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newSchoolWorks.com › VoTechWorks › Health & Personal Services › Optician / Optical Technician › Contact Lens Basics › Lesson 7
HLT-OPTI68-05 · Lesson 7 · 18 clock hours · lab

Soft Contact Lens Fitting Procedure

Objective. Students will select a soft lens trial pair, apply and remove lenses on a cooperative subject, evaluate fit using slit-lamp criteria, and perform an over-refraction.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.

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Soft Contact Lens Fitting Procedure

Fitting soft contact lenses requires you to integrate the patient's prescription data, corneal measurements, and wearing goals into a systematic clinical procedure. Every step—from trial lens selection through over-refraction—builds toward a safe, comfortable, and optically correct final lens order.

Understanding the Patient Profile

Before touching any lens, review the patient profile card. The card provides the spectacle refraction (sphere, cylinder, axis), keratometry (K) readings, and the patient's wearing goals (daily wear, extended wear, daily disposable, toric, etc.). Keratometry values describe the curvature of the cornea in two meridians and are expressed in diopters (D) of curvature or millimeters of radius. The average flat and steep K values guide your base curve selection.

Spectacle prescriptions must be converted to ocular plane (contact lens) power before you select a trial lens. For powers beyond approximately ±4.00 D, apply vertex distance compensation. Most soft lens fitting guides provide conversion tables; for powers within that range, spectacle power is often used directly as a starting estimate.

Selecting the Trial Lens

Soft lens fitting sets are organized by base curve radius (BCR, in mm) and power. Manufacturers publish fitting guides that recommend a starting base curve based on average K values. A common starting principle: choose a base curve 0.5–1.0 mm flatter than the flat K reading, but always follow the specific manufacturer's guide because lens materials and designs vary.

Document your rationale on the fitting form before opening a trial lens package. Note the patient's K readings, the calculated or estimated ocular power, and why you chose the specific base curve and power. This habit is essential in clinical practice and is tested on the NCLE exam.

Sterile Insertion Technique

Safety begins with hand hygiene. Wash hands thoroughly with a non-oily, fragrance-free soap and dry with a lint-free towel before handling any contact lens. Verify the trial lens for correct eye (right vs. left), inspect it for tears or debris, and ensure it is not inside-out (the correct orientation produces a bowl shape with edges that flare outward slightly when placed on a fingertip).

Inform the volunteer subject what you are about to do. Retract the upper lid with one hand and the lower lid with the other. Place the lens on the lower sclera or directly on the cornea, release the lids gently, and have the subject look straight ahead and blink normally. Allow the lens to settle for 15–20 minutes before performing the formal fit evaluation. Do not rush this step; an unsettled lens gives misleading slit-lamp findings.

Evaluating Fit at the Slit Lamp

Slit-lamp evaluation of soft lens fit focuses on three primary criteria:

Centration: The lens should center over the cornea with minimal decentration. A well-centered lens covers the limbus evenly in all gaze positions.

Movement on blink: With each blink, a properly fitted soft lens should move approximately 0.25–0.5 mm. Too little movement (tight fit) risks hypoxia and discomfort; too much movement (loose fit) reduces visual stability and centration.

Limbal coverage: The lens edge should extend approximately 0.5–1.0 mm beyond the limbus (the corneal-scleral junction) in all directions. Inadequate coverage leaves the corneal periphery exposed.

Record your findings on the standardized fitting form immediately. Use the accepted descriptors: centration (central, slightly superior/inferior/nasal/temporal), movement (adequate, reduced, excessive), and coverage (full, partial). If fit is unacceptable, document the problem and the corrective action—for example, a tighter base curve to reduce excessive movement.

Over-Refraction

An over-refraction (OR) finds the additional lens power needed on top of the trial lens to give the patient best-corrected visual acuity. Place a trial frame on the patient, set the PD to approximate monocular distance, and refract one eye at a time using loose lenses while the contact lens is on the eye.

Start with a spherical fogging technique: add plus power until vision blurs, then reduce plus (add minus) in 0.25 D steps until the endpoint. Record the over-refraction result as a sphere power (and cylinder if needed, though many soft toric fits require a separate process).

Calculate the final lens power by algebraically adding the over-refraction result to the trial lens power:

> Final power = Trial lens power + Over-refraction power

If the trial lens is −2.00 D and the over-refraction is −0.75 D, the final order is −2.75 D. Compare this to your original trial lens selection and note any discrepancy. A discrepancy of more than ±0.25 D should prompt you to re-examine your vertex conversion or refraction technique before ordering.

Safety Reminders

Key terms9

Base curve radius (BCR)The radius of curvature, expressed in millimeters, of the posterior surface of a contact lens; matched to corneal curvature to achieve proper fit.
Keratometry (K readings)Measurement of the corneal curvature in the two principal meridians, expressed in diopters or millimeters, used to guide contact lens base curve selection.
Over-refraction (OR)A refraction performed while a contact lens is on the eye to determine the additional power needed to achieve best-corrected visual acuity.
Vertex distance compensationMathematical adjustment applied to spectacle prescriptions, typically for powers beyond approximately ±4.00 D, to account for the difference in distance between the spectacle lens and the eye versus the contact lens and the eye.
CentrationThe position of a contact lens relative to the center of the cornea; a properly fitted soft lens should center over the cornea in all gaze positions.
Limbal coverageThe extent to which the contact lens edge extends beyond the limbus (corneal-scleral junction); adequate coverage protects the corneal periphery.
Lens movement on blinkThe amount a soft contact lens shifts with each blink, typically 0.25–0.5 mm for an ideal fit; used to assess whether a lens is too tight or too loose.
Trial lens fitting setA manufacturer-supplied collection of diagnostic lenses in various base curves and powers used to evaluate fit and vision before ordering the final lens.
Scope of practiceThe legally and professionally defined range of tasks an optician or optical technician is permitted to perform, which does not include diagnosis or treatment of eye disease.

