Optics, Refraction, and Contact Lenses
Optics fundamentals
Light bends at an interface according to Snell's law. The eye has about 60 D total power: cornea about 43 D and lens about 20 D. Myopia focuses light anterior to the retina and is corrected with minus lenses; hyperopia focuses behind the retina and needs plus lenses. Astigmatism has two principal meridians and is corrected with a cylinder. Presbyopia is loss of accommodation, treated with a near addition. Spherical equivalent = sphere + ½ cylinder.
Refraction and transposition
Transpose by adding the cylinder to the sphere, changing the cylinder sign, and rotating the axis 90°. Record the original prescription before transposing. Vertex distance matters above about ±4.00 D: moving a minus lens closer to the eye increases its effective minus power, while moving a plus lens closer reduces its plus effect.
Contact lenses
Keratometry gives corneal curvature; an RGP base curve is usually fitted flatter than the flat K (often about 0.1 mm) so the tear lens and edge clearance are adequate. Soft lenses are specified by base curve, diameter, power, and Dk/t (oxygen transmissibility); teach daily wear and replacement schedules. Complications include microbial keratitis, contact-lens acute red eye, giant papillary conjunctivitis, and neovascularization. Scleral lenses vault the cornea and are useful in irregular astigmatism.
Worked example
K readings are 44.00 @ 180 and 45.50 @ 090. Corneal cylinder = 1.50 D; because the vertical meridian is steeper, this is with-the-rule astigmatism. Spherical equivalent = (44.00 + 45.50) / 2 = 44.75 D. If over-refraction is −0.50 sphere, add that to the lens power when ordering.
Traps
Do not transpose without rotating the axis 90°; do not ignore vertex distance in high prescriptions; do not fit an RGP exactly on the flat K without assessing fluorescein; do not use tap water with lenses; do not promise zero risk with extended wear.