The Base Curve (BC) is a fundamental measurement for contact lenses that determines how the lens fits on the eye’s surface. It represents the curvature of the lens’s back surface, the part that rests directly against your cornea. This measurement relates to the lens’s physical shape, not its vision-correcting power, making it a measurement of comfort and eye health. Getting this dimension correct ensures the lens sits properly, providing both clear vision and a comfortable wearing experience.
Defining the Base Curve Measurement
The Base Curve is a technical measurement that reflects the radius of curvature of the lens’s central posterior surface. This value is measured in millimeters (mm) and is chosen to match the unique curvature of the eye’s front surface, the cornea. For most soft contact lenses, the BC typically falls between 8.0 mm and 10.0 mm, with values like 8.4 mm or 8.6 mm being very common choices.
The number assigned to the Base Curve measurement is inversely related to the steepness of the lens. A lower number, such as 8.3 mm, signifies a steeper or more curved lens, designed to fit a more rounded cornea. Conversely, a higher number, such as 9.0 mm, indicates a flatter lens intended for a less curved corneal shape. Even a small difference of 0.2 mm can significantly affect how the lens feels and functions on the eye.
The Critical Role of Proper Lens Fit
A correctly selected Base Curve governs the relationship between the contact lens and the delicate corneal tissue underneath. The lens must center well over the pupil and maintain a precise degree of movement with each blink to function optimally. A well-fitted lens should float slightly on the eye’s natural tear film, which helps it remain stable and ensures vision remains consistently clear.
This slight movement is essential for the continuous exchange of tears and the flow of oxygen beneath the lens surface. Since the cornea has no blood vessels, it relies on the tear film and the atmosphere for its oxygen supply. When the fit is correct, tears circulate naturally, washing away debris and supplying the cornea with oxygen necessary for long-term corneal health. Without this proper fit, the health of the eye can be compromised.
Identifying Symptoms of an Incorrect Base Curve
When the Base Curve measurement does not match the eye’s shape, the resulting poor fit can lead to two distinct sets of uncomfortable symptoms.
Too Steep (Tight Lens)
If the lens is too steep (BC is a lower number than required), the lens fits too tightly and is often referred to as a “tight lens.” This tight fit creates a suction-like seal on the eye, restricting the flow of tears and oxygen underneath the lens. Symptoms include reduced movement on the eye, a feeling that the lens is sticking, and potential redness noticeable after the lens is removed. Because oxygen is restricted, a tight fit can sometimes lead to corneal swelling and irritation due to the lack of tear exchange.
Too Flat (Loose Lens)
If the lens is too flat (BC is a higher number than required), the lens will fit too loosely on the eye. This results in the lens moving excessively, sliding around the eye when you blink. A loose fit can cause intermittent blurring of vision because the lens fails to remain centered over the pupil. In extreme cases, a lens that is too flat may feel unstable or have a tendency to easily dislodge.
Locating Base Curve on Your Contact Lens Prescription
The Base Curve is a required piece of information on a contact lens prescription and is usually listed clearly with a specific abbreviation. You will most often find it labeled as “BC” on the written prescription or the contact lens box. Occasionally, it may also be labeled as “CR” or “BOZR” (Base Optic Zone Radius).
This measurement is always expressed in millimeters and is distinct from other specifications on your prescription, such as the Diameter (DIA), which measures the width of the lens, and the Power (PWR or SPH), which is the refractive strength. An eye care professional determines the correct Base Curve by using specialized instruments, such as a keratometer or corneal topographer, which measure the precise curvature of your cornea. The correct Base Curve for a specific eye and lens combination should never be guessed or substituted without professional consultation.