Tilting the lens compensates the cylinder?!

Hello everyone,

I was just playing with my test lens kit and I’ve noticed something really strange.

I need about -2.25 SPH and -0.75 CYL for close up. (Just for context)

The axis of the CYL is 170° (nearly 180° which is key for what I’m about to show you).

So looking only through a SPH lens I see directional blur (of course). Picture 1

But whenever I start to tilt the lens upwards Picture 2 the directional blur start to disappear until it is gone completely.

The same thing applies whenever I tilt my head upwards and look through the bottom part of the lens. Picture 3

I don’t know how exactly CYL works behind the scenes but I assume I was able to emulate a CYL correction when tilting the lens upwards at around (180° which is pretty close to my axis - 170°).

But why is this important? Well, because my computer screen is below eye level, The bottom part of the screen is crystal clear unlike the rest which makes it harder to work out my differentials… Any input would be appreciated, would love to start a discussion about this!

When you tilt a lens you change the vertex distance of part of the lens (distance between lens and eyeball), increasing or decreasing the power of that part of the lens. This could provide the equivalent of a small cylinder correction in the lens - depending on angle of tilt, as you discovered.

It might be a good idea to work with your computer screen at eye level, and not below, and this might also help you to find the correct diffs. I also have better vision below eye level, even when checking on the Snellen chart (which I can do going up the steps of my stairs). Changing your screen to eye level will also change your posture, and you will have to make sure you do not angle your head too far back to create the same downward gaze, as this could create neck and back problems. I have compromised by placing my screen only slightly below eye level.


A monitor mount is the next in my TODO list. So for now when choosing the diffs while playing with the test lens kit I’ll just elevate the monitor on some books to prevent myself from cheating with this “downward gaze”… Just wanted to know that I am indeed compensating the CYL and not that I don’t need it in the first place…

Made a rough and ready one from scraps of wood myself, and it does the trick nicely, raising my laptop 12cm above my external keyboard. Typing on my laptop keybord was what got me too used to the downward gaze. .

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tilting the lens also increases the distance the light travels through the lens to your eye, this seems to increase the apparent power of the lens so it of course improves vision on the weaker axis as well


Thanks - nice new bit of information (at least new to me).

@Lajos Dipped a toe into the refraction of light, and quickly pulled it out again. I will have to take this on trust.


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I always though it was the other way around - when you bring the lens closer to you eye it becomes stronger, that’s why when converting from glasses to contacts you reduce by 0.25…

you don’t have to take it on trust Han, just try it with your own glasses - if you tilt the lens, the view becomes sharper

This is irrelevant - you are talking about something else

yes, bringing the lens closer decreases the vertex distance and increases the power
and no, you don’t simply reduce by 0.25 when converting from glasses to contacts, you use the formula here https://en.wikipedia.org/wiki/Vertex_distance (so you don’t change much below -3 and you change more than 0.25 above around -6 )

I was talking about increasing the length of the path of light through the lens material - a diagonal line across a rectangle is longer than a straight line through it from one side to the other (pythagoras), so yeah, you can see if you tilt the lens you increase the light’s path length within the lens, but you might also decrease the vertex distance of part of the lens

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Will do so.

Oh, I see what you mean now. :+1:

Are you just tilting the lens, or are you also looking through a different part of the lens? By “tilting”, I’d imagine you still look through the same part of the lens, just with the lens itself tilted.

The reason I ask is that most of my glasses have lenses that are slightly astigmatic when not looking through the optical center. I had one pair (the last ones I ever got from an Opto) where the astigmatism was rather high when looking to the side. It’s annoying because the effecive sphere correction goes higher off-axis, meaning stuff appears sharper to the side, but had the obvious sign of astigmatism (the sharpness was biased in one direction).

If that is happening to you, it’s possible that the lens-induced off-axis astigmatism is aligned perfectly to counter your eye’s astigmatism.

Maybe not just small - when I tilt high prescription glasses around their horizontal axis (they were with less cylinder than needed), I end up with so much cylinder that all around looks like in curved mirror (and I get very severe opposite angle directional blur). The same is with -4 lenses without astigmatism.