How to Calculate Monovision Contact Lens Powers

What is monovision in contact lenses?

Monovision is a contact lens correction in which one eye is fitted for distance and the other eye for near, rather than both eyes being fitted to see at both distances.

According to Parekh and colleagues, Multifocal versus modified monovision corrections (Indian Journal of Ophthalmology, 2023), in monovision correction one eye is fitted with a contact lens for distance correction — the dominant eye — and the non-dominant eye for near correction. That paper lists the practical arguments for the strategy as stated in the literature it reviews: it is straightforward to fit, it costs less than the alternatives, and because each lens carries a single power it is independent of pupil size, so the complications specific to simultaneous-vision multifocal designs do not arise. The same paper names the cost side in the same breath: monovision can reduce stereopsis and contrast sensitivity, and it is problematic in demanding distance and near tasks, mostly at higher add powers.

Two single-vision lenses, two different jobs. That is the whole design. There is no bifocal zone, no aspheric power gradient, no centration-dependent optic — the binocular system is being asked to take distance from one eye and near from the other and suppress the blurred image on the other side.

This page is contact lens monovision only; monovision planned around intraocular or refractive surgery is a surgical decision with different arithmetic, made by the operating surgeon, and it is not covered here.

How do you calculate monovision contact lens powers?

You calculate monovision powers by converting both distance powers to the corneal plane, giving the dominant eye that converted power, and giving the non-dominant eye that same power plus the near add.

The sequence has seven steps, and only two of them are arithmetic:

1. Refract. Record the distance prescription for each eye and the near add.
2. Test ocular dominance at the chair. This is an observation, not a calculation.
3. Convert both distance powers from the spectacle plane to the corneal plane. Vertex compensation applies at or above about ±4.00 D, using a default 12 mm working vertex distance and the formula Fc = Fs ÷ (1 − d × Fs).
4. Order the dominant eye at its converted distance power. It carries no add.
5. Order the non-dominant eye at its converted distance power plus the near add.
6. Insert both lenses and check distance and near acuity binocularly, with both eyes open.
7. Over-refract binocularly in 0.25 D steps, one change at a time.

Published method pages agree on step 4 and step 5 and skip step 3. VisionPlus Magazine, Fitting Guidelines For Monovision (Chaudhry, Shah, Sengupta and Thakur, 11 March 2020) instructs the fitter to fit the dominant eye with the maximum distance correction as per the spectacle-to-contact-lens calculation, and the non-dominant eye with the least plus that provides clear near vision. Review of Optometry, Pearls for Fitting the Presbyopic Patient (Joe Yager, OD, 15 September 2002) states the same standing rule: almost always start by using the dominant eye for distance and the non-dominant eye for near.

That article also publishes a first-lens formula with a small bias in each direction: add +0.25 D to the power on the distance eye, and on the near eye take the distance correction plus the add and then apply a further −0.25 D. Its worked instance is a distance refraction of −2.50 D in both eyes with a +1.50 D add, giving a distance lens of −2.25 D and a near lens of −1.25 D. That ±0.25 D bias is that article’s rule, published in 2002. It is quoted here because it is the source’s arithmetic, not adopted here as this site’s rounding: the ordering step on this site is 0.25 D, and a deliberate bias is a fitting preference a practitioner applies knowingly.

Worked example: a monovision contact lens prescription

The example below is worked at the corneal plane, because that is the step the published guides leave out.

A presbyope refracts to −4.50 DS in each eye with a +1.50 D near add, and the right eye is dominant.

• Both distance meridians sit at or above the ±4.00 D gate, so vertex compensation applies. At a 12 mm working distance, −4.50 D at the spectacle plane becomes about −4.27 D at the cornea, which is −4.25 D at a 0.25 D ordering step.
• OD (dominant) — distance: −4.25 D. No add.
• OS (non-dominant) — near: −4.25 D + +1.50 D = −2.75 D.
• The ordered pair now differs by 1.50 D, which is the add. That difference is the strategy and it is also the cost.

