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Astigmatism 101: Soup spoon vs. Teaspoon
One of the most mystical terms used in ophthalmology is astigmatism.
Well, what is it?
Uncorrected astigmatism is a frequent reason why people cannot see clearly.
Okay, but what is it?
Astigmatism keeps many people from wearing soft contact lenses.
Enough already! What is astigmatism?!?
First things first. Let's start with a perfect uncorrected 20/20 eye. This eye clearly "sees" an object because the eye is precisely focusing light rays that come from the object. Follow me so far?
The outer surface of the normal, healthy, clear cornea is smooth, even, and round. We call the surface spherical because its shape is just like half of a sphere - like half a racquetball. We've previously mentioned that two-thirds of the eye's focusing power occurs along the front surface of the cornea. Curvature of the cornea contributes greatly to that focusing power.
Light rays bombard the cornea from every conceivable angle: straight down the middle (axial), horizontal, vertical, and every oblique angle in-between. Since the curvature of the spherical cornea is uniform and identical along every conceivable meridian (any line that passes through the center of the cornea and connects opposite points at the edge of the cornea, like 0 and 180 degrees - better known as 3 and 9 o'clock), every ray is equally focused.
What happens when it is not?
At long last. Astigmatism loosely means "not spherical"*. Eyes (corneas) with astigmatism have an uneven curvature. Let's stick with the ball analogy. Imagine cutting a football in half tip-to-tip along its long axis. Right away you can appreciate how the curvature of this 'hemi-football' is flatter along the longer meridian and steeper along its shorter meridian. For all nonspherical surfaces there is a steepest meridian and a flattest meridian. The difference in curvature between these two meridians creates cylindrical error, the quantity of astigmatism.
Want to easily demonstrate some astigmatism on yourself? Look at your reflection in a round silver soup spoon and compare what you see with an oval teaspoon.
Don't be frustrated if you had trouble absorbing all of this information. Next time we're together we'll apply some of these optics to everyday real-world vision correction.
* Here's the fine print: The etymology for astigmatism more accurately means "without a point". Spherical surfaces focus the image at a single point. Uneven curvature prevents that from happening. Less frequently, the crystalline lens can also generate some astigmatism.
Related Topics: Eye on New Vision Procedures , Eye Cell Implants Help Parkinson's
Technorati Tags: astigmatism, contacts, glasses, vision
Well, what is it?
Uncorrected astigmatism is a frequent reason why people cannot see clearly.
Okay, but what is it?
Astigmatism keeps many people from wearing soft contact lenses.
Enough already! What is astigmatism?!?
First things first. Let's start with a perfect uncorrected 20/20 eye. This eye clearly "sees" an object because the eye is precisely focusing light rays that come from the object. Follow me so far?
The outer surface of the normal, healthy, clear cornea is smooth, even, and round. We call the surface spherical because its shape is just like half of a sphere - like half a racquetball. We've previously mentioned that two-thirds of the eye's focusing power occurs along the front surface of the cornea. Curvature of the cornea contributes greatly to that focusing power.
Light rays bombard the cornea from every conceivable angle: straight down the middle (axial), horizontal, vertical, and every oblique angle in-between. Since the curvature of the spherical cornea is uniform and identical along every conceivable meridian (any line that passes through the center of the cornea and connects opposite points at the edge of the cornea, like 0 and 180 degrees - better known as 3 and 9 o'clock), every ray is equally focused.
What happens when it is not?
At long last. Astigmatism loosely means "not spherical"*. Eyes (corneas) with astigmatism have an uneven curvature. Let's stick with the ball analogy. Imagine cutting a football in half tip-to-tip along its long axis. Right away you can appreciate how the curvature of this 'hemi-football' is flatter along the longer meridian and steeper along its shorter meridian. For all nonspherical surfaces there is a steepest meridian and a flattest meridian. The difference in curvature between these two meridians creates cylindrical error, the quantity of astigmatism.
Want to easily demonstrate some astigmatism on yourself? Look at your reflection in a round silver soup spoon and compare what you see with an oval teaspoon.
Don't be frustrated if you had trouble absorbing all of this information. Next time we're together we'll apply some of these optics to everyday real-world vision correction.
* Here's the fine print: The etymology for astigmatism more accurately means "without a point". Spherical surfaces focus the image at a single point. Uneven curvature prevents that from happening. Less frequently, the crystalline lens can also generate some astigmatism.
Related Topics: Eye on New Vision Procedures , Eye Cell Implants Help Parkinson's
Technorati Tags: astigmatism, contacts, glasses, vision
