of like an autofocus SLR camera. The IOL cannot do that, with the exception of a few that are trying, but are not really succeeding.
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I think the best way to look at the defocus curve is to consider that the whole curve shifts as you change the target to the right. If you look at the Alcon monofocal in the IOL Review: 2021 Newcomers article, you will see that the curve intersects the 0.2 LogMAR (20/32) in two places. On the left it intersects at +1 D, and on the right at -1.5 D. If you can accept 20/32 for distance vision, you can slide the whole curve 1 D to the right. That lets you see 20/32 at -2.5 D or . That is a pretty good range from infinity down to 16". And if you only make the shift with one eye, you retain 20/20 at infinity. And many push it more by leaving one eye at 20/20 and pushing the curve 1.5 D to the right. The distance vision (in one eye) degrades down further than 20/32, but you can now see reasonably well down to 12".
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So, yes your thinking is correct if you only consider one eye. To get a wide range of high quality vision you need to push only one eye to the right.
jay3210 RonAKA
Posted
thank you!
greg59 jay3210
Posted
Ron gives a good explanation. Most defocus curves are averages of lots of people in a study - so your mileage may vary. Those included in studies typically have less than 1 diopter of corneal astigmatism. If you are left with -0.75D or more of astigmatism, macular edemas, or other bad surgical outcomes, then your results will likely not be as good as the curves suggest.
When reading the defocus curves, it's helpful for me to convert diopters to feet/inches or meters. -1D means the focal point of maximum vision is one meter (39 inches) away. -2D means 1/2 meter (19.5 inches), -3D is 1/3 meter (33 cm or 13 inches) etc. 0D is essentially infinity. "Plus"D is farsighted beyond infinity, not really helpful. Most IOLs will have good vision (20/30 or 6/9) within 1D of the focal point. Some IOLs can do much better than that.
On the vertical axis, logMAR values are often used which can be translated into Snellen vision roughly as:
20/20 (feet) or 6/6 (meters) - logMar 0
20/25 or 6/7 - logMar 0.1
20/30 or 6/9 - logMar 0.2
20/40 or 6/12 - logMar 0.3
While the area under defocus curves should remain roughly constant for a given lens material, lens designers seemingly can create products with just about any shape desired. A longer flatter defocus curve above 20/30 (6/8) or so might lead to a maximum depth of focus that permits far, intermediate and perhaps near vision (like multifocals or EDOFs). However, reducing the hump in the curve would let less light in for distance vision and probably negatively impact contrast sensitivity and low light / night vision. So there are tradeoffs when you flatten the hump in the curve in efforts to eliminate spectacles for near and far.
With my limited understanding of defocus curves, I'm struggling to understand why any surgeon would target plano (0D) or even "plus" values with any lens. As Ron suggests, shifting these curves to the right until your distance vision is 20/25 or other reasonable target gives you the best chance at decent intermediate or near vision along with decent distance vision. Of course, targets can be missed by 0.5D and 20-40% of the time even more than that. So, ideally you have a plan for any outcome within 1/2 diopter of the target. For people with relatively normal vision before cataracts, I suppose getting good distance vision is most important and might justify targeting something near plano.
I've been highly myopic my entire life so wearing glasses for distance wouldn't be a shock. I'm probably more willing than most to take chances by shifting the defocus curve a far enough to the right to achieve intermediate/phone/near vision. If targets aren't met and the curve is shifted further to the right than planned, I'm risking wearing glasses to drive but might have better near vision than planned as well. I could live with that but some wouldn't like it.
Perhaps surgeons try to avoid this outcome by targeting plano. To me, that just wastes the good vision available in the lens to the left of plano which can be about 1D of 20/30 (6/9) or better vision. Low-light distance vision might suffer a bit as well, I suppose. Depending on the lens, that 1D of wasted vision can be the difference between useful computer/phone vision and computer/phone readers.
Dave13852 greg59
Posted
Ron, Jim, and Greg,
Thanks for your informative replies to Jay.
Greg can you expand upon a bit regarding "Most IOLs will have good vision (20/30 or 6/9) within 1D of the focal point." Say for example the best focal point is set at 19.5 inches (-2D for near/intermediate), what does "within 1D" equate to in terms of distance in inches?
Thank you.
greg59 Dave13852
Posted
The distance that corresponds to "within 1D" depends on where you set your focus. If set to -1D, then "within 1D" would go from -2D to 0D (1/2m = 50cm to infinity). This might provide decent distance and intermediate vision.
If the focus is set at -5D, like my natural eyes, then "within 1D" would go from -4D to -6D (1/6m = 16cm to 1/4m = 25cm). This isn't good for reading, intermediate or distance but does allow you to view lots of details in items held very close (pill bottles, etc.)
If the focus is set at plano (0D), then "within 1D" would be from -1D to +1D (1m to infinity.) In this case, you wouldn't have much intermediate vision, only distance.
Dave13852 greg59
Posted
So 1D on each side of the focal point.
I wasn't sure if it was a total of 1D i.e.
half a D on each side. Thank you very much for the explanation Greg!
RonAKA Dave13852
Posted
This range of 1 D on both sides is based on 20/32 being acceptable vision. At the peak point you may be close to 20/15. So it is range based on a tolerance for less than the very best vision.
Dave13852 RonAKA
Posted
I see Ron thanks for the clarification.
jimluck jay3210
Posted
Side comment: A bit of residual astigmatism gives the patient some extended depth of focus, albeit soft focus. (According to the 2 ophthalmologists I have consulted.)
RonAKA jimluck
Posted
Yes, it is considered when calculating spherical equivalent by adding 50% of the astigmatism to the sphere. I did a contact lens trial using the spherical equivalent formula to determine the non toric contact lens power. It worked quite well, but not quite as good as a toric contact that corrected both the sphere and cylinder to the optimum amounts.
CooperVision provides an on line calculator that determines contact power when selecting a toric or non toric (for low amounts of astigmatism). It also makes a correction to the contact sphere power when there is a larger amount of correction. Google this to find it:
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CooperVision OptiExpert CONTACT LENS PRESCRIPTION CALCULATOR
Dave13852 jay3210
Posted
Jim,
I also read somewhere about residual astigmatism giving some extended depth of focus but I can't recall where I read about it. Any idea how much residual astigmatism they'll talking about? Also does "extended depth of focus" mean the same thing as "the range of focus" in terms of ability to see along say inches if talking about near/intermediate vision? What does it mean when it's said that there's no "accomodation" with IOLs? Finally what is meant by "contrast" with IOLs.
Thank you.
jimluck Dave13852
Edited
With astigmatism, one has two focal points. If you look at a glasses prescription, the first number ("sphere") tells you where one focal point is and the second number ("cylinder") added to the first tells you where the other one is. So, if your glasses prescription is 0.0 (sphere) and -1.5 (cylinder), you have one focal point at plano (great for, say, 6 feet to infinity distance) and one at -1.5 diopters (good at around 2 feet -- 67 cm to be exact). Neither one will be real sharp, but they may be good enough and you may have gotten used to ignoring the softness of the focus. If you correct the astigmatism so that there is only one focal point at plano, then you'll only have sharp distance vision and something 2 feet away may be too fuzzy to read. So, the astigmatism gives extended depth of focus (sharpish distance and sharpish 2 feet away).
"Accommodation" is the ability to change the focal point of your eyes using little muscles in the eye to change the shape of the lens. Young people have lots of accommodative ability. Old people have very little -- maybe one diopter -- not enough to go from infinity to reading distance, but still a useful amount. Once your natural lens has been replaced by an artificial lens (cataract surgery) you have none at all. The lens shape is fixed. Your ability to see with sharp focus at different distances depends entirely on the depth of focus of the lens, and/or on multi-focal properties of the lens engineered into it. Extended depth of focus (yes, your could say "range of focus") and multifocality come with tradeoffs, like loss of contrast in low light, and/or seeing junky stuff that should not be there, like starbursts around lights. You lose all ability to change the shape of the lens.
They are working on artificial lenses with accommodating ability, and I guess some have been marketed, but without great success so far.
Contrast sensitivity means your ability to distinguish things from their background -- will you see that bicyclist riding along the side of the road with no lights at dusk or will be just blend into landscape until you are just about to hit him? His clothing is a different color, texture and brightness than the woods in the background, but will your eyes have enough sensitivity to that subtle contrast to make him visible to you in the dim light of dusk? That's contrast sensitivity. Your cataracts are reducing your contrast sensitivity. A plain old monofocal IOL will do the best job of restoring contrast sensitivity (in general, there may be exceptions). In general, multifocal and extended depth of focus lenses don't restore contrast sensitivity as well. The Eyhance, which is just slightly EDOF, claims to be good on contrast sensitivity. The makers of the IC-8 pinhole IOL claim that if you put it in one eye and a standard monofocal in the other, there is no loss of contrast sensitivity when both eyes work together, relative to having monofocals in both eyes. The Vivity lens carries an FDA warning about loss of contrast sensitivity. Etc.
Dave13852 jay3210
Posted
Jim thanks for your detailed explanation. I think I understand the terms much better now but will continue to reread your post throughout the day. Appreciate it!