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The tear lens (TL) is an often ignored and under-appreciated component of rigid contact lens success. The tear lens is formed by the back surface, or base curve (BC), of the contact lens and the front surface of the cornea (K).

The 8.4mm base curve is still the likely best fit for the majority of eyes. In instances when the 8.4mm lens is too steep, the 8.8mm lens allows a flatter option. This is more likely needed in smaller eyes, and possibly in some very flat corneas.

If you wear or want contact lenses, you'll need to get a contact fitting in order to get a prescription. A contact fitting involves your eye doctor identifying what strength is needed for your eyes and what type of contact lenses will correct your specific refractive error.

Spherical Lenses Spherical contact lenses are an ideal choice for those who are nearsighted(myopia) or farsighted (hypermetropia). These are the common types of contact lenses prescribed for myopia or hyperopia.

Simple Rule for estimating lens base curve on a PLUS POWER lens For plus power use the spherical equivalent (SE) and add 4.00 diopters to that. For example, if you have an Rx of + 2.00 sphere, the base curve for the lens will be approximately 6.00. Rx +2.00Sph -> [+2.00 +4.00D] = 6.00BC.

Sag value: When talking about the scleral lenses, Sag value can be defined as the space between cornea and the lens or the Vault. Sag value can be manipulated in many ways by different lens manufacturing companies.

The lens should align with the cornea and the conjunctiva, and not indent conjunctival vessels. Indentation would indicate stagnation of tears in this region and reduced oxygen supply to the limbus. Similarly, the lens should also show no edge standoff (fluting), which would lead to discomfort.

Power. The power of the tear lens is approximately 0.25 D for every 0.05-mm radius-of-curvature difference between the base curve of the contact lens and the central curvature of the cornea (K), and this power becomes somewhat greater for corneas steeper than 7.00 mm.

The Dioptre formula is used to calculate the optical power of a lens or curved mirror. The dioptre is the unit for a measure of the refractive index of a lens. The power of a lens is specified as the inverse of the focal length in meters, or D =1/ f, where D is the power in dioptres.

?Lag? refers to the difference in distance from contact lens edge to limbus in the same location (nasal, temporal, superior, or inferior) when looking in primary gaze compared to peripheral gaze.