Spectacles (eyeglasses) sit ~12-14 mm in front of cornea; contact lenses rest directly on cornea. Due to this distance difference, spectacle and contact lens powers differ for same optical correction. Formula: F_contact = F_spectacle / (1 - d·F_spectacle), where d = vertex distance (m). Example: -8.00 D glasses @ 12mm vertex → contact lens ≈ -7.41 D (weaker). Physics: posterior spectacle lens is further from retina than contact lens—must be stronger to create same retinal image. Myopia (negative power): strong myopic (-8 to -10D) patients benefit from contacts (less minification, larger apparent image). Hyperopia (positive power): less effect on image size. Vertex distance primarily matters above ±4.00 D (higher powers show larger effect). Clinical pearl: as vertex distance d increases, contact power F_c converges toward F_s. Conversely, as d decreases, difference increases. Extreme example: d=0 (ideal contact position on retina) → F_c = F_s exactly. Practical considerations: patient comfort varies between spectacles and contacts; some patients prefer contacts for sports (no frames falling off), cosmetics (no glasses visible), or field of view (full peripheral). Hyperopic presbyopia (age 40+): bifocals or progressive lenses in spectacles; multifocal contacts provide alternative. Contact lens fit optimization: minor power adjustments (±0.25 to ±0.50 D) may occur due to tear film, corneal shape changes, contact lens material properties.

Advanced clinical considerations: high myopia (-12 to -20D) exhibits significant vertex difference (0.5 to 2+ diopters)—contact conversion essential for accurate Rx. Astigmatism compounds vertex effect; toric contacts required (axis alignment critical). Presbyopia management: monovision (one eye distance, other near) or multifocal contacts offer alternatives to bifocals. Rigid gas permeable (RGP) lenses better for high astigmatism/myopia; soft lenses more comfortable. Corneal topography influences contact power selection—irregular astigmatism (keratoconus) may require specialty RGP fitting. Post-refractive surgery (LASIK/PRK): contact fitting becomes complex; surface curvature changes necessitate re-evaluation. Myopic shift with aging requires periodic vertex re-evaluation (especially ages 6-18). Orthokeratology: overnight rigid lenses reshape cornea, reducing myopia during day without correction—complex vertex considerations for day vision post-removal.