On Wed, 22 Nov 2017 12:38:17 -0000, "Jim GM4DHJ ..." coalesced the vapors of human experience into a viable and meaningful comprehension...
who remembers putting the lion into the cage at the eye clinic? ...... >
I have sometimes wondered if an instrument exists so a (half intelligent) patient could determine his own astigmatism metrics, by manipulation of a joystick to make an apparent ellipse circular.
The way the optician traditionally determines this by repeatedly asking the patient "better or worse" seems prone to error.
Astigmatism isn't the distortion of circles into ellipses. It's the fact that vertical lines and horizontal lines are focussed at different planes within the eye (or a camera, for that matter) because the lens is not part of a perfect sphere. Objects remain circular but the sides are more blurred than the top and bottom (or vice versa).
A machine that allowed the user to apply variable levels of correction until maximum sharpness is achieved in all planes would have to alter cylindrical focus rather than spherical focus. I'm not sure whether this could be achieved optically by varying the position of a cylindrical lens in the same way that altering spherical focus can be achieved by varying the position of a spherical lens.
At present opticians achieve it by putting a variety of different powers of cylindrical lenses into the optical path, together with a spherical lens to achieve optimum overall focus.
I've always admired the way that opticians can build a prescription for the correct lenses, given that any small adjustment of focus (certainly spherical focus) will be corrected for by the eye "accommodating" - changing the power of its own lens - to within the limits of its ability to do so, which declines with age.
Astigmatism is when the focus is different in different planes. You can also get an effect where you seem to have a second image just above below or to the side of the main one. You can also have chromatic aberration mixed in with it so the astigmatism varies with colour.
Yes, caused by flattened eyeballs! All sight defects that I know of are caused by distorted eyeballs.
If the distortion alters the length from the optimum circular shape to something shorter or longer, the eye focuses the image either in front of or behind the retina, causing short or longsight.
If it is flattened sideways, it changes the lens curvature and, hence, the focus in two planes, one in line with the distortion and the other at right angles to it. Thus the focaus is different in the two planes, as Bill said and, obviously changes continually between the two.
I wore glases from the age of 14, primarily for astigmatism, with some short sightedness thrown in. It tended to vary over the years and got progressively worse at one stage.
Somebody asked me one day why I wore glasses. When I said astigmatism nhe didn't know what I meant so I took of my glasses, told him to hold them in front of himself a look through the right lens, which was the worst, at the clock on the wall. Then slowly rotate the glases though 90 degrees.
The reaction as he saw this grotesque distortion was quite spectactular and he quickly handed back ny glasses at arms length as if he might contract this strange 'disease' if he stayed too close to them!
The best part of 20 years ago I had cataract operations on both eyes about 18 momths apart. I assume that the plastic lenses are stiffer than the natural ones they replaced because I now have 20:20 distance vision.
Although I now no longer need to wear glasses all the time, I do, in fact, werar varifocals. When I first had reading glassses the always ended up on my desk or my lab bench - which were three floors apart - when I needed them(!) so never again. Much easier to wear the same pair of glasses from the time I get up until the time I go to bed!
Two ophthalmic conditions seem to have become conflated in this thread. The bit about "Is this better or is *this* better?" *is* part of the optician's patter during the astigmatism test. However, the question about putting the lion in the cage raised by our resident "Billy Bunter-a- like" is, as you correctly said, all about testing for the condition known as Strabismus (crossed eyes).
The test with lion and cage is obviously one designed to detect Strabismus in early childhood, presumably only if there's any suspicion of the condition since I don't recall playing such a game with the optician when I was finally diagnosed as being short sighted at age 11.
Not afaicr ever having played this game, just to satisfy my suspicion and curiosity about it, I tried googling for more info before landing on the following:-
Having read that wikipedia article, which failed to make any mention of this test, I tried looking harder for results on the search phrase "the lion in the cage eye test" before finally finding this link;-
which is a pdf named "17 Ophthalmic Instruments and Diagnostic Tests" revealing that this lion in the cage test is a subjective angle of squint test using an instrument known as a Synoptophore.
With all of that said, I hope to have provided this thread the "closure" required to lay the question well and truly to rest. Of course, this hope doesn't take into account this group's claim to fame for its propensity to topic drift so I won't be waiting with bated breath to issue its last rites. :-)
I always used to have almost perfect vision - just very slight short-sightedness which meant that distance glasses made road signs look very slightly sharper (so small that I decided not to pay for glasses).
Then as I reach my late 40s I found it progressively harder to read at normal book distance, which was exacerbated when I had a heart attack and my brain was deprived of a normal blood flow for the two hours (yes, really) that it took the ambulance crew to get my heart beating properly on its own - I survived on CPR alone. How I'm still alive is a miracle.
After that, my close vision is worse and the distance vision is slightly worse (though I can still drive without glasses, I find that distance glasses do help a bit with reading words on signs).
All down to loss of accommodation - the lens's ability to alter its shape to focus at different distances, which gets worse at around age 45-50.
I tried varifocals when I found I needed both reading and distance glasses but I found them terrible (and I did persist for a week or so to give my brain chance to re-learn) because when I moved my head from side to side or an object passed across my field of vision, vertical edges sloped one way when it was on the left, became vertical in the centre and then sloped the other way when it was on the right of my field of vision. The optician said they'd never heard of that symptom before.
