Multi-focal spectacles?

food for thought: how can myopia (short-sightedness) and hyperopia (long-sightedness) occur simultaneously in a person? my mom recently got multi-focal spectacles, which is why i'm wondering.

from what i understand, myopia occurs because the image falls in front of your retina, while hyperopia occurs because the image falls at the back of your retina. if they were both to the same degree, wouldnt they cancel each other out? perfect vision again then, haha :)

otherwise, if your myopia is to a higher degree than your hyperopia or vice versa, wouldnt one kind of lens suffice? as in, you're either long sighted, or short sighted coz they cancel each other out. no two ways about it, no?

any opthalmologists or optometrists? thank you in advance.
Answer:
Multifocal spectacles are usually for compensating for a progressive degeneration of the optic lens called presbyopia (old eye). Presbyopia typically gets bad enough to become noticeable at about age 40 or so and is quite common in middle aged and older adults. In this phenomenon, the lens loses its ability to change shape as it is tugged and squeezed by the eye muscles. As a result, the person typically loses the ability to focus clearly on close-by objects.

To correct for this, an extra amount of convexity is put into that part of the lens used to focus on closeby objects and left out of the part used for viewing more distant objects. In fancier versions, more than one amount of compensation is included for viewing objects at several distances. In older or less expensive versions of multifocal spectacles, there is an abrupt transition between these two parts of the lens. Wearing multifocal lenses is a sign of advancing age, so some people prefer a more gradual transition to disguise the fact that they are wearing such lenses.

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No, because you have two different types of lenses, concave and convex.
Concave lenses are used to treat shortsightedness, and convex are for farsightedness.
Bifocals have concave on top and convex below.
The reason why they don't cancel each other out is because the lenses that refocus far-sight actually decrease your near-sight and you have to compensate with a magnifier.

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3 kinds of "vision problem"
1) myopia (shortsightness) eyeball too long
2) hyperopia (long sightness) eyeball to short.
3) presbyopia (readng problems) focusing lens within eyeball hardens and muscles weaken

so what you mum experience is shortsightness plus presbyopia which occurs as early as late 30s. glasses for dist allows her to see distance but she will not be able to see near because her muscles are not strong enough to change focus to a macro view( very much like camera lens) so multifocal enables that to happen.

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