SCFHS Optometry Complete Review Guide 2026–2027

4. An autorefractor is only a starting point refined by subjective refraction; it can give false myopia from near-focus accommodation.
5. Keratometry: K = 337.5 ÷ r (mm); e.g. 7.80 mm gives 43.27 D; it samples about 3 mm of the front surface only; a steeper vertical meridian is with-the-rule astigmatism.
6. MEM retinoscopy at 40 cm: normal lag is about +0.25 to +0.50 D; a lag of +1.25 D with near symptoms points to accommodative insufficiency.
7. Fogging at the start of subjective refraction relaxes accommodation; the endpoint is maximum plus (least minus) for best visual acuity (MPMVA).
8. A ±0.25 D Jackson cross cylinder equals +0.25 / −0.50 with zero spherical equivalent; equal blur in both positions means the cylinder power is correct.
9. Duochrome: red clearer means add minus for a myope; the red and green foci are about 0.50 D apart; if equal is not reached, leave red slightly clearer.
10. Keeping the spherical equivalent when changing cylinder: reducing the cylinder by 0.50 D shifts the sphere by 0.25 D; e.g. −2.00 / −1.50 becomes −2.25 / −1.00.
11. Acuity equivalents: 6/12 = 20/40 = 0.5 decimal = 0.30 logMAR; each letter on a 5-letter logMAR chart is worth 0.02; testing at 3 m on a 6 m chart is recorded 3/6 = 6/12.
12. Changing the cylinder axis (e.g. 180 to 160) can tilt the floor; move the axis partly back toward the old one and use a trial-frame walk-around in older patients.
13. Vertex distance: −6.00 D at 14 mm becomes −5.86 D at 10 mm; the strongest spherical lens goes in the back cell of the trial frame because it is closest to the eye.
14. Urgent signs: new flashes and floaters, an acute −2.50 D myopic shift after topiramate (angle closure), or a new hyperopic shift with distorted vision (macular elevation).
15. Presbyopia is caused by lens stiffening; emmetropes notice it around 40–45, and uncorrected hyperopes earlier because accommodation is already used for distance.
16. Near add = demand − half the amplitude; e.g. amplitude 3.00 D at 40 cm gives 2.50 − 1.50 = +1.00 D; the add reaches about +2.00 to +2.50 D in the late 50s.
17. Amplitude: push-up = 1 ÷ near point (m); minus-lens method = lens power + target demand; the endpoint is the first sustained blur.
18. Balancing the add: add + (NRA + PRA) ÷ 2; e.g. +1.50 with NRA +1.00 and PRA −0.50 gives +1.75 D; check the range of clear vision at the working distance.
19. Bifocals: image jump is absent when the segment optical centre is at its top line (executive type); a 22 mm round segment with +2.00 D add gives 2.2Δ of jump.
20. Anisometropia causes vertical prism imbalance at near (Prentice); manage with slab-off on the more minus lens, or contact lenses.

Common mistakes in this chapter

• ✕ A +3.00 D neutralising lens at 67 cm is not +3.00 D hyperopia; subtract +1.50 for a net of +1.50 D, and at 50 cm a plus gross finding can still be myopic.
• ✕ Autorefractors can give false myopia (instrument myopia) in young people; if acuity is 6/5 and retinoscopy and subjective refraction are plano, prescribe nothing.
• ✕ In transposition do all three steps together: add sphere and cylinder, change the cylinder sign and rotate the axis by 90 degrees.
• ✕ Red clearer on duochrome means the myope is under-corrected, so add minus; over-minus shows as smaller, blacker letters without better clarity.
• ✕ If +1.00 D added binocularly still leaves 6/6, the prescription is over-minused; an accurate endpoint drops acuity to about 6/12–6/18.
• ✕ Do not default to a full add: an add that is too strong brings the clear zone closer so the patient holds print very near; +2.75 D at age 50 is often too strong.
• ✕ An uncorrected −2.50 D myope reads at 40 cm without glasses because the far point is at 40 cm; this does not mean presbyopia is absent.
• ✕ Do not prescribe while glucose is high and unstable; refraction shifts with lens hydration, so defer until control is stable.