Compare that with the example VisionPlus publishes: a 43-year-old with RE −3.50 DS, LE −2.75 DS, a +1.00 D add in both eyes, and the left eye dominant. Their order is RE −2.50 D for near (the non-dominant eye: −3.50 + 1.00) and LE −2.75 D for distance, then fine-tuning in 0.25 D steps, binocularly. Both of those powers sit below the ±4.00 D gate, which is exactly why that example can be ordered straight off the spectacle numbers without a conversion step appearing anywhere in it. Run the same method at −6.00 D and the missing step is worth more than a quarter diopter per eye.

Calculate the converted distance powers on the Contact Lens Conversion Calculator, or isolate the plane change on the Contact Lens Vertex Calculator and Distance Chart. Then allocate.

Which eye takes the distance power?

The dominant eye takes the distance power in almost every monovision fit, and dominance is established by a chairside test, not by a calculation.

VisionPlus gives the optical reason: the visual system is able to suppress an image in the non-dominant eye better than in the dominant eye, and it is for that reason that ocular dominance is assessed. The blurred image has to go somewhere the brain will ignore it. Two assessment methods are named on that page:

• Pointing. The patient clasps their hands with the forefingers extended, keeps both eyes open, and points at one of the practitioner’s eyes from across the room. The practitioner sees which of the patient’s eyes the fingers line up with. That is the dominant eye.
• Sighting. The patient holds a card with a hole in it at arm’s length and views a distant object through it. The practitioner again looks to see which eye is lined up with the object.

Eyes On Eyecare’s presbyopic fitting guide (Hay) adds a third and instructs that dominance is always tested when fitting monovision or multifocals: a single large letter on the chart, the patient’s overlapping hands leaving a small gap, and the hands brought back toward the face.

Review of Optometry names two situations in which the standing rule is set aside:

• The task fixes the side. Where an occupation requires that the near or distance eye be chosen by the direction the patient works in — a screen sitting to one side, for instance — that eye is fitted for near and dominance is disregarded.
• The dominant eye carries the most astigmatism. In that case that eye is generally made the near eye, because at distance the more astigmatic eye has the most fluctuating vision even when it is corrected with a toric soft lens, and patients tolerate that poorly.

No tool on this site computes dominance. The Multifocal Contact Lens Calculator treats it the same way — a label you test and record, applied when a later over-refraction needs a one-eye change — and this page does not compute it either. Sphere and add contain no information about which eye leads.

How much add does the near eye carry?

The near eye carries the whole near add and the distance eye carries none of it — the asymmetry is the correction.

That is the one structural difference between monovision and a multifocal fit. A multifocal starts with equal adds in both eyes and unequal adds are an endpoint rather than a starting point; monovision starts unequal by definition, with the entire add on one side. Nothing about the add’s value changes because you are fitting monovision. How a spectacle near add becomes a starting contact lens add is the procedure on the Multifocal Contact Lens Calculator, and the LOW / MED / HIGH designation that manufacturers print instead of a diopter is defined on ADD on a contact lens prescription. Resolve the add there; allocate it here.

Two practitioner cautions travel with the allocation. VisionPlus specifies the least plus that provides clear near vision rather than the full comfortable maximum. Hay is conservative with plus at near for the same reason, and states the rule directly: do not add extra minus to the distance eye to sharpen far vision and then extra plus to the near eye to help near work — it is not needed, and it widens the gap between the eyes for nothing.

Monovision vs multifocal contacts: when is each one chosen?

Monovision is chosen when a single power per eye is preferable to two powers in front of one pupil — most often because distance acuity has to stay sharp, because pupil size or centration makes a simultaneous-vision design unreliable, or because a cylinder has to be corrected in a design where a toric multifocal is not available.

The comparative evidence is narrow but it is direct. Parekh and colleagues ran a double-masked, prospective comparison in 19 presbyopic neophytes aged 40 to 48, all with spherical equivalents within ±3.00 D, astigmatism of 0.50 D or less and a near add of +1.50 D or less. Modified monovision gave superior high-contrast distance visual acuity; the multifocal correction performed better for stereopsis; low-contrast acuity, near acuity and contrast sensitivity were comparable between the two. Near stereoacuity was significantly reduced against spectacles under every contact lens condition tested, including the multifocal one.

