The upper part of your glasses simply acts as a stronger lens by tilting it (albeit with less perfect focusing properties than the center of your glasses, the lower part should also work like that - try tilting your glasses a bit, then you get a thicker lens). Don’t do it, that’s the equivalent of just putting stronger glasses on, but with worse aberrations due to the off-center focus (point is you could get astigmatism with aberrations - they’ve already tested using non-uniform glasses on animals and the shape of their eyeballs distorted). You can play with this (pick beam) and see the aberrations caused off center + the slightly translated focal plane: https://benedikt-bitterli.me/tantalum/tantalum.html
@FMR, I notice very sharp vision when I tilt my head up. Normal:20/30+3(20/20 line) but with little shadows, with tilting my head up: crisp 20/20, or sometimes even 20/15. Note that, I have recently dropped my CYL, so I thought it is either due to Astigmatism or the effect is due to thicker glass range while tilting head up. My glasses are RE: -8.5, LE:-8
It happens to me on the bottom part of the lens and might just be that the edge has more power than the center because it’s thicker
It is due to the lens being thicker when you tilt it as explained - thicker lens = stronger lens. However since it’s off center it’s not perfectly focused, as you can see if you play with the interactive raytracing example I’ve posted. Basically you get astigmatic aberration.
Well, it turns out I was basically looking through the bottom 3rd of the lens before. I thought I was centered but wasn’t.
Fixed it and got rid of the astigmatic blur that had been bothering me for weeks. Things are almost crystal clear now, just a hint of myopic blur now, as it should be.
It’s still slightly sharper if I tilt my head, but then the astigmatism sets in.
So it was two issues going on…the lenses being too high on my face, and the undercorrection.
I solved the former.
Thanks for all the help.
@Aparna Could it be that the optical centres of the lenses (i.e., where there is least deviation from the lens prescription) are too low, so that only when you tilt your head up (or gaze downwards) do you actually look through them?
Perhaps try adjusting the nosepad to sit the glasses higher up?
And when replacing its lenses next time, perhaps check with the optician if the lenses’ OC height needs to be increased? I wrote more about this issue here: What should be the PD for Differential?
@FMR Could it be that the optical centres of the lenses were too high? I.e., the opposite of Aparna’s problem? Just a guess
No, I feel the explanation provided by @420Mamut is more correct. I also read the other post written by you which you linked…Thanks. Because, the optical Center is defined by PD which was carefully measured by Opto, so it cant be wrongly placed. Moreover, this thing (clarity while tilting my head up) is there in all glasses.
Also, I do not find it clear when I hold the glasses up or down. Its only when I am wearing glasses normally and tilt my head up. One more thing is my astigmatism is also in that direction, so perhaps the stronger lens corrects that. I have a habit of taking my head little forward and tilting up while working on PC. I am trying to correct the posture problem whenever aware.
Hi Aparna, great that you seem to have figured out the problem; all the best with your posture correction, etc.
This isn’t accurate: your (total) PD specifies only the horizontal distance between the optical centres of the two lenses; it is independent of the lenses’ OC (optical/ocular centre) height, which is measured from the bottom of the frame. If the OC height is not specified, the lens-cutting machine defaults to a certain setting.
But don`t the Optos make a mark on frame where Pupil is? I thought its pretty accurate as otherwise it would cause strain to eyes? I am stressing this assumption because if OC is off, it cannot be off in all the glasses, right!
Regarding relation between Posture and astigmatism, I saw a video by Nathan oxenfeld and Esther, where they explain about horizontal/vertical astigmatism, which has interesting info. Here is the link:
After seeing this video, I noticed my posture while working on laptops, as for typing on the keyboard, I tend to move my head forward and lift the chin up. so, perhaps thats the reason for astigmatism. Now, with glasses, if I tilt my head and chin upwards perhaps through the stronger lenses light rays fall at an angle on cornea rather than straight and thus clearing my astigmatism. I hope the explanation is understandable.
Anyways, its good that we are having discussion, it really improves the knowledge! Thank you!
Hi @Aparna , I don’t really understand your last sentence above.
While ideally the optician checks the client’s gaze through the new spectacles - to judge if the OC height needs to be adjusted for the usage purpose (book/computer/distance) - before sending it off to the lab for lens-cutting, I’ve rarely had an optician do that for me, certainly not in the last decade. Of course they do always measure your PD, but again PD (horizontal) and OC height (vertical) are two independent components of OC position.
I don’t think eyestrain necessarily occurs if the OC is too high/low or the PD is a few mm’s too small/big.
If your optician has precisely and correctly specified the OC on your leses, then everything I’ve said about OC height does not apply to your case.
Yes I viewed this video a couple of weeks ago and found it interesting too, particularly the part saying a minus cylinder along a roughly-horizontal axis suggests that the medial & lateral rectus muscles are getting more workout than the other extraocular muscles.
This seems to be based on or related to Elliott Forrest’s theory/work, but I haven’t read enough of the literature to have a considered opinion on this. I would love to hear what more knowledgeable folks in this EM forum think about this.
In particular, if my minus cylinder’s axis is indeed horizontal, am I (1) gazing left-right too much, or (2) gazing up-down too little? The video seems to suggest the former (i.e., that I should instead move my head left-right when reading a looong tine of text, instead of keeping my neck stiff), but if I do that, aren’t I then limiting the use of my peripheral vision, which is also bad??
p.s. I created a new thread because I’m going off-tangent in this thread!: Esther's video on reversing astigmatism
Ok, there is a new complication and it’s just weird now.
I took off my differentials and experimented. Guess what. The downward head tilt still improved my vision without glasses on.
There is something going on here. It’s innocuous, but still something. I have a suspicion of a couple of possible causes.
Can you guys do the same experiment?
That may be some element of astigmatism at play. The downward tilt doesn’t work for me, but I still have relatively high myopia (-4.50).
Well I did (without glasses, hold phone at edge of blur and start tilting my head down). And it definitely got better, but it’s definitely because of the head tilt my eye is just getting closer to the screen I’m watching. You don’t tilt your head from your eyes, but way more down from the neck. So as you tilt your head, your eyes get closer to the screen. It’s not obvious from distance, but it is doing it at the edge of blur. If I compensate for this motion then there is no difference.
Dr. Elliott Forrest’s research on astigmatism described @15:00 to 22:00 made a lot of sense. I sometimes lean my head on my supported left hand while reading the screen.
I have high Myopia -8.00, so my answer to this experiment may not have much significance. But, I still tried holding a text near to me about 12 cm, I had a different result: with glasses when I tilt head upwards vision improves, but without glasses when I tilt head downwards vision is better? I am not sure what is happening here.
I’m trying to become more conscious of head angle when reading. I think people build some extra plus power into the bottom of the cornea from reading habits. What other explanation is there?
I think you’re on to something with your assessment. I can’t explain it, but I think your right. If you think of progressive lenses, this concept works the same way.
A question I have been waiting for. My gifted kid age 8 does the same thing when she’s too far from the board or in the school audi. No idea how she figured it out. For her it’s the lower part of the lens frame. She tilts the glass downward. Claims it helps. It can’t be good for active focus right? She’s wearing a 20/50 correction -1.5 both eyes, no astigmatism, no bifocal, no progressive . I have never needed glasses so ‘getting’ her on this one is hard. But there is probably some reason why it works. She also figured her eyesight was better through her tears all by herself. Hmm. If kids find a tool they use it. Thinking.
Did she confirm to you it was 20/50 on a Snellen? Does she know how to active focus? If so, she’s already ahead of the game!
I am joining this conversation a bit late, but it interests me as I also have the tendency to tilt my head upwards to get a downwards gaze when working at my computer, and this is without glasses. When I started using the Snellen, I found that a downwards tilt also gave me better vision. I discovered this because my 6m Snellen is opposite the stairs going to my mezzanine. 6m is sitting on the 3rd step of the stairs, which gives me a direct gaze from eyes to chart. Sitting 5 steps higher up is 1m further and 1m higher, giving me a downward gaze to the chart. I can see almost as well sitting on the 8th step as on the 3rd. If I stand on the 3rd step, I also get the downward gaze (and better vision), but it is more comfortable sitting on the steps when doing AF for any length of time I figured out that this is because of my droopy eyelids (from an early age) which interfere with a direct gaze but not a downward gaze. It is the same when I measure cm distance, both for the stronger eye and the more myopic one - a downward gaze along the 60cm ruler gives slightly better distance to blur. It costs me extra effort to raise my eyelids. I am also astigmatic (oblique, mirror image) at 50 and 110, but cannot really see a reason why this would make me prefer a downward gaze. So for me I suspect it has nothing to do with glasses , but with eyelids (and probably also many, many years of reading many, many books )