With the rule astigmatism is a common type of astigmatism characterized by specific corneal shape and refractive correction needs.
Astigmatism is an eye condition where the cornea (the front surface of the eye) or the lens inside the eye has a slightly different curve in one direction than the other. This prevents light from focusing properly on the retina, leading to blurred vision at various distances. "With the rule" is one of the main classifications of astigmatism, indicating a particular orientation of this curvature difference.
Understanding With the Rule Astigmatism
According to the provided reference, in with the rule astigmatism:
- Principal Meridians: The eye has two main curves (principal meridians) that are at a 90-degree angle to each other.
- Meridian Shape: The vertical meridian is steeper than the horizontal meridian. Imagine looking at a football where the vertical curve is tighter (steeper) than the horizontal curve.
- Correction: This specific curvature difference typically requires a particular type of lens correction.
How is With the Rule Astigmatism Corrected?
The method of correcting with the rule astigmatism depends on whether a concave or convex cylindrical lens is used:
- It requires a concave cylinder placed at an axis of 180 ± 20 degrees.
- Alternatively, it requires a convex cylinder placed at an axis of 90 ± 20 degrees.
This specific axis orientation (around 180 degrees for concave or 90 degrees for convex) is a defining characteristic of with the rule astigmatism correction.
Key Features Summary
Here's a quick overview of the key features of with the rule astigmatism:
- Principal Meridians: Right-angled
- Steeper Meridian: Vertical
- Flatter Meridian: Horizontal
- Concave Cylinder Axis: Near 180 degrees
- Convex Cylinder Axis: Near 90 degrees
With the Rule vs. Other Types
While "with the rule" is most common, astigmatism can also be "against the rule" (horizontal meridian is steeper) or "oblique" (principal meridians are between 30-60 degrees or 120-150 degrees). The classification helps eye care professionals determine the most effective lens prescription.