Practice has already made this trade. The same paper cites an international survey in which, on average, 29% of presbyopic contact lens wearers are fitted with multifocal lenses and 8% with monovision.

Older references and patient-facing pages often say bifocal where current practice says multifocal. Monovision vs bifocal contacts is the same comparison under an older name: two focal points in front of each pupil, against one power per eye.

This page states what differs. It does not rank the two, price them, or name a design.

What is modified monovision?

Modified monovision puts a multifocal on one eye and a different correction on the other, keeping the between-eye division of labour while giving one eye back some range.

Parekh and colleagues record that the term covers several arrangements:

• The dominant eye is fitted with a single-vision distance correction, spherical or toric, and the other eye with a multifocal design.
• A low-add multifocal in the dominant eye for distance and a high-add multifocal in the other eye to optimise near.
• A centre-distance design in the dominant eye and a centre-near design in the other.

Review of Optometry describes the first of those as the step to try when a patient has not adapted to plain monovision, before moving to a bifocal correction in both eyes: in early presbyopes with an add up to +1.50 D, correct the dominant eye for distance with a sphere and fit the non-dominant eye with a bifocal carrying full distance correction and an add equal to the spectacle add.

It is a middle position between the two strategies above, not a third product category.

Does monovision work with astigmatism?

Monovision works with astigmatism, but the cylinder still has to be corrected in each eye, so the strategy is limited by whether the design being fitted is available as a toric.

Splitting distance and near between the eyes does nothing to a cylinder. Each eye needs its own sphere, cylinder and axis, and the near eye’s sphere is the one that carries the add. Where a toric is available in the design, monovision and toric correction coexist; where it is not, the choice is between leaving cylinder uncorrected, taking a spherical equivalent, or changing design — and that is a fitting decision, not an arithmetic one. Calculate starting sphere, cylinder and axis on the Toric Contact Lens Calculator; see spherical equivalent for when collapsing a cylinder into sphere is defensible and when it is not.

Cylinder also pushes some patients toward monovision. Hay states that a patient with −0.75 D of cylinder in the dominant eye tends to go into monovision rather than a multifocal, and that spherical equivalent must be accounted for with cylinder as low as −0.25 D. Review of Optometry’s soft bifocal rules of thumb say the same thing from the other side: do not fit a patient with 1.00 D or more of astigmatism into a soft bifocal. The same article’s second exception applies here too — when the dominant eye carries the most astigmatism, that eye is generally the one made the near eye, because a fluctuating distance image is what patients notice.

How much difference between the eyes does monovision tolerate?

The dioptric difference monovision creates between the eyes equals the near add, and the tolerance for it falls as the add rises.

Three published limits bound the strategy:

• A blur ceiling. Review of Optometry advises trying not to blur the near eye more than 20/40 at distance, describing that as a tolerable amount of imbalance for most patients. Early presbyopes are the readier candidates precisely because the difference created is small.
• An add ceiling. VisionPlus states that the more near add there is, the more stereopsis is lost, that the approach works best at adds below +1.50 D, and that higher adds are difficult to adapt to for fine work.
• A measured cost. Parekh and colleagues found near stereoacuity significantly reduced under monovision-type correction compared with spectacles, and record the literature’s finding that monovision is most problematic in demanding distance and near tasks at higher adds.

That is a real functional consequence, not a comfort footnote. VisionPlus is explicit that careful advice must be given about driving and other vocational tasks that may be affected by monocular blur and impaired stereopsis, and that patients should not carry out those potentially hazardous tasks until they have adapted and feel their visual function is not impaired. Say it at the fitting, not at the follow-up.

How do you verify a monovision trial with over-refraction?

A monovision pair is verified with binocular loose-lens over-refraction, both eyes open, outside the phoropter — never by checking each eye on its own.

