The tear film is the first and most directly influential factor in the fitting process. It represents the initial point of interface between the contact lens and the eye. The tear film covers the entire front surface of the eye and has a multitude of important functions.

Among these are:

  • Lubrication of the sclera and cornea
  • Removal of debris
  • Oxygenation of the cornea
  • Provides a smooth surface of refraction
  • Protection of the eye from infection


The tear film is made up of three separate layers each playing its own part in maintaining the system. The anterior-most layer is composed of lipids that slow evaporation, keeping the cornea hydrated between blinks. The middle layer is aqueous containing salts and antibacterial substances to fend off infections. The inner layer is mucoid. Its primary function is to anchor the tear film. A breakdown in any one part of this system can result in dry eyes. A patient with dry eyes represents a challenge as a contact lens fit. Why is this? When a lens rests on the eye, be it rigid or soft, it is actually floating on the tear film. This allows it to sit and move comfortably during blinking. The cornea demands oxygen filtered through the tear film to maintain clarity as well. If tears cannot flow under the lens, metabolism will be compromised and corneal edema will result. For this reason observing lens movement is an important step in the fitting process. Too much movement may produce irritation and poor vision but too little would create the above mentioned condition. The tear/lens relationship is not only affected by the dimensions of the contact but also by the material as well. A soft lens flexes to match the shape of the eye creating a uniform thickness tear film beneath. A rigid contact will produce a different circumstance. In a lens that is steeper than the corneal surface, more tears will pool in the greater space beneath the central portion than the periphery. This is popularly referred to as the 'tear lens.' A tear lens that is thick centrally and thin peripherally will effectively add plus power to the over all correction. For this reason the power of the tear lens must be considered when selecting the power of the lens.

Consider, for clarification, the following example. A woman with -5.00 diopters of myopia is fit with a GP lens that is slightly steeper than her cornea. The resulting tear lens produces +0.50 diopters of refractive correction.

In this case the tear lens is counteracting the correction somewhat and must be compensated for by increasing the minus in the contact correction. The reverse situation may be true as well though when a lens is fit flatter than the corneal curvature.