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Anisometropia means the eyeglasses prescription is significantly different between each eye. |
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It is normally not a problem until a 20% difference exists. Then it becomes difficult to adapt to the eyeglasses because of the significant difference in each lens. It can lead to fatigue, eyestrain, distortion of objects, headaches, nausea, and even double vision. It gets even worse when a bifocal is needed.
Why does this happen? The answer is that all eyeglass lenses that have prisms, and the power of the prism gets stronger the closer to the edge you look. A prism moves an object away from its original position in space. It makes the object appear to move in the direction of the thin part of the prism (in this case the lens). You may have noted this in the shrinking of a person’s cheeks when looking at the image of their cheek’s trough their glasses.
This affects us most in a bifocal and can cause double vision with one image appearing above the other. When you use a bifocal whether containing a line or a PAL (no line) you do not look in the center of the lens. In the center of the lens the prism power is zero but it increases toward the edge. Since one lens is stronger than the other there is more prism power in one eye than the other at that location. That location is where the bifocal is placed in the bottom section of your glasses. Let's pretend the right eye is more nearsighted by more than 20% . That makes the image seen by the right eye through the bifocal lower than that of the left. than the left. Then you are unable to merge the images and see double.
How do we solve this? There are three solutions:
How do we solve this? There are three solutions:
1. A slab-off prism in the eye that has the higher power is one solution. The portion of the lens where the prism is started is barely visible. It is similar to a bend in the design of a car. The bend is there but it is not outlined. You can’t see the junction but you can feel it as you rub your finger across it. The slab-off neutralizes the stronger prism and you do not see double when reading.
2. A Shaw lenses are precisely fabricated lenses with varying curves designed to eliminate the distortion that causes a double image. It works but is expensive up to 5 times more expensive than regular glasses. The lenses are named after Peter Shaw, a Canadian optometrist who has a mathematical and logical mind that led to the invention of the lenses. He graduated from my college Pacific University in Oregon.
3. Contact lenses are the best and usually the least expensive method to eliminate the distortion. The center of the lens, any lens including contact lenses, have no prism power in the center of the lens. Since the contact lens is always in the center of your eye, there is no distortion or doubling.