Well, can a person have nearsightedness and farsightedness in the same eye? I also want to know if people get bifocals to correct problems like this. Or is there another reason for essentially having two different prescriptions for the same eye?
My current assumption is that people get bifocals if they have myopia, but then as they start getting older, they get presbyopia too, so they need the bifocals to correct the problems. Or, they were farsighted and nearsighted in the same eye, so they needed bifocals to correct the problem.
Are there any other reasons for wearing bifocals besides what I just stated above?
I don't think this problem applies to me--I am just curious. Yes, it is possible, as Judy B states, with the condition called "mixed astigmatism" where one meridian of the eye has a positive RX and one has a negative.
+2.00 DS /-3.00 DC ax 75, for example.
If someone has -1.00 distance Rx but needs a +2.00 reading addition, yes, bifocals for that will have + and -components in the spectacle lens, but that wouldn't really be considered "farsighted and nearsighted in the same eye" as the prescription elements are powered to work for tasks at different distances, rather than the eye having two Rx's at the same time.
There is one rare condition (which I 've seen a few times) where the eyes actually are entirely short and longsighted at the same time. In Marfan's syndrome some individuals develop partially dislocated lenses, which often have some degree of cataract.
Here, with the lens only covering part of the pupil, it's possible to obtain a prescription in the region of +12.00 and -8.00 in the same eye, depending on which part of the pupil the refraction is taken through.
It's usual to do both, and see which the patient prefers.
But not use + in one eye and - in the other.
There will also be a reading addition in such cases!
Medical picture below
Anwering your last question, there are two other reasons for giving bifocals, and these in young people.
There is a theory followed by some practitioners that reducing accommodative effort can reduce the progress of myopia. A reading addition is thus given on a myopic base precription even though there is still adequate accommodation. This is often combined with prismatic elements.
Separate to this, cases of over-convergence associated with accommodation may also benefit from bifocals to assist binocular stability at near.
Optometrist, retired. The only way to be farsighted and nearsighted is if you have significant astigmatism. Then you could be farsighted for vertical lines and nearsighted for horizontal lines. You would not need bifocals to correct that problem
Most people get bifocals because they are presbyopic, which is not the same as farsighted. Farsighted (hyperopic) means the eye does not have enough power to focus and extra power is gained through accommodation of the lens. Non hyperopic people use accommodation to change the focus of the eye for near viewing but hyperopes use it to clear the distance and then use even more to clear near vision.
Accommodation declines with every year and by age 40 or so most people can no longer accommodate enough to see clearly at near and need reading glasses. Hyperopes will then need glasses to clear distance vision and even stronger glasses to clear near so get bifocals. Myopes (nearsighted), as you have already figured out need bifocals too.
Reading glasses blur the distance vision so even people who are neither myopic or hyperopic will get bifocals for the convenience of not taking reading glasses off and on. Not really sure what you mean by 'farsighted and nearsighted in the same eye".
But there are conditions where people need bifocals because they have trouble focusing up close, no matter what their distance eyeglass prescription is. So if a person is nearsighted, meaning they need glasses to see far, they could have a problem, where, when they put their glasses on, they can't see near. So they need bifocals or they need to take their glasses off to see up close.
If a person is truly farsighted, meaning they need glasses to see up close, then they can't be nearsighted too. |
