Diplopia can also occur when viewing with only one eye; this is called monocular diplopia, or, where the patient perceives more than two images, monocular polyopia. While there rarely may be serious causes behind monocular diplopia symptoms, this is much less often the case than with binocular diplopia. The differential diagnosis of multiple image perception includes the consideration of such conditions as corneal surface keratoconus, subluxation of the lens, a structural defect within the eye, a lesion in the anterior visual cortex or non-organic conditions, however diffraction-based (rather than geometrical) optical models have shown that common optical conditions, especially astigmatism, can also produce this symptom.

Many people with amblyopia, especially those who only have a mild form, are not aware they have the condition until tested at older ages, since the vision in their stronger eye is normal. People typically have poor stereo vision, however, since it requires both eyes. Those with amblyopia further may have, on the affected eye, poor pattern recognition, poor visual acuity, and low sensitivity to contrast and motion.

Amblyopia is characterized by several functional abnormalities in spatial vision, including reductions in visual acuity, contrast sensitivity function, and vernier acuity, as well as spatial distortion, abnormal spatial interactions, and impaired contour detection. In addition, individuals with amblyopia suffer from binocular abnormalities such as impaired stereoacuity (stereoscopic acuity) and abnormal binocular summation. Also, a crowding phenomenon is present.

These deficits are usually specific to the amblyopic eye. However, subclinical deficits of the "better" eye have also been demonstrated.

People with amblyopia also have problems of binocular vision such as limited stereoscopic depth perception and usually have difficulty seeing the three-dimensional images in hidden stereoscopic displays such as autostereograms. Perception of depth, however, from monocular cues such as size, perspective, and motion parallax remains normal.

Temporary binocular diplopia can be caused by alcohol intoxication or head injuries, such as concussion (if temporary double vision does not resolve quickly, one should see an optometrist or ophthalmologist immediately). It can also be a side effect of benzodiazepines or opioids, particularly if used in larger doses for recreation, the anti-epileptic drugs Phenytoin and Zonisamide, and the anti-convulsant drug Lamotrigine, as well as the hypnotic drug Zolpidem and the dissociative drugs Ketamine and Dextromethorphan. Temporary diplopia can also be caused by tired and/or strained eye muscles or voluntarily. If diplopia appears with other symptoms such as fatigue and acute or chronic pain, the patient should see an ophthalmologist immediately.

Amblyopia has three main causes:

- Strabismic: by strabismus (misaligned eyes)

- Refractive: by anisometropia (difference of a certain degree of nearsightedness, farsightedness, or astigmatism), or by significant amount of equal refractive error in both eyes

- Deprivational: by deprivation of vision early in life by vision-obstructing disorders such as congenital cataract

The earliest sign of exotropia is usually a noticeable outward deviation of the eye. This sign may at first be intermittent, occurring when a child is daydreaming, not feeling well, or tired. It may also be more noticeable when the child looks at something in the distance. Squinting or frequent rubbing of the eyes is also common with exotropia. The child probably will not mention seeing double, i.e., double vision. However, he or she may close one eye to compensate for the problem.

Generally, exotropia progresses in frequency and duration. As the disorder progresses, the eyes will start to turn out when looking at close objects as well as those in the distance. If left untreated, the eye may turn out continually, causing a loss of binocular vision.

In young children with any form of strabismus, the brain may learn to ignore the misaligned eye's image and see only the image from the best-seeing eye. This is called amblyopia, or lazy eye, and results in a loss of binocular vision, impairing depth perception. In adults who develop strabismus, double vision sometimes occurs because the brain has already been trained to receive images from both eyes and cannot ignore the image from the turned eye.

Additionally in adults who have had exotropia since childhood, the brain may adapt to using a "blind-spot" whereby it receives images from both eyes, but no full image from the deviating eye, thus avoiding double vision and in fact increasing peripheral vision on the side of the deviating eye.

When observing a person with strabismus, the misalignment of the eyes may be quite apparent. A patient with a constant eye turn of significant magnitude is very easy to notice. However, a small magnitude or intermittent strabismus can easily be missed upon casual observation. In any case, an eye care professional can conduct various tests, such as cover testing, to determine the full extent of the strabismus.

Symptoms of strabismus include double vision and/or eye strain. To avoid double vision, the brain may adapt by ignoring one eye. In this case, often no noticeable symptoms are seen other than a minor loss of depth perception. This deficit may not be noticeable in someone who has had strabismus since birth or early childhood, as they have likely learned to judge depth and distances using monocular cues. However, a constant unilateral strabismus causing constant suppression is a risk for amblyopia in children. Small-angle and intermittent strabismus are more likely to cause disruptive visual symptoms. In addition to headaches and eye strain, symptoms may include an inability to read comfortably, fatigue when reading, and unstable or "jittery" vision.

Concomitant esotropia – that is, an inward squint that does not vary with the direction of gaze – mostly sets in before 12 months of age (this constitutes 40% of all strabismus cases) or at the age of three or four. Most patients with "early-onset" concomitant esotropia are emmetropic, whereas most of the "later-onset" patients are hyperopic. It is the most frequent type of natural strabismus not only in humans, but also in monkeys.

Concomitant esotropia can itself be subdivided into esotropias that are ether "constant," or "intermittent."

- Constant esotropia

- Intermittent esotropia

A patient can have a constant esotropia for reading