Contact Lens Quick Reference: Vertex Table, LARS, RGP and Keratoconus
Last updated: October 2026
Everyday contact lens fitting: converting spectacle power, toric lens rotation, rigid lens base curves, options for irregular corneas, and the safety advice every wearer needs.
Spectacle to contact lens power (vertex 12 mm)
| Minus spectacle | Contact lens | Plus spectacle | Contact lens |
|---|---|---|---|
| −4.00 | −3.75 | +4.00 | +4.25 |
| −4.50 | −4.25 | +4.50 | +4.75 |
| −5.00 | −4.75 | +5.00 | +5.25 |
| −5.50 | −5.25 | +5.50 | +6.00 |
| −6.00 | −5.50 | +6.00 | +6.50 |
| −6.50 | −6.00 | +6.50 | +7.00 |
| −7.00 | −6.50 | +7.00 | +7.75 |
| −7.50 | −7.00 | +7.50 | +8.25 |
| −8.00 | −7.25 | +8.00 | +8.75 |
| −9.00 | −8.00 | +9.00 | +10.00 |
| −10.00 | −9.00 | +10.00 | +11.25 |
| −11.00 | −9.75 | +11.00 | +12.75 |
| −12.00 | −10.50 | +12.00 | +14.00 |
| −13.00 | −11.25 | +13.00 | +15.50 |
| −14.00 | −12.00 | +14.00 | +16.75 |
| −15.00 | −12.75 | +15.00 | +18.25 |
| −16.00 | −13.50 | +16.00 | +19.75 |
| −18.00 | −14.75 | +18.00 | +23.00 |
| −20.00 | −16.25 | +20.00 | +26.25 |
Corneal-plane power = F ÷ (1 − dF), rounded to the nearest 0.25 D. Convert each principal meridian separately for astigmatic prescriptions, or use the transposition and vertex tool. Below ±4.00 D the difference is negligible.
Soft toric lenses: LARS
- Look at the orientation mark on the eye. If the lens has rotated to the examiner's left, add the rotation to the prescribed axis; if to the right, subtract it (Left Add, Right Subtract).
- Estimate the rotation from the mark spacing for that brand (often 10° or 15° between marks).
- If rotation is unstable from blink to blink, change the lens design rather than the axis.
- Spherical soft lenses usually cope with up to about 0.75 D of astigmatism; above that, consider toric lenses.
Rigid gas-permeable (RGP) lenses
- Start with a base curve on, or 0.05–0.10 mm flatter than, the flattest K; choose a total diameter of about 9.0–9.6 mm for regular corneas.
- Tear lens rule (SAM-FAP): Steeper Add Minus, Flatter Add Plus; each 0.05 mm change in base curve ≈ 0.25 D.
- Ideal fluorescein pattern: light central touch or alignment, mid-peripheral bearing, and a clear 0.4–0.6 mm edge band; the lens should move 1–2 mm on blinking.
- RGP lenses correct corneal astigmatism through the tear lens; residual astigmatism comes from the crystalline lens.
Keratoconus and irregular corneas
| Lens | Best for | Notes |
|---|---|---|
| Small RGP / keratoconus designs (e.g. Rose K type) | Mild to moderate cones | Aim for three-point touch; first choice in many clinics |
| Piggyback (RGP over soft) | Poor RGP comfort | Higher oxygen-permeable soft lenses only |
| Hybrid lenses | Centred cones, comfort problems | Rigid centre, soft skirt; costlier |
| Scleral lenses | Advanced cones, post-graft astigmatism, severe dry eye | Vault the cornea; fill with preservative-free saline |
| Soft toric or custom soft | Very mild cones | Limited vision quality in irregular corneas |
Safety advice for every wearer: no sleeping in lenses unless prescribed; never use tap water, saliva or homemade solutions; no swimming or showering in lenses; replace cases monthly; remove lenses and seek help the same day for a painful red eye or blurred vision (risk of Pseudomonas and Acanthamoeba keratitis).
Related: contact lens clinic, contact lens care (for patients), topography and keratoconus card.
Key sources
- Efron N. Contact Lens Practice. 4th ed. Elsevier; 2023
- Phillips AJ, Speedwell L. Contact Lenses. 6th ed. Elsevier; 2018
- Stapleton F et al. Risk factors for moderate and severe microbial keratitis in daily wear contact lens users. Ophthalmology 2012;119:1516–21
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