Resolving power of eyes vs glasses?

[Deleted member 4644]

I just realized than when I am 1 foot away from my laptop, without my glasses on I can see individual pixels and even the little "lines" or "spaces" between them on my LCD laptop.

With my glasses on, I really can't see either. (They are distance glasses).

Is that a focus issue, or something about the resolving power of the actual lenses involved?

 

[bobsmith1492]

Your eyes have a limited range over which they can focus... when you're young your eyes are more flexible so the overall range is better. As you get older, the range of focus can diminish. A coworker here has glasses that set his eyes so one can focus further away and one can focus up close so he can see in each case.

Anyway I would bet your eyes can naturally focus better up-close and the glasses interfere with that, shifting your "average" focal point further away.

 

[CycloWizard]

it is probably a little bit of both. Since you have lenses which correct your vision to see at longer distances, things close by will be out of focus when you are wearing them. Your eye may correct for this by accommodating (changing its focal length by adjusting the shape of the lens), but this ability is lost gradually with age. When not wearing glasses, your eyes are disaccommodated at relatively near distances. The degree of disaccommodation depends on your prescription. The loss of resolving power of the glasses is a result of optical aberrations: since the lens cannot be made perfectly, some resolution is always lost when an optical element (i.e. lens in your glasses) is added to the system.

In other words, when viewing through "distance" glasses, the effective optical power of your eye is decreased. Accommodation attempts to increase it to bring near objects into focus by changing the lens in your eye. If the glasses weaken the eye more than the accommodation can strengthen it, then things will be blurry.

 

[Ticky]

Originally posted by: CycloWizard
it is probably a little bit of both. Since you have lenses which correct your vision to see at longer distances, things close by will be out of focus when you are wearing them. Your eye may correct for this by accommodating (changing its focal length by adjusting the shape of the lens), but this ability is lost gradually with age. When not wearing glasses, your eyes are disaccommodated at relatively near distances. The degree of disaccommodation depends on your prescription. The loss of resolving power of the glasses is a result of optical aberrations: since the lens cannot be made perfectly, some resolution is always lost when an optical element (i.e. lens in your glasses) is added to the system.

In other words, when viewing through "distance" glasses, the effective optical power of your eye is decreased. Accommodation attempts to increase it to bring near objects into focus by changing the lens in your eye. If the glasses weaken the eye more than the accommodation can strengthen it, then things will be blurry.

I have an objection to your definition of aberrations. Even a perfectly made optical system will still have aberrations. It's not a manufacturing defect, the problem is that with only a single piece of glass, and with only spheric surfaces, you're really limited in what you can accomplish. Now, there are certainly also manufacturing errors to be taken account of. Also, your average pair of glasses isn't going to be exactly what your eye's need. It will simply be the closest available prescription that the shop can grind. Usually it's accurate to a quarter diopter.

 

[CycloWizard]

Originally posted by: Ticky
I have an objection to your definition of aberrations. Even a perfectly made optical system will still have aberrations. It's not a manufacturing defect, the problem is that with only a single piece of glass, and with only spheric surfaces, you're really limited in what you can accomplish. Now, there are certainly also manufacturing errors to be taken account of. Also, your average pair of glasses isn't going to be exactly what your eye's need. It will simply be the closest available prescription that the shop can grind. Usually it's accurate to a quarter diopter.

I'm not really sure why you interpret, "he lens cannot be made perfectly," to imply only manufacturing defects, as this should also obviously include design difficulties. However, I will point out that, in principle, it is possible to produce perfect, aspheric lenses that have no aberrations in and of themselves. In practice, near-perfect lenses have been produced for some systems (e.g. telescopes), though they're obviously expensive and time-consuming, so they won't be used for glasses any time soon. The only aberration that can't be accounted for by perfectly designing and manufacturing the lens' geometry is chromatic aberration.

It is also interesting to note that the optical surfaces of the cornea and lens are not aspherical, and that changes in the lens shape during accommodation actually minimize certain types of aberrations which are important to the focal distance at which the eye is viewing. The natural optics also have chromatic aberration. The retina and brain can overcome a lot of the optical shortcomings to give a very sharp, apparently analog image from discrete photoreceptors receiving an input with significant errors.