Worked examples4

1. A patient's spectacle prescription for the right eye is −2.50 DS. The trial lens selected is −2.50 D. After allowing the lens to settle, you perform an over-refraction and find −0.50 D gives best acuity. What is the final contact lens power to order?
  1. Identify trial lens power: −2.50 D
  2. Identify over-refraction result: −0.50 D
  3. Add algebraically: (−2.50) + (−0.50) = −3.00 D
  4. The final ordered power is −3.00 D
Answer: Final contact lens power = −3.00 D
2. A patient's spectacle prescription for the left eye is −1.75 DS. The trial lens on hand is −2.00 D (the nearest available power). The over-refraction is +0.25 D. What is the final contact lens power?
  1. Identify trial lens power: −2.00 D
  2. Identify over-refraction result: +0.25 D
  3. Add algebraically: (−2.00) + (+0.25) = −1.75 D
  4. The final ordered power is −1.75 D, which matches the original spectacle Rx — confirming the refraction is consistent
Answer: Final contact lens power = −1.75 D
3. A patient's spectacle prescription is +3.25 DS. You select a trial lens of +3.25 D. The over-refraction yields +0.50 D. What is the final lens power, and what does the discrepancy between your trial selection and the final power tell you clinically?
  1. Identify trial lens power: +3.25 D
  2. Identify over-refraction result: +0.50 D
  3. Add algebraically: (+3.25) + (+0.50) = +3.75 D
  4. Compare final power (+3.75 D) to original trial selection (+3.25 D): discrepancy is +0.50 D
  5. A discrepancy greater than ±0.25 D suggests the trial lens power or the vertex conversion may not have been adequate; re-examine refraction technique or consider whether vertex distance compensation was needed
Answer: Final contact lens power = +3.75 D; the +0.50 D discrepancy warrants review of vertex compensation and refraction technique.
4. A patient's flat K is 43.00 D and steep K is 44.50 D. The fitting guide for the lens brand you are using recommends selecting a base curve 0.8 mm flatter than the flat K value expressed in millimeters. The flat K of 43.00 D corresponds to a radius of approximately 7.85 mm. What starting base curve should you select?
  1. Identify flat K radius: 43.00 D ≈ 7.85 mm (using the standard conversion: radius in mm = 337.5 / K in D: 337.5 / 43.00 ≈ 7.85 mm)
  2. Apply fitting guide rule: starting BCR = flat K radius + 0.8 mm
  3. 7.85 + 0.80 = 8.65 mm
  4. Select the trial lens with a base curve of 8.60 mm or 8.70 mm (whichever is closest in the fitting set) and document the rationale
Answer: Starting base curve ≈ 8.65 mm (select nearest available in the fitting set, typically 8.60 or 8.70 mm).

The math in this lesson1

Learners add trial lens power and over-refraction, e.g., −2.50 + (−0.50) = −3.00 D.
7.NS.1
Brush up in the math halls at newSchoolWorks.com:
In class: Using a standardized patient profile card (Rx, K readings, wearing goals), select the appropriate trial lens from a fitting set, document rationale, and apply the lens to a volunteer following sterile insertion technique. · Evaluate lens fit at the slit lamp for centration, movement on blink, and limbal coverage, then record findings on a standardized fitting form; instructor observes and provides real-time feedback. · Perform a monocular over-refraction using a trial frame and loose lenses, calculate final lens power, and compare the order to the fitting set selection, noting any discrepancy and its clinical meaning.
Materials: Soft lens trial fitting sets (multiple base curves and powers); trial frame; loose lens set; slit lamp; fitting assessment forms; lens storage cases; multipurpose solution; disinfection supplies
Check: Completed fitting form for one subject graded on a 15-point rubric; student must score 12/15 with correct over-refraction arithmetic and no omitted safety checks.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Contact Lens Fitting and Patient Safety
1. Before inserting a soft trial lens on a volunteer subject, which action is the most critical first safety step?
Soft Lens Fitting Evaluation
2. A soft lens shows 0.1 mm of movement on blink and decenters inferiorly between blinks. Which fitting description best applies?
Soft Lens Fitting Evaluation
3. During slit-lamp evaluation, what is the generally accepted minimum lens edge extension beyond the limbus for a properly fitted soft lens?
Contact Lens Power Calculations
4. A patient wears a −3.00 D trial lens. The monocular over-refraction is +0.75 D. What final contact lens power should be ordered?
Optics and Lens Power
5. When should vertex distance compensation most likely be applied when converting a spectacle prescription to contact lens power?
Scope of Practice and Patient Safety
6. A student performing a soft lens fitting observes unexpected corneal staining and injection when evaluating the volunteer at the slit lamp. What is the correct action?
Contact Lens Fitting Principles
7. Which keratometry value is most commonly used as the primary reference when selecting a starting soft lens base curve?
Soft Lens Fitting Evaluation
8. After inserting a trial soft lens, how long should you typically allow the lens to settle before performing the formal slit-lamp fit evaluation?
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