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There are two general categories of contact lenses ? soft and rigid gas permeable (RGP).

Although traditional training and fitting guides recommend assessing lens movement on right and left gaze (known as lag, figure 5) and on up gaze (known as sag, figure 6), studies indicate that these measures have little predictive value in deciding if a lens fit is ideal or not,8,10 though movement on horizontal gaze ...

?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.

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.

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 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.

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.