Binocular Vision A-Z
A comprehensive reference glossary of binocular vision terminology for eye care professionals.
Accommodation
The ability of the crystalline lens to change its focus when looking from distance to near objects, maintaining a clear retinal image at varying distances.
Accommodation Amplitude
The maximum range of accommodation available to an individual - the difference between the far point and near point of accommodation.
Accommodation Flexibility
The speed and ease with which the eyes can shift focus between near and far targets, measured in cycles per minute.
Amblyopia
A developmental condition (commonly called lazy eye) where one eye has reduced vision that cannot be corrected by glasses alone, caused by abnormal visual development early in life.
Aniseikonia
A condition in which there is a difference in the perceived image size between the two eyes, often causing discomfort and difficulty with binocular fusion.
Anisometropia
A significant difference in refractive error between the two eyes, which can lead to amblyopia if uncorrected during visual development.
Associated Phoria
The residual fixation disparity under binocular conditions - the prism required to eliminate any fixation disparity.
Binocular Vision
The ability to use both eyes simultaneously to create a single, unified, three-dimensional image of the world.
Binocular Acuity
Visual acuity measured with both eyes open, which is typically better than monocular acuity due to binocular summation.
Binocular Summation
The improvement in visual performance that occurs when both eyes are used together compared to either eye alone.
Convergence
The inward movement of both eyes to maintain single binocular vision when viewing a near object.
Convergence Insufficiency
A common binocular vision disorder where the eyes struggle to work together when focusing on near objects, leading to eye strain, headaches, and double vision.
Colour Vision
The ability to perceive and distinguish different wavelengths of light as different colours, mediated by cone photoreceptors in the retina.
Depth Perception
The visual ability to perceive the world in three dimensions and judge the distance of objects, enabled by stereopsis and other visual cues.
Diplopia
Double vision - the simultaneous perception of two images of a single object, caused by misalignment of the two eyes.
Divergence
The outward movement of both eyes to maintain single binocular vision when changing fixation from a near to a distant object.
Esophoria
A latent inward deviation of one eye that is controlled by fusional vergence, only becoming apparent when binocular fusion is disrupted.
Esotropia
A manifest inward deviation of one eye (convergent squint) that is present even with both eyes open.
Exophoria
A latent outward deviation of one eye that is controlled by fusional vergence, only becoming apparent when binocular fusion is disrupted.
Eye Teaming
The ability of both eyes to work together as a coordinated team, maintaining proper alignment for comfortable and efficient binocular vision.
Fixation Disparity
A small misalignment of the eyes during binocular viewing, where the images do not fall exactly on corresponding retinal points despite maintained fusion.
Fusion
The process by which the brain combines slightly different images from each eye into a single, unified perception.
Fusional Reserves
The range of vergence movements available to maintain single binocular vision - measured as positive (convergent) and negative (divergent) fusional reserves.
Heterophoria
A latent deviation of the eyes that is held in check by fusional vergence. Becomes apparent only when binocular fusion is interrupted.
Hyperphoria
A latent vertical deviation of one eye, where one eye tends to deviate upward when fusion is disrupted.
Myopia
Short-sightedness - a refractive error where distant objects appear blurred because light focuses in front of the retina rather than on it.
Myopia Management
Clinical strategies aimed at slowing the progression of myopia in children and young adults, including optical, pharmaceutical, and behavioural interventions.
Phoria
A latent deviation of the eyes from proper alignment that is controlled by fusional vergence and only becomes manifest when fusion is disrupted.
Prism
An optical element used to deviate light - in optometry, prisms are prescribed to correct binocular vision problems by altering the perceived direction of objects.
Stereopsis
The ability to perceive depth and three-dimensional structure from the slight differences in the images seen by each eye (binocular disparity).
Stereo Acuity
The smallest detectable difference in depth between two objects, measured in arc seconds - a measure of stereoscopic sensitivity.
Strabismus
A manifest misalignment of the eyes (commonly called a squint), where the eyes point in different directions even when both are open.
Suppression
The brain's active process of ignoring or blocking out the image from one eye to prevent confusion or diplopia when the eyes are misaligned.
Vergence
Simultaneous movement of both eyes in opposite directions to maintain single binocular vision - includes convergence (inward) and divergence (outward).
Visual Acuity
A measure of the eye's ability to resolve fine detail, typically measured using a Snellen chart and expressed as a fraction (e.g. 6/6 or 20/20).
Visual Therapy
A programme of eye exercises and activities designed to improve binocular vision, eye coordination, focusing, and visual processing skills.