Lensometer (Focimeter) Step by Step: Single Vision, Prism, Bifocals and Progressive Lenses

Last updated: November 2026

RPC Rounds for Optometrists · B.Optom study guide

Every optometrist must be able to read a pair of glasses accurately in under a minute. The lensometer (focimeter, vertometer) measures the back vertex power of a lens: sphere, cylinder, axis, prism and add. It is the first step in checking new glasses and in solving a patient's "my new glasses are wrong" complaint.

Before you start

1. Focus the eyepiece for your own eye: turn it fully anticlockwise (plus), then turn back slowly until the reticle is just sharp. Do this every session; otherwise every reading carries your own refractive error.
2. Check the instrument reads zero with no lens in place.
3. Place the spectacles with the back surface against the lens stop (temples away from you) so you measure back vertex power, the power that matters for the eye.
4. Read the right lens first, with the frame bottom resting on the table so the axis is referenced to the horizontal.

Reading a single vision lens (manual, minus-cylinder method)

1. Centre the target in the reticle by moving the lens. The centre where the target sits exactly in the middle is the optical centre; mark it with the inking device.
2. Turn the power wheel from high plus towards minus until the first set of lines (the sphere lines) comes into focus. Rotate the axis wheel so they are continuous and sharp. Record this power as the sphere and note the axis.
3. Keep turning towards minus until the perpendicular (cylinder) lines are sharp. The difference between the two readings is the cylinder (negative in minus-cylinder form); the axis reading at this point is the axis.
4. Example: sphere lines sharp at −1.00 with axis 90°, cylinder lines sharp at −2.50 → −1.00 / −1.50 × 90.

If all lines focus at once, the lens is spherical. Plus-cylinder instruments read the other way; transpose to compare with a minus-cylinder prescription.

Measuring prism

• Mark the optical centre of each lens, then compare with the patient's pupil centres (marked on the lens or measured with a PD rule). The distance between the optical centres should match the prescribed PD.
• Prism at the patient's pupil position: place that point at the centre of the stop and read the displacement of the target on the reticle rings (each ring is usually 1Δ). The direction of displacement shows the base direction (base in, out, up or down).
• Use Prentice's rule to estimate unwanted prism from decentration, and compare with tolerances (see spectacle dispensing reference).

Bifocals and progressive lenses

Automated lensometers

Faster and print the result, including prism and PD. Still check that the lens is seated correctly and centred, that the right lens is read first, and that the correct mode is selected (single vision, progressive, contact lens). They struggle with high powers, scratched lenses and some progressive designs.

Checking new glasses against the prescription

Compare sphere, cylinder, axis, add, PD and prism with the order and with tolerance standards (for example ISO 21987 or national standards): axis tolerance is tighter for higher cylinders, and power tolerance is wider for higher powers. If a patient complains, recheck the glasses first, then the fitting (PD, heights, tilt), then the refraction.

Test yourself: flip cards

Quick quiz

Related: How to Read Your Glasses Prescription: SPH, CYL, AXIS, ADD, Spectacle Transposition and Contact Lens Power Tool, Prentice's Rule: Induced Prism With Worked Examples, Spectacle Dispensing Reference: PD, Fitting Height, Tolerances, Eyeglass Lens Coatings Guide Ahmedabad. Back to the optometry curriculum.

Key sources

• Brooks CW, Borish IM. System for Ophthalmic Dispensing. 3rd ed. Butterworth-Heinemann; 2007
• Jalie M. Ophthalmic Lenses and Dispensing. 3rd ed. Butterworth-Heinemann; 2008
• ISO 21987: Ophthalmic optics, mounted spectacle lenses (tolerances)