The opticians re-checked their measurements of the exact spacing of my pupils etc and made up a new pair of glasses with slightly tweaked measurements, but that made no difference so I took up their standard offer of a pair of distance and a pair of reading glasses for the same price as I'd paid for the varifocals. Because the prescription for my reading glasses changed a couple of years later, I now have two pairs of those - one stays at home for reading (eg in bed) and the other lives in the car in case I need to read while I'm out (eg killing time waiting for my wife to come out of work etc).
I know the "where have I put my reading glasses down" problem very well :-)
Those of us with simple sperical correction can afford to solve that problem by having a pair of cheap reading glasses in each place they might be needed.
More accurately, it?s to measure ?latent squint? which is the resting position of the eyes when the extra-ocular muscles aren?t working. Everybody has some degree of this but the greater it is, the more the muscles have to work.
As a student I made a ?Maddox wing? latent squintometer out of an old cereal packet and visitors to our flat all got their latent squints measured. Oh the fun we had... ;-)
I have a hypothesis that the same sort of 'adaptive corrections in wetware' apply to all of the eye's many minor imperfections, including astigmatism. Compared to even a cheap camera, the eye falls far short of that 'perfection' but since the eye/brain system overall forms an adaptive optics system, the many minor defects are readily corrected out until age takes its toll (ever stiffening lens reducing not only accommodation but also efficacy of astigmatic correction).
In my case, mild myopia and astigmatism, I eventually found the need to remove my glasses to read text (books and computer displays) once I reached my early fifties. Of course, this results in the lens Ciliary muscle having to work harder to correct out the astigmatism as well as fine tuning the focus.
Even today, some 15 years on, this trick still works for reading distances in the 50 to 70 cms range when my eyes aren't tired. For close in work, reading glasses, magnifying glass or jeweller?s loupe, the working range of focus is sacrificed to the need to include astigmatic correction. Strangely, prescription reading glasses, which include astigmatic correction, don't seem to improve the situation as much as I was expecting (if at all).
With my "distance glasses" on, I can accommodate down to a couple of metres where it becomes a toss up as to whether I can see just as clearly sans glasses which covers the range down to about half a metre before the use of non-prescription reading glasses becomes necessary.
Interesting but a lot of links to follow up to fully appreciate the article in my case. Ophthalmics seems a very complex subject to get to grips with when you only think of the eye (if you think of it at all) as a simple camera without considering all of the infrastructure that controls it in order to feed the slightly misnamed[1] "Visual Cortex" which might be better described as the brain's own "Virtual Reality Suite" fed with inputs not only from the eyes but all the other senses as well.
[1] I suppose it's not too 'misnamed' when you consider that the primary source of sensory data feeding the brain's VR Suite comes from the eyes to the extent that the visual cortex handles all the tasks of focus, pan. tilt, rotate (and blanking out nonsense data on rapid pans between points of interest in the visual landscape) as well as convergence for binocular sensing of distance and so on and so forth.
What you think you're seeing as a direct image presented by your eyes, is in fact, mostly a memory, albeit continuously refreshed, of what the visual cortex assembled from the various visual scans it ordered the eyes to carry out in response to the tiny bit of the brain that thinks it's in charge of everything, the Id or "You".
The Id, is in fact little better than the boss of a major corporation who cannot possibly know every detail of how his enterprise actually functions but who does know enough to issue the right orders to get things done that will preserve the whole organisation as a viable business operation. As long as the boss understands the limits and refrains from asking the impossible, the enterprise will continue to thrive.
Luckily for most of us "Ids", we generally have a superior track record compared to bosses of actual corporations. Those that fail to the extent often seen in the commercial world are usually considered for a Darwin Award (e.g. crossing busy roads without looking or trying to leap onto a departing ferry without considering their athletic limits and other such stupid stunts).
Some of those "Ids" decided to task their brains with the goal of researching how their brains actually function. We call them, and similarly inquisitive "Ids" who want to understand how stuff *actually* works, scientists and engineers. :-)
I am old but have got by so far on plastic reading glasses only. I tried someone else's varifocals once; they worked as intended, but I decided never to get varifocals because of the godawful distortion you describe which offends my artistic sensibilites.
I have been tempted to buy adjustable glasses which have a knob on each side that slides two aspheric lenses sideways to create a variable strength result, but have never tried them.
I think this very much depends upon your particular prescription, and the actual lens detail. They are not all created equal.
In my case I have in the past been aware of slight distortion, though not found it difficult to adapt to, but my most recent pair have top of the range lenses (1), which greatly improve matters.
A great many pairs are made by simply cutting the right sized section from one of a large number of blanks held in stock - that is why they can be provided by some opticians in a very short time.
So, they are custom made to the extent that an existing lens is cut to suit the chosen frame.
Because varifocal optics are very much more complicated, I don't believe that the more expensive ones are ever just provided from pre-existing blanks, but would be interested to know more.
You will find me on the back row, wearing my first pair of wire-framed National Health specs, and "short back & sides" haircut. These, to my chagrin, were certainly not regarded as fashionable. How times have changed.
HomeOwnersHub website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.