Checking one eye at a time measures the wrong thing. Monovision is a binocular arrangement; the eye you occlude is the one doing the suppressing. Hay performs the over-refraction outside the phoropter with both eyes open, using hand-held lenses or flippers over the dominant eye first to see whether distance vision improves, then repeating the same check for reading, and makes as few changes as possible at the initial visit — if the refraction was on target, no change should be needed. Moran CORE, Contact Lens Over-Refraction runs its sequence in 0.25 D steps and confirms binocularly, and states that any meridian at or above about ±4.00 D should be vertexed. The same gate this page’s step 3 uses.

Five rules govern what happens next:

• Change one power at a time.
• Change in 0.25 D steps.
• Allow at least three weeks of adaptation before adjusting a power, and three weeks again after each adjustment (Review of Optometry).
• If there is still no adaptation after two months, reverse the distance and near lenses (Review of Optometry).
• Some patients end up with the non-dominant eye taking distance; VisionPlus notes that crossing over depends on adaptation, which is another reason the dominance test is a starting label rather than a verdict.

Verify the pair with binocular over-refraction on the Over-Refraction Calculator for Contact Lens Parameters.

Starting monovision powers are not a prescription

Monovision powers calculated from a written refraction are starting parameters for a trial pair, not a prescription, because the arithmetic cannot tell you whether this particular binocular system will accept the arrangement.

Monovision does not suit every presbyope, and whether a given wearer adapts to it is settled on eye over weeks by the practitioner who fitted it, not by any calculation on this page.

Read Starting Contact Lens Parameters Are Not a Prescription for the bound that applies across this site. Convert spectacle prescription to contact lens parameters for both distance powers. Calculate vertex-compensated contact lens power where the ±4.00 D gate applies. Allocate the add to one eye. Verify power with over-refraction after the trial lens. Contact lens parameters at the corneal plane stay unfinished until that sequence is done.

Constants used above — a 12 mm working vertex distance, an approximately ±4.00 D compensation threshold, and a 0.25 D ordering step — are the same values the calculators use, imported from the site’s conversion module rather than restated. How we calculate.

Sources

Clinical claims on this page are attributed to the publications below.

• VisionPlus Magazine, Fitting Guidelines For Monovision(Prof Monica Chaudhry, Sushant Shah, Roshni Sengupta and Kamal Thakur; 11 March 2020) — suppression rationale for testing dominance; the pointing and sighting methods; dominant eye at maximum distance correction and non-dominant eye at the least plus giving clear near; the 43-year-old worked example; stereopsis loss rising with the add and the below-+1.50 D observation; the driving and vocational caution; crossover.
• Review of Optometry, Pearls for Fitting the Presbyopic Patient(Joe Yager, OD; Vol. 139:09, 15 September 2002) — dominant eye for distance as the standing rule and its two exceptions; the ±0.25 D first-lens formula and its worked instance; the 20/40 blur ceiling; three weeks before adjusting and 0.25 D per adjustment; reversing the lenses after two months; modified monovision in early presbyopes; the 1.00 D astigmatism limit for soft bifocals.
• Parekh D, Asokan R, Purkait S, Iqbal A. Multifocal versus modified monovision corrections: a non-dispensing comparison of visual assessment in presbyopic neophytes(Indian Journal of Ophthalmology 2023;71:1837–1842) — the definition of monovision and of modified monovision’s three forms; pupil independence; the comparison results for high-contrast acuity, stereopsis, low-contrast acuity, near acuity and contrast sensitivity; the 29% / 8% fitting-share survey figure.
• Eyes On Eyecare, Guide to Fitting Contact Lenses for the Presbyopic Population(Hay) — always test dominance for monovision or multifocals; the overlapping-hands method; binocular loose-lens over-refraction outside the phoropter, dominant eye first; conservative plus at near; the instruction not to add minus at distance and plus at near; −0.75 D cylinder in the dominant eye pushing toward monovision and spherical equivalent from −0.25 D.
• Moran CORE, Contact Lens Over-Refraction— the 0.25 D binocular over-refraction sequence; vertexing meridians at or above about ±4.00 D. Cited identically on the multifocal and over-refraction pages so the three agree.