Saturday, January 02, 2010

Varifocal Contact Lenses - How Do They Work?

In 2009 I got put on a contact lens trial. My eyes have a difficult prescription, and I've been told that, as middle age progresses, I will eventually need both lenses and spectacles. Now my optician has put me on a commercial trial of varifocal contact lenses, and they seem to fix the problem.

The only trouble is the very existence of varifocal contact lenses seems to contradict a lot of what I thought I knew about optics and vision. And I can't find any good explanation of them anywhere.

My eyes

My eyes aren't the easiest to work with, but opticians usually get interested when they see them. Their strange prescription has pushed me towards contact lenses instead of glasses - but in recent years, I've been told that eventually lenses alone won't be enough.

About twenty years ago I had a recurrent corneal erosion problem. I was told I'd never wear contact lenses. Luckily contact lenses got better, because what happened to my eyes ten years ago meant I needed them.

I had cataracts in my right eye lens, so I had it removed, and an implant put in. It's a standard procedure, but - maybe because of the difficulty of working through my tiny pupil, maybe because of some other bit of birth defect - it didn't work too well for me. The old lens came out, but the implant fell from the pocket left for it, and I was left with no lens in the right eye.

That's better than it sounds - an eyeball with no lens in it can still bend light - most of the focusing is done by the cornea - and a short-sighted eyeball will do better in this situation than a long-sighted one. I came out with a very long-sighted right eye, with a prescription of about +5.

The two eyes had such different prescriptions that correcting both eyes with spectacles wouldn't work. The left eye needs a concave lens, while the right one needs a convex lens; put them in glasses, and there's a distance between the lenses and my eyes, so they act as a reducing and a magnifying glass, respectively.

The result is two images which are so different in size, that my brain can't put them together. I tried it in the optician's shop, and it really, really didn't work.

So I tried a pair of glasses with plain glass in the left lens, and then moved onto contact lenses, when daily disposables became gentle enough for my previously-t00-sensitive corneas.

That has been excellent. The combination of my two eyes corrected has been better than any vision I had in my life, including before my cataract problem.

Recently, my eyes have been changing, apparently, and I've had to have the prescription changed - the left eye gets a 4.0 lens, leaving it slightly short-sighted, so it can compensate for the right eye. I've been pushed towards monocular vision - the right eye most of the time and the left eye for close work.

(I guess this is like the problems most people have as their eyes age, except I wouldn't have thought I'd get presbyopia - age-related-shortsightedness caused by stiffening of the lens - as my left eye is already short sighted, and my right eye is no longer using its natural lens.)

Whatever the caue of my changing prescription. I've been warned that as it progresses, the current set-up will stop working eventually, and I'll have to have glasses and contacts. After ten years with no glasses, I'm not looking forward to that.


I was interested to be offered varifocal contact lenses (fortnightly Acuvue lenses using the Oasys material).

  • it sounds like a way to avoid glasses
  • it's a commercial trial, before the full launch, so very few opticians can offer them
  • it's a concept I'm having trouble understanding.

A quick Google search gives us an explanation of varifocal spectacles work. On the Specsavers site, you can see that they have different zones for different distances. For any given thing, you look through the part of the lens you need.

So far I haven't been able to find an explanation of how varifocal contacts work. Sites offering them tend to follow up the question "How do they work?" with non answers, like this "This means that the wearer does not have to change lenses when switching between reading and driving, but care should be taken when driving in bright sunlight."

And yet, it works!

Just try them, said my optician. So I put them in, and went for a walk round Brixton market. I crossed a road, bought a newspaper and read the front page.

After a quick check to see how I was doing with them - and yes I could read the bottom line of the chart. I was having the odd bit of blurring, but already I could see the improvement in my depth perception - so I cycled off home. And since then, I've been (mostly) stunned by how good they are.

It's sometimes patchy, and the picture improves (and sometimes gets worse again). My first stint at a screen was horrible. I had multiple images and couldn't get them under control. But I broke off from that, spoke to someone in the room for twenty minutes, and when I looked back the screen was perfectly readable.

I've found my office screen is hard to read for the first twenty minutes of the day in the office - after I've cycled there in cold weather.

Sometimes, after looking at something for a long time and having trouble with it, it springs into focus.
So what is going on? Well, I'm still frustrated by that fact that I have seen no proper explanation how this works, but I'm guessing it might be something like this. Suppose the varifocals do indeed create multiple images in each eye, and then the two eyes work together to choose the best one?

That's a new way of working (and one that is completely different from the way normal degree-level optics pictures images forming on the retina). That would explain the way my eyes sometimes cope, and sometimes seem to go backwards.

My right eye is still the strongest - and with its tiny pupil has a nice depth of field. So in many situations, I can still try and do things in the monocular way. The sudden blurring is when the information from the left eye starts to look useful, and my brain has to work to put it together.

So the first day I had the lenses, when I sat at my screen, I was so used to working in a monocular way, that I automatically tried it again, and was confused by the multiple images in my right eye. Looking round the room, and talking to someone got me back into using both eyes again, and that helped when I went back to the screen.

I've found night vision is much better from the start, and I see further in the distance, right form the first.

Does that explain it?

That seems to explain some of it. By this explanation, I should find that the first few days are a process of acclimatisation, and I get more used to the lenses thereafter. Certainly, I've noticed that I settle into the lenses much quicker in the mornings now than I did in the first few days.

I've tried looking at things with one eye, too, to see if I can see the sort of multiple-focus images I'm talking about, and I can't. I certainly see better when I use both eyes - and I'm less monocular than in the past, but I'm leaning heavily on my right eye.

So I've tried things my optician says would be wrong: one day, after a week of the varifocals, I put a pair of the old lenses in, thinking I might find the old way hard to do after I'd got used to the varifocals.

In fact, the monocular vision struck me as easy, and very clear, and in some ways a relief. There was no effort to keep the screen in focus, I was just using my right eye. However, I didn't have such good depth of field, at all.

After a couple of hours, I switched to the varifocals, and again was surprised by how easy the transition was. I just went back to the screen and moved around the house as before.

I did have another bad time acclimatising to the screen - but that was the next day, after a whole day wearing the varifocals.

I've also noticed a couple of other things. Driving on a sunny day, I realised my left eye was closed - it dazzles easily - but the right eye, with its varifocal lens, was making out just fine. It seems that with practice I don't actually need to use both eyes to get benefit from the lenses.

Later that day, I focussed a telescope on the moon, and saw a lot of detail in the craters. Again, that was with one eye at a time. I realise now that I only used my right eye. It's a reflex as it's been my best eye, but I should have had a go to see how the left eye did!


My ideas about how this works don't really stand up, and I want to know more, but it seems that people making varifocal contact lenses don't put information on the web about how they work.

I know my prescription is fairly unusual, so I wouldn't expect other people's experience to match, but I'm interested to hear any other thoughts on varifocal lenses.