There is no universally agreed to definition of near, intermediate and distance. These are guidelines. You must understand that a monofocal IOL or a monofocal toric IOL inserted correctly in an eye with a corresponding amount of astigmatism there is a precise distance that the IOL will be focused anything closer or further away will be somewhat blurred. Due to the pupil changing size there is a "depth of focus" where within that range things will be pretty clear.
Distance: This means 20 feet (6 meter) or further
Near: Generally this means about 12 to 18 inches which is where most people read.
Intermediate: this is the most imprecise. Generally it means 2-5 feet away. Within this distance are using a computer, seeing a dashboard on a car, seeing shelves at a store.
This leaves a distance of 5-20 feet that falls between the gaps. This often includes 8-18 feet away which is the distance that many people watch television and without glasses on there can be problems reading the news/weather scroll and scores of ball games.
Thank you for your answer. This is how I understand that answer. Is my understanding correct?
Say I got an IOL for distance. Say this IOL gave me perfect vision at 30 feet; clear, sharp, focused, I could read a newspaper.
Without contacts or glasses, items closer than 30 feet will be out of focus, and items farther than 30 feet will be out of focus.
Wearing contacts or glasses will bring into focus items that are closer than/farther than the 30 feet "in focus" distance.
Thank you again.
Forgot to ask.
1. Is the cataract surgeon's plan to make my bad eye equal to my good eye a good or not so good idea?
2. Why or why not?
3. Before the cataract surgery, can I try a contact lens in the bad eye that will approximate what the prescribed IOL will give me vision wise?
4. I already wear the strongest Alcon contact available in my bad eye (-10.0). How is or is not a contact that approximates the prescribed IOL power work different from my current -10.0 Alcon contact?
One other thing that is hard for people to think about is what a 20/X figure means in terms of how well they can see. This page:
gives a description of the sorts of print sizes you can see with different 20/X visual acuities like newsprint and books. Unfortunately your situation is complicated by the fact that the other eye issues may lead your vision to be worse than the typical results quoted for an IOL for different distances.
Your eye has a range of focus around where it is targeted, with the visual quality going down the further in or out you go from that. Usually targeting a tiny bit of myopia, like -0.5D, still leaves fairly decent distance vision. Choosing a lens power isn't an exact formula, so there is some chance the lens power might be off (even if for most people with low prescriptions its fairly accurate). If you target 0D then there is a chance the IOL power might wind up being off in the farsighted direction, e.g. +0.5D, which could noticeably reduces your closer in vision without a real benefit. If you target -0.5D and its off by +0.5D that merely leaves your eye target at distance, 0D. If its off in the other direction by -0.5D that leaves you at -1D which reduces distance vision a little bit in that one eye, while giving you more near, and is still a rather small amount of monovision in the range almost every one adapts to well.
The higher the level of mononvision, the more impact there is on stereopsis (3D perception), though most people don't notice it or get used to it, and the more monovision the more near you get.
When they talk about focusing an eye at a certain diopter measure, e.g. -0.5D, the diopter units can be converted to a distance. If your eye is focused at X diopters then its focused at (100 / -X) centimeters. So a -0.5D focus is targeted at (100 / 0.5) = 200 cm = 2 meters = 6.6 feet. How much further in you see than that depends on the IOL you get (and the person, the results of studies are only averages).
Also your vision is coming from both eyes, so the visual acuity at a certain distance can sometimes be better than either eye individually (though it is sometimes only as good as one of the eyes, usually the best but sometimes the worst) so its hard to guess what you'll get with monovision if there isn't a study with that IOL, and I hadn't hunted for one.
Most IOLs have information online someplace giving a "defocus curve" which indicates the visual acuity (on average) you get going further in or out from the best focal point. Often monofocals don't have that on their own web page, but multifocals from the same manufacturer often show the monofocal comparison. This third party study uses an Alcon monofocal as a control:
Although the figures are behind a paywall, Google search turned up their graph of the defocus curves:
Which seems to show the Alcon hits around 20/40 at -1D focus. That would indicate that if your eye were focused at -0.5D that at -1.5D = 66.7 cm = 26.2 inches you'd still have 20/40 vision (if your eyes were average, your other eye issues make that less likely).
I haven't hunted for various sources to see what to trust regarding their defocus curves since the manufacturer itself uses a study that is much more optimistic. Alcon shows their monofocal in a comparison on this page:
which claims that even 2 diopters in from the target you'll get 20/40 vision (though that seems optimistic based on other data to my recollection, but I hadn't checked). That would mean if you hit the target of -0.5D that at -2.5D you'd still have 20/40 vision (were your eye like average eyes), which would be (100 / 2.5) = 40cm or 15.7 inches. With premium lenses there tend to be multiple studies to check to compare their visual acuity results, but I hadn't looked at the Alcon monofocal data to know whether the studies tend to be usually closer to the manufacture's figures or the other study above.
Some defocus charts like one I used above use a different visual acuity measure like LogMar rather than 20/X, this page has a conversion chart:
I posted that before reloading the page to see the other questions. I need to get to other things, but I'll note that of course a contact lens test now would be impaired by the cataract reducing your visual quality, but might still be of use.
Thank you so very much for your reply. I've read some of your previous posts and I've been hoping to hear from you.
Would you please tell me if my understanding of what you said is correct? Here's my stats for reference:
OS UCVA 20/40
OS BCVA 20/20 with -0.50 contact = good vision 10' and out
OD UCVA 20/400
OD BCVA 20/80 with -10.0 contact = blurry, distorted
1. What does it mean when you say "....if you target -0.5D..."? Does that mean the IOL chosen is one that will require a -0.5D contact lens to read 20/20 Snellen?
2. My good eye uses a -0.5D contact lens. It gives me 20/20 Snellen. It also gives me clear and sharp vision from about 8 feet and beyond, but I need cheaters (+2.0) to read and use the computer. I'm satisfied with these results. With that in mind, and also considering my bad eye's current and poor UCVA and BCVA, is the cataract surgeon's plan to target a post-op contact lens of -0.50 in my bad eye (to match my good eye) a good idea?
3. You said "The higher the level of mononvision, the more impact there is on stereopsis (3D perception)..." Do you mean higher monovision = less depth perception?
4. Regarding stereopsis and monovision, what do you mean by "....and the more monovision the more near you get."?
5. Can you please explain monovision and mini-monovision, and the good and bad of each? I've read up on them but can't quite understand the pros and cons of each, or when one is preferable over the other.
6. The Alcon chart. Is it showing that with an IOL targeted for 0.0 (meaning the IOL alone, with no glasses or contacts will give the patient 20/20 Snellen), but ends up being off -2.0, will produce uncorrected VA of 20/40?