A
AC/A Ratio
Ratio describing the relationship between accommodation and convergence. It indicates the amount of convergence produced when accommodation changes. A high AC/A ratio means convergence varies significantly with changes in spherical power, whereas a low AC/A ratio indicates little influence of accommodation and may require the use of prisms or binocular vision exercises.
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Accommodation
Natural mechanism of the eye involving a change in the curvature of the crystalline lens, allowing clear vision at different distances, particularly for near vision. Accommodation acts like an automatic focusing system that enables the eye to maintain clear vision at various distances.​
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Accommodative Lag
Delay of accommodation relative to accommodative demand. The accommodation produced is lower than the required accommodation, resulting in slight blur in near vision.
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Accommodative Lead
Advance of accommodation relative to accommodative demand. The accommodation produced is greater than the required accommodation, which may lead to over-accommodation.
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Associated Phoria
Optical correction using sphere, addition, or prism to neutralize fixation disparity. It is measured using tests such as the Mallett unit, polarized cross, half-square polarized target, Schober test, Worth test, and red filter test.
B
BCC (Binocular Cross Cylinder)
Technique used in near vision to determine the near addition in presbyopic patients. The binocular cross cylinder is placed in front of both eyes, and the patient compares two positions in order to adjust the power until equal clarity is achieved.
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Binocular Balance
Step of the eye exam aimed at balancing accommodation between both eyes to avoid excessive dominance of one eye. It can be performed using prism dissociation (line of letters dissociated with prisms), polarized letter lines (using polarized filters), or a line of letters with an occluder.
C
Convergence
Inward movement of the eyes that allows fixation on a near object. Four types of convergence are distinguished: tonic (muscle balance), accommodative (linked to accommodation), proximal (linked to the perception of nearness), and fusional (used to compensate for a phoria and maintain eye alignment).​​​​​
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Cross Grid
Near vision test using a grid target composed of horizontal and vertical lines, used to determine the near addition in presbyopic patients.
D
Defogging
Technique consisting of adding minus lenses (-) to the spherical correction in order to reduce blur and progressively refine the prescription.
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Diplopia (Double Vision)
Perception of two images of a single object.​
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Dissociated Phoria
Total deviation between the visual axes revealed under dissociated conditions. It is measured using tests such as the cover test, prism tests (Von Graefe / Howell), Maddox rod, and Thorington test.
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Divergence
Outward movement of the eyes that allows fixation on a distant object.
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Dot Target
Target composed of multiple dots used during subjective refraction to detect and refine astigmatism, particularly the axis and cylinder power.
E
Esophoria
Latent deviation of the visual axes inward, which appears when binocular vision is dissociated.
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Exophoria
Heterophoria characterized by a tendency of one eye to deviate outward, toward the temple, when binocular vision is dissociated.​​​​​
F
Fixation Disparity (FD)
The symptomatic portion of a dissociated phoria that is poorly compensated by fusional reserves, which may lead to line skipping or visual discomfort. It is evaluated using tests such as Schober, Worth, red filter test, Mallett unit, polarized cross, and half-square polarized targets.
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Fogging
Technique consisting of adding plus lenses (+) to the spherical correction in order to create intentional blur and relax accommodation during the eye exam.
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Fusional Reserves
Ability of the visual system to compensate for a phoria and maintain binocular vision. Fusional reserves include positive fusional reserves (convergence) and negative fusional reserves (divergence).
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Fusion
Second degree of binocular vision, corresponding to the ability of both eyes to perceive a single image. It includes motor fusion (maintaining single vision) and sensory fusion (achieving a clear and stable image). In the absence of fusion, diplopia or suppression of one eye may occur.
H
Habitual Reading Distance (REVIP)
Near working distance corresponding to the patient’s habitual reading distance. It allows the eye exam to be adapted to real-life viewing conditions.
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HARMON Distance
Near working distance corresponding approximately to the distance between the elbow and the thumb when the reading material is held naturally. It is used to standardize the reading distance during the eye exam.
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Heterophoria
Latent deviation of the visual axes that is compensated by binocular fusion. It becomes apparent when binocular vision is dissociated, such as during occlusion of one eye. Depending on the direction of the deviation, types include esophoria, exophoria, right hyperphoria, and left hyperphoria.
J
JCC (Jackson Cross Cylinder)
Technique used during subjective refraction consisting of flipping a Jackson cross cylinder in front of the patient’s eye to compare two visual perceptions. It allows refinement of the cylinder power and/or axis of astigmatism by asking the patient which position provides better clarity.​
L
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M
Maddox Test
Test used to detect and measure horizontal and vertical heterophorias. It relies on total dissociation: the eye viewing through the Maddox rod perceives a line of light, while the other eye sees a point of light.
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Motor Dissociation
Dissociation obtained by separating the images perceived by each eye, usually using prisms, which displace the retinal image.
N
-
O
Objective Refraction
Method used to estimate the optical correction without subjective responses from the patient.
P
Partial Dissociation
Partial separation of the images perceived by each eye, with shared elements allowing binocular fusion to be maintained.
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Phoria
Latent deviation of the visual axes compensated by binocular fusion. It becomes apparent only when binocular vision is dissociated.
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Phoropter
Instrument used during the eye exam to modify the optical correction (sphere, cylinder, axis, prisms) in order to determine the best prescription. There are manual and automated phoropters.
Polarized Number Lines
Target used to perform binocular balance, allowing accommodation to be balanced between both eyes. This test uses dissociation through polarized filters.
R
Red/Green Test
Test based on longitudinal chromatic aberration of the eye, using chromatic dissociation with red and green filters. It allows refinement of the spherical correction and binocular balance. It is used in presbyopic patients to determine the addition and in non-presbyopic patients to assess the accommodation-convergence relationship.
→ Learn more about the Red/Green Test
S
Sensory Dissociation
Dissociation obtained by presenting different images to each eye, typically using colored or polarized filters.
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Simultaneous Vision
First degree of binocular vision, corresponding to the ability to perceive images from each eye at the same time. It is typically optimized during monocular refraction.
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Stereopsis
Third degree of binocular vision, corresponding to the fine perception of depth and relief provided by binocular vision.
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Subjective Refraction
Method used to determine the optical correction based on the patient's responses during various tests performed with the phoropter.
Swaine’s Rule
Model relating visual acuity to ametropia. This rule is used for visual acuities between 1/10 and 5/10. It allows approximate estimation of ametropia from visual acuity, and vice versa. It is expressed as follows: Ametropia ≈ 0.25 / Visual Acuity
T
Total Dissociation
Complete separation of the images perceived by each eye, with no common elements, preventing binocular fusion.
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Tropia
Manifest deviation of the visual axes present even in binocular vision, corresponding to strabismus.​​​​​​​​​​​
V
Visual Acuity
The ability of the eye to distinguish fine details and separate two closely spaced points. It is measured using optotypes (letters, numbers, or symbols for children) and is typically expressed as a fraction (e.g., 20/20). A visual acuity of 20/20 corresponds to normal vision, while some individuals may achieve 20/15 or better.
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Von Graefe Test
Test used to detect and measure horizontal and vertical heterophorias. It relies on motor dissociation using prisms, which shift the retinal image and break binocular fusion. Dissociation is performed in the vertical plane to measure horizontal phoria and in the horizontal plane to measure vertical phoria.
W
Worth Test
Binocular vision test using partial sensory dissociation with red/green filters to evaluate simultaneous vision and fusion. It can also reveal suppression or fixation disparity.
Why consult this optometry glossary?
This optometry glossary is a practical reference designed for optics and optometry students, as well as eye care professionals.
It allows you to quickly find, in just a few clicks, a definition, a forgotten term, or a concept used during classes or the eye exam.
Some definitions are complemented by detailed articles to further explore key concepts.
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Keep this glossary within reach to improve your knowledge and become a better professional.