Difference in focus between lenses and glasses

2018-06-05 08:21:08

I have both glasses and contact lenses.

The prescriptions are both recent and up to date.

I am 46, so I'm starting to have a harder time to focus on things that are very close to my eyes, compared to 10 years ago.

With lenses, I notice it takes some efforts to read text very close to my eyes, but with my glasses, I don't have the same problem at all.

Does the distance between the eye and the lens explain this? and, if yes, how? or could it be other factors (lower material quality, lenses rounded by .25, etc).

Your eye plus the corrective lens (whether on the surface, or some distance away) makes an "effective" lens with a focal length that depends on the distance between the eye and the lens.

When you are using contact lenses, that distance is fixed; when you have glasses, that distance is variable (by sliding the glasses towards the tip of the nose, or closer to your eyes).

Now typically the effect of glasses gets stronger as the lenses slide towards the tip

  • Your eye plus the corrective lens (whether on the surface, or some distance away) makes an "effective" lens with a focal length that depends on the distance between the eye and the lens.

    When you are using contact lenses, that distance is fixed; when you have glasses, that distance is variable (by sliding the glasses towards the tip of the nose, or closer to your eyes).

    Now typically the effect of glasses gets stronger as the lenses slide towards the tip of your nose: if you have positive diopter lenses (because you are far-sighted), you can see things closer up by sliding the lens further away; if you have negative diopter lenses (as you state), then pushing the lens closer will help.

    Your optician selected lenses that had a certain curvature, expecting that they would give appropriate correction at a particular distance from the eye; but your glasses are working better than the contacts when you are looking close up, which tells me that you probably push your glasses a little cl

    2018-06-05 08:42:17
  • I believe it is because of the effectively smaller aperture when using glasses. Blurriness depends not only on how close the image distance is to the location of the retina, but also on the size of the aperture. A pinhole doesn’t even have a focal length, but can form a reasonably clear image by restricting the aperture. If you don’t have your glasses, you can see distant objects clearly by looking through a small hole. The focal length of the lens in your eye is unchanged, but by eliminating all the rays far from the central axis (the ones that require a larger angle of refraction) you get rid of the blurring.

    Now, moving a concave lens in front of your eye further away does effectively the same thing. Any rays far from the central axis are bent outward to the point that they no longer enter the pupil. As the distance grows, the total amount of light entering the eye decreases, but the incorrect focal length produces less blurring.

    2018-06-05 08:58:21
  • First thing might be presbyopia. You need bifocal contacts.

    Or you could have a focusing problem, where vision therapy helps.

    Or what you need to know is that eyglasses prescription is mostly different from lenses prescription, simply because lenses are not available in all power.

    You need to know too that lenses are not fixing astigmatism (cylinder). Your question does not say whether you are near or far sighted. If you are farsighted (can't see far) then I have the same problem with the lenses. It is just optics, lenses are making things look smaller and since they are close to your eyes, your eyes cannot fix this downsizing. When you are using glasses, your eyes can still fix this problem a little bit.

    2018-06-05 09:08:21