Refraction and Contact Lenses
This section covers refraction and contact lenses: retinoscopy, cycloplegic refraction, spherical equivalent, vertex distance, keratometry, lens fit, and contact-lens complications. The exam favours practical conversion and safety.
Core rules and concepts
- Cycloplegic refraction with cyclopentolate 1% or atropine is essential in young children and accommodative esotropia; retinoscopy is performed after adequate cycloplegia.
- Spherical equivalent equals sphere plus half the cylinder. Transposition changes the sign of the cylinder and adds it to the sphere while rotating the axis by 90°.
- For spectacle powers above about 4 D, convert to contact-lens power using F_c=F_s/(1−dF_s), where d is vertex distance in metres.
- Rigid lenses provide sharper vision and better oxygen transmission when fitted well; soft lenses require correct base curve, diameter, and Dk/t.
- Red-eye contact-lens patients need corneal assessment, lens removal, and scraping when microbial keratitis is suspected.
Worked example
A spectacle refraction is −4.00/−1.00×180. The spherical equivalent is −4.50 D. At a 12 mm vertex distance, the contact-lens power is −4.50/[1−0.012(−4.50)]=−4.50/1.054≈−4.27 D. The axis remains 180° because the cylinder is unchanged. A trial lens or over-refraction confirms the final power.
Traps that cost marks
Do not prescribe contact lenses from an uncorrected spectacle power in high myopia. Never omit cycloplegia in a child with esotropia. Distinguish contact-lens-induced red eye from sterile infiltrates and microbial keratitis; both need lens removal, but keratitis needs urgent scraping and treatment.