Difference in focus between lenses and glasses [closed]

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).

closed as off-topic by StephenG, sammy gerbil, Kyle Kanos, Bill N, Cosmas ZachosJun 24 '18 at 18:10

• This question does not appear to be about physics within the scope defined in the help center.
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• I'm voting to close this question as off-topic because this is really a question that should be addressed to an optician familiar with the person involved. – StephenG Jun 4 '18 at 22:09
• Same eyeballs, two different sets of lenses, two results; what does it have to do with the person? it's the only part that is not a variable. – Thomas Jun 4 '18 at 22:12
• @StephenG This is a basic question about optics, not a personal request for medical advice. – PM 2Ring Jun 4 '18 at 22:31
• Anyone with the knowledge to answer this question will realise you're a little short-sighted, but it wouldn't hurt to mention the strength of your glasses/ lenses (a rough approximation is sufficient, there's no need to reveal personal details). My guess is around -2 to -4 dioptres. – PM 2Ring Jun 4 '18 at 22:41
• @Thomas An eyeball in contact with something is fundamentally different from an eyeball with a correcting lens at some distance from it. – StephenG Jun 4 '18 at 23:16

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 closer to the eye than the optician was expecting.

You can experiment with this and let us know - does sliding the glasses towards the tip of your nose make it harder to read close-up?

The equation describing the effect (assuming two thin lenses with focal length $f_1$, $f_2$ and separated by a small distance $d$) is the Back Focal Length

$$\rm{BFL}=\frac{f_2(d-f_1)}{d-(f_1+f_2)}$$

• As a long time glasses wearer, I can assure you that this is backward: negative focal length lenses are functionally stronger when closer to the eye and weaker when further. That’s why glasses prescriptions are significantly stronger than contacts prescriptions. – Ben51 Jun 5 '18 at 6:13
• @Ben51 it’s late and you are probably right that I misstated that. I will edit this when I am fully awake... – Floris Jun 5 '18 at 6:21

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.