My spectacles defy the laws of physics

[Steve4Physics]

TL;DR Summary

Why do my spectacles apparently violate the Helmholtz reciprocity principle?

For curiosity (obviously not having anything better to do with my time) I turned my spectacles the ’wrong way round’ - so that they were upside down, with the arms pointing outwards - and looked though them. (This gives the correct lens for each eye of course.)

The image is noticeably worse than with the spectacles the right way round.

I tried different lens-eye distances but this doesn’t affect the image enough change to account for the difference. I tried looking through different parts of the lens but this made virtually no difference.

I can’t explain this. I thought the 'Helmholtz reciprocity principle ('principle of reversibility') would ensure that a lens did the same whichever way round it is. But maybe I’m missing something.

Any explanations welcomed!

(The lenses are for distance and corrected for some astigmatism. )

 

[Ibix]

The axis of the astigmatism is flipped - if it looks like \ it becomes / under the reversal. Mine improve to near perfection if, after flipping them, I rotate them in the plane of the lenses until (I presume) the astigmatic axis is correct. I'm not a snail, so I can only do that one eye at a time, obviously...

 

[Steve4Physics]

The axis of the astigmatism is flipped - if it looks like \ it becomes / under the reversal. Mine improve to near perfection if, after flipping them, I rotate them in the plane of the lenses until (I presume) the astigmatic axis is correct. I'm not a snail, so I can only do that one eye at a time, obviously...

Aha - it's obvious now you've said it! Thankyou - I will now be able to sleep at night!

 

[Redbelly98]

I would imagine that spherical aberration is much worse with the glasses pointing the wrong way. There's a reason why both the inner and outer lens surfaces both generally have the concave side toward your face.

 

[Steve4Physics]

I would imagine that spherical aberration is much worse with the glasses pointing the wrong way. There's a reason why both the inner and outer lens surfaces both generally have the concave side toward your face.

That sounded interesting so I tried an experiment.

With the glasses 'the wrong way' I rotated them to get the astigmatism axis correct (for one eye at a time of course). Vision in each eye was then (at least subjectively) the same as when wearing the glasses normally.

I also tried it with some (non-prescription) reading glasses. Having the glasses the wrong way made no perceptible difference.

I presume any spherical aberration/concavity effect is small enough to be neglected.

 

[Ibix]

I rather suspect your eye itself is the major contributor to the aberration in your optical system, even with your glasses correcting the worst of it. You might be able to see a difference in the appearance of a point source in a very dark room with your glasses either way around, but my guess is that the differences are too tiny to be visible.