Choosing the correct contact lens size is a precise process that involves two primary measurements: the diameter (DIA) and the base curve (BC). Both measurements are determined exclusively by an eye care professional through a comprehensive examination and fitting. You cannot choose these measurements yourself.
The "size" of a contact lens refers to specific parameters tailored to your eye's unique anatomy to ensure comfort, clear vision, and eye health.
1. Diameter (DIA)
The diameter is the total width of the contact lens from one edge to the other, measured in millimeters (mm). This measurement ensures the lens properly covers your cornea.
- Average Diameter: The average contact lens diameter size is around 14mm, given that the average human cornea is about 12mm in diameter. This allows for proper coverage and centering.
- Individual Variation: Your specific eye size influences the ideal diameter.
- If you have smaller eyes, you may be closer to the 14mm diameter size.
- For those with bigger eyes, the diameter may land more towards the 14.5mm end of the range.
- Importance of Fit: A correctly sized diameter ensures the lens sits securely on your eye without causing discomfort or moving excessively. If a lens is too small in diameter, it might feel like it's sliding around or causing irritation. Conversely, a lens that's too large could wrinkle or be difficult to insert and remove.
2. Base Curve (BC)
The base curve measures the curvature of the back surface of the contact lens. It's designed to closely match the natural curvature of your cornea, ensuring a comfortable and stable fit. This measurement is also in millimeters (mm). A lower BC number indicates a steeper curve, while a higher BC number indicates a flatter curve.
- Matching Corneal Shape: The base curve is critical for the lens to rest properly on your eye.
- Too Flat: If the lens's base curve is too flat for your cornea, the lens will fit loosely, move excessively with blinks, and can lead to discomfort, blurred vision, or even corneal abrasion from friction.
- Too Steep: If the lens's base curve is too steep, it will fit too tightly against your eye, restricting the tear exchange and oxygen flow to your cornea. This can cause redness, irritation, blurry vision, and increase the risk of eye infections.
Why Professional Fitting is Essential
It is impossible to self-determine your correct contact lens size. A licensed eye care professional (optometrist or ophthalmologist) must perform a comprehensive eye exam and a specialized contact lens fitting.
During a professional contact lens fitting, your eye care provider will:
- Measure Your Eyes Accurately: They use advanced instruments, like a keratometer, to precisely measure the curvature of your cornea and assess your eye's overall dimensions.
- Evaluate Tear Film: The quantity and quality of your tears are assessed, as this impacts lens material choice and comfort.
- Provide Trial Lenses: Based on your measurements and visual needs, you'll be given trial lenses. Your eye care professional will then carefully observe how these lenses fit on your eyes, how they move, and evaluate your vision with them.
- Monitor Eye Health: They will ensure the lenses are not negatively impacting your eye health over time, often scheduling follow-up appointments.
Signs of an Ill-Fitting Contact Lens
Wearing contacts that are the wrong size can lead to various problems. Seek professional advice if you experience any of the following:
- Persistent Discomfort: The lenses consistently feel scratchy, dry, or irritating.
- Eye Redness: Your eyes appear frequently red or bloodshot.
- Variable Vision: Your vision is intermittently blurry or fluctuates, especially with blinking.
- Excessive Lens Movement: The lens slides around too much, making it difficult to maintain clear vision.
- Lens Feels "Stuck": The lens is difficult to remove or feels uncomfortably tight.
- Increased Light Sensitivity: Your eyes become more sensitive to light than usual.
Quick Reference: Contact Lens Parameters
| Parameter | Abbreviation | What It Measures | Typical Range | Impact of Improper Fit |
|---|---|---|---|---|
| Diameter | DIA | Total width of the lens | 13.8 - 14.5 mm | Too loose (slides, irritation) or too tight (restricts tears) |
| Base Curve | BC | Curvature of the back of the lens | 8.3 - 8.9 mm | Too flat (excessive movement, discomfort) or too steep (oxygen deprivation, tightness) |
| Power | PWR / SPH | Corrective strength (for vision) | Variable | Blurry or distorted vision |
In conclusion, choosing contact lens size is a critical and specialized process performed by an eye care professional, ensuring your lenses are perfectly matched to your eye's unique diameter and curvature for optimal vision, comfort, and eye health.
