I have lots of cheap eye glasses which it would be good to sort out and need a more certain way to test the dioptre, than trying them on.
I wear various dioptres for different purposes, reading, TV, driving etc., and I am rather rough with them, so I have quite a collection.
I'm thinking along the lines of a lamp, then focusing the lamp onto a surface then measuring the respective distances, but then how to work out the dioptre?
You'd be better using sunshine. That's a parallel beam, and measure the distance from the lens to the point of focus, in metres. Take the reciprocal and you have the dioptre.
Convex or concave? Could get complicated if there are varifocals and/or ones that correct for astigmatism too. Opticians do it by holding the glasses against ones that eliminate the refraction; I think they call it "neutralising'.
Oh, and a dioptre is the reciprocal of the focal length in metres, negative for concave.
The sun is parallel for most practictical purposes in fact I reckon a lamp the other side of a room will be good enough. Assuming convex lenes just focus an image of the lamp and measure the distance lens to screen. ok for 2 dipotres you might not get a distance of exactly
0.5 m but as lenes are in 0.25 dipoter steps and 1.75 is 0.57m and
2.25 0.44 m it ought to be obvious what the lens is. I'd try it but we are myopic...
Myself I'd use a local standard. IE why do you need such a technical measurement? As long as you can grade them as to what you can see with them does it matter to anyone else. The spec you retaking about assumes a light source which is a point, like an infinite distance away. I being old fashioned have no idea how this measurement is actually done, but as I say, you don't need that, you need a number scheme that you can relate to yourself when you use them. Brian
If sunlight was all truely paralell shadows would have sharp edges, they don't. This is beacuse the sun is a disc in the sky and you get rays from opposite sides of that disc.
Now if you took the light of a single star you'd be getting closer.
My eyes are just not that sensitive to wearing the wrong dioptre for the wrong purpose. It takes me a while to realise I am wearing the wrong ones and I notice the eye strain.
A few years ago, I devised a scheme of colour marking them. Remember the coloured, numbered as used on cables in electrical control panels? Those slip neatly onto the arms of the glasses.
The pair I'm wearing now, serve me for most purposes. At the moment I am (obviously) typing on my laptop and watching TV at the far side of the room, but they are useless for close up. They sort of work for longer distances, but I wear a different dioptre for driving.
I try to keep a full range of dioptres in the car, in the caravan and in the house - hence my confusion.
The fact that the sun is not at infinity makes very little difference to where a lens produces an (in-focus) image of the sun. Plug the numbers into the thin lens equation and for typical convex specs the difference is less than a nanometre. Even a source 5 metres away only shifts the focus by a few mm from that so as you said a lamp on the other side of a room is good enough.
If you need distance correction (for driving etc), that should really be done properly by an optician. Not like it's that expensive. They will also be fine for TV - unless you sit on top of the set.
I have contact lenses that correct for distance. But do use ready made for reading etc. Two strengths are fine for most things. One for this computer screen and slightly stronger for reading.
And I'm of an age where my accommodation is near zero.
If you are noticing any eye strain, it means something is not right.
My accommodation is not as good as it was. I'm very slightly short-sighted: although I can see fine in the distance and to read road-signs when driving, my distance glasses just sharpen things up a bit. Driving without my glasses is not a problem, if I forget them.
My close eyesight, for reading, has got dramatically worse over the past couple of years (I'm now in my early 50s), and this seems to have started after I had a heart attack and cardiac arrest: maybe one of the only after effects of being technically without a pulse for an hour or so (other than my wife and the ambulance crew giving me CPR) is that my lens muscles have got weaker. I definitely need reading glasses for anything smaller than about 12 point print. The computer screen - at a distance of about a metre, is in between. Without glasses, it's definitely a bit blurred in one eye (but fine in the other - so I think my brain probably ignores my left eye for it) whereas my eyes feel as if they are straining if I wear my reading glasses for the computer. Maybe I'll have to bite the bullet and get a third pair of in-between glasses for the computer.
When I first found that I needed reading glasses as well as the distance glasses I already had, I was offered varifocals but I didn't get on at all well with them. Although I persisted with them for a couple of weeks, I found that I got weird parallelogram distortion: when I moved my head from side to side, vertical lines tilted one way or the other, depending on whether I was panning from right to left or left to right. The optician had never heard of this side-effect and re-tested me to make sure that she really had measured the strengths and the exact position of the centre of my eyes correctly. She then offered me two pairs of single-vision glasses (reading and distance) at no extra cost, which is a standard arrangement for those people who can't adjust to varifocals.
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