Lensometry Technique
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Direct answer
Lensometry (focimetry) measures the back vertex power, axis and prism of an unknown lens — every spectacle order is verified this way before dispensing. On a manual focimeter, first focus the eyepiece graticule for your own eye against the unmetered zero, then bring the mires to focus: the triple-line target first to read the sphere, the single line second, and the difference between the two readings is the minus cylinder, its direction giving the axis. The addition is verified separately as the difference between near and distance vertex powers measured with the lens reversed (convex side toward the instrument), and prescribed prism is read from the displaced mire centre against the graticule rings, each ring usually representing one prism dioptre.
What you must remember
- Eyepiece first: focus the graticule at zero power, no lens, for the operator's own accommodation — skip it and every later reading inherits your uncorrected error.
- Mire logic: focus the triple line — that reading is the sphere; then the single line — sphere plus cylinder; cylinder = second minus first, axis from the drum.
- Reading example: single line at −3.00, triple at −1.25 records −1.25 DS/−1.75 DC at the observed axis.
- Addition check: measure distance and near as front vertex powers (lens reversed); the add is the difference — a +2.00 add must read +2.00, not +1.75.
- Prism reading: centred mires mean no prism; each graticule ring is typically 1 Δ, base direction read from target displacement with the lens facing as worn; the prism-compensating device verifies prescribed values precisely.
- Marking: dot the optical centre or major reference point before edging; on segmented lenses, verify segment height and near centre placement.
- Progressive verification: locate the engraved circles 34 mm apart, align the layout card, verify distance power above the fitting cross and the addition at the near reference circle.
- Autolensmeters: fast and accurate for routine single visions, but unreliable with small segments, high prism and decentered progressives — manual neutralisation arbitrates disputes.
Neutralising a progressive order, step by step
An order arrives: right −3.25 DS/−0.75 DC × 175, add +2.00, fitting height 22 mm. Begin with the eyepiece. Rest the right lens convex-down on the stop, find the distance area just above the fitting cross, and focus: −3.25 on the triple line, −4.00 on the single at axis 175 — cylinder −0.75 confirmed. Move to the near circle some 12-14 mm below and read with the lens reversed: expect −1.25/−0.75 × 175, exactly add +2.00. Now geometry: the fitting cross marked on the lens should coincide with the pupil centre; the engraved circles should sit 34 mm apart and level, the temporal circle carrying the addition digit. Any mismatch — add reading +1.50, circles tilted — sends the job back before the patient tries it.
Prism follows the same discipline. A prescription of 2 Δ base-in right must show the mire centre displaced two rings nasal on the graticule; if instead the optical centre sits 3 mm off the geometric centre of a −6.50 lens, the dispenser has decentered to manufacture prism — by Prentice, 0.3 × 6.5 ≈ 2 Δ — acceptable only if ordered that way and marked correctly.
Where marks are lost
Practical examiners dock an unfocused eyepiece above all else, because it poisons every number after it. The second loss is measuring the addition with the lens the wrong way round: vertex effects make high adds read short as back-vertex measurements. Third is prism direction — reading base without stating which way the lens faces invites sign errors. And reporting a progressive addition from an unspecified point in the corridor, rather than at the near reference circle, reveals the candidate never found the engravings.
Frequently asked questions
Why must the focimeter eyepiece be focused before use?
So the graticule is sharp for the operator's own eye at zero power; otherwise the operator's accommodation adds a systematic error to every power reading that follows.
How are the mires used to separate sphere and cylinder?
Focus the triple line for the sphere, then the single line for the combined power; the difference is the minus cylinder, with the axis taken at the single-line focus.
How is a bifocal addition verified?
Measure distance and near powers with the lens reversed (convex toward the instrument) so both read as front vertex powers; the addition is the difference.
What does one graticule ring represent on a standard focimeter?
One prism dioptre of image displacement — a rapid prism check, with exact prescribed values set on the prism-compensating device.
How do you verify an unmarked progressive lens?
Locate the hidden engravings 34 mm apart, reconstruct the fitting cross and near circle with the layout card, then verify distance, cylinder, axis and addition at their reference points.