Optometrist Registration cheat sheet

Snellen acuity is recorded as the test distance over the letter size distance (6/6 or 20/20); a 6/12 line subtends 5 minutes of arc at 12 m but is read at 6 m, so acuity fraction = test distance / letter distance.

Spherical equivalent = sphere + (cylinder / 2); use it to estimate the power needed when a toric correction is unavailable, e.g. +1.00/-2.00 x 180 gives -0.00 D.

Vertex distance correction: Fc = F / (1 - dF), where d is the change in vertex distance in metres; moving a +12.00 D lens 12 mm closer to the eye reduces effective power by roughly 2 D.

Tear film break-up time (TBUT) below 10 seconds supports evaporative dry eye; Schirmer I below 5 mm in 5 minutes indicates aqueous-deficient dry eye.

Intraocular pressure normal range is approximately 10-21 mmHg by Goldmann applanation tonometry; central corneal thickness alters readings, with thin corneas underestimating true pressure.

Grading of diabetic retinopathy: mild non-proliferative shows microaneurysms only; moderate adds haemorrhages or hard exudates; severe shows >20 intraretinal haemorrhages in 4 quadrants or venous beading.

Age-related macular degeneration: dry form shows drusen and geographic atrophy; wet form shows choroidal neovascularisation with subretinal fluid, and Amsler grid distortion is a red-flag symptom.

Pupillary light reflex pathway: afferent travels via the optic nerve and chiasm to the pretectal nucleus, then bilaterally to Edinger-Westphal nuclei; a relative afferent defect indicates optic nerve or severe retinal disease.

Amplitude of accommodation declines with age; Hofstetter's formula gives minimum amplitude = 15 - 0.25 x age, average = 18.5 - 0.3 x age, and maximum = 25 - 0.4 x age.

Prentice's rule: prism dioptres = decentration in centimetres x lens power in dioptres; a 2 mm decentration of a +5.00 D lens induces 1 prism dioptre.

Minimum blank size = frame effective diameter + 2 x decentration + 2 mm allowance; if the result exceeds the stock lens diameter, the lens cannot be edged.

Refractive index determines thickness: higher index (1.67, 1.74) gives thinner lenses but lower Abbe value and more chromatic aberration; CR-39 has index 1.498 and Abbe 58.

Lens power tolerance for dispensing typically allows +/-0.13 D for powers up to 6.00 D and +/-0.25 D above that, per common standards; verify with a focimeter before delivery.

Pantoscopic tilt of 8-12 degrees and face-form wrap of 5-10 degrees optimise optical alignment; excessive tilt induces unwanted astigmatism and prism.

Vertex distance should be recorded on every prescription above +/-4.00 D because changing it alters effective power and can cause adaptation symptoms.

Impact resistance: polycarbonate and Trivex meet higher impact standards and are indicated for children, monocular patients and sports eyewear.

Pupillary distance must be measured monocularly (each eye to bridge centre) for progressive lenses; a binocular-only PD risks unwanted prism and non-tolerance.

Frame fitting: the bridge should rest on the nose without slipping, temples should follow the mastoid contour, and the frame front should sit level with the pupils near the top third of the lens.