Glasses for DIY

[The Natural Philosopher]

I don't think you understand what astigmatism is dennis.

[Brian Reay]

In simple terms, it occurs due to the eye ball being more like a rugby ball shape than a football shape. The cornea of the eye, the thick layer which contains it all, is distorted. As a result the optical path through the eye isn't correct for all parts of the image are focused at incorrect places with respect to others.

[The Natural Philosopher]

Exactly. So there are two things that need specification. The ratio of te major to minor focal lengths, and the orientation of the ball axis.

Neither of those are rotationally symmetrical nor necessarily the same in both eyes.

So flipping astigmatically correct glasses upside down is never going to work unless the astigmatism is the same in both eyes and either totally horizontal or totally vertical.

Prismatic correction is almost never applied to glasses - eyes can move independently enough to resolve that unless there is a total squint .

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[dennis]

I don't think you do if you don't understand what I said. Maybe you don't know what prism is?

[Tim Lamb]

In message , Brian Reay writes

Brain pick alert:-)

Brian. Can you explain why, at night, I now see self illuminating objects (stars, car rear number plates e.g..) as vertical pairs?

The effect is from both eyes although specsavers tell me I have the beginnings of a cataract on one.

Difficult to estimate the angular displacement but for a star maybe 0.5 deg.

I have a distance vision prescription but, so far, only use them for TV as I find the frames obscure a lot of peripheral vision.

[dennis]

That would be about correct and is why its really bad when some idiot puts the lenses in 90 degrees out.

I don't think TNP has a clue from his claim that what I said was wrong.

I have astigmatism of about 4 dioptres so i know exactly what its like.

[The Natural Philosopher]

Of course I know what a prism is.

Its not applied to glasses however

[The Natural Philosopher]

Astigmatism

[Tim Lamb]

Hmm.. why was I unaware of any problem before getting to 65 or so? Why is the effect only apparent at low background light levels.

For the record, both my daughters were found to have astigmatism issues when tested as children.

[The Natural Philosopher]

Because that's what happens at our age. Lenses get cloudy, and stiff we can no longer focus, and the jelly starts to dry out so we get floaters and the retina starts to get patches the don't work.

Because the same as any lens, the defects show up when the lens is opened wide up stop wise.

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[Tim Lamb]

So, to check astigmatism, I should lie on my side and see if the stars are now side by side?

Also, the effect is local. I only get the star in focus doubling.

[DJC]

I started to notice that after I had had a cataract op to fix the cataract created by fixing a detached retina. Vertical lines such as the window panes on the houses across the street started to double up.

[polygonum]

I have astigmatism in both eyes. (Nothing unusual about that!)

Never had double-vision at all until a few years ago. Two separate times I had monocular diplopia for a few days. In both eyes - but right eye much more pronounced. First thing I noticed was red kites flying in perfectly matched pairs. :-)

Hindsight (which is definitely not diplopic) explained it as an obscure effect of insufficient thyroid hormone.

Haven't yet reached 65.

[Brian Reay]

I suspect that is a form of chromatic aberration.

Basically, in a perfect lens, the light isn't split into its component colours and all the colours take the same path through the lens and end up in the same place.

If the lens is imperfect, the light is split, different colours take a different route, and you get more than one image.

I assume, as you see two images, you only see a dominant second image- possibly something to do with the way the eye responds to colours at night.

The above is based on my rather rusty A level Physics and O level Biology, so don't take it as gospel.

The good news is, everyone I know who has had the cataract op swears by the results. When the time comes for yours, I wish you well.

As an aside, during my last visit to Specsavers, I was told cataracts were all but inevitable. I don't have any yet, I'm not yet 60, but few escape them it seems. The conversation was prompted when I asked about laser surgery and/or those other lenses to correct short sight.

[Brian Reay]

A friend had an op to fix a detached retina, in fact a series of ops, including having the fluid drained and replaced with oil. He was told that, following all of this, the instances of cataracts rose dramatically so they would pop a new lens in as part of the process. Thankfully, he is fine now but the process seemed to take ages.

[Tim Lamb]

I think imperfect lens/astigmatism is the answer. Prompted by TNP I tried looking at a night time point light source, with and without my distance prescription glasses.

Cloudy and half moon so no stars but the Christmas LED lamps on the barn resolved from twins to singles with the glasses.

I have held off wearing glasses, other than TV or reading/close work, on the basis that I don't suffer from eyestrain/headaches but am concerned that my sight will deteriorate more rapidly with their use.

[dennis]

They wont deteriorate any faster if you wear specs. You may notice it happening sooner than someone that doesn't get eye tests.

[Tim Lamb]

Is that established fact?

Glasses are a pain in the proverbial for my occupation, rain, dust masks, safety hat for strimming and chainsaw, etc.

I assumed that not exercising the muscles around the lens would make them less effective and might encourage hardening.

I go along for the 2 year routine testing and then resist the sales chat. My TV glasses are now 3 years old, my reading ones 5 years and the pair used for the workshop 7 years. Specsavers are very unwilling to re-lens old frames.

[Vir Campestris]

Yes, silly me. You'd have to swap the lenses too.

Rotating the lens by 180 would likely have little effect.

Andy

[Vir Campestris]

It must be, all the opticians say it. And of course they'd have no vested interest in you needing glasses sooner, would they?

My computer glasses are about 5 years old, and held together with mod wire. The pair I'm wearing now were 2 quid in a charity shop (ready readers) about 2 years ago, and aren't really quite strong enough for reading any more - but fine at screen distance. I've got some pound shop ones for reading. I really must book another test... but they'll try to sell me some hundred quid glasses, and I don't think they'll be hundred times better.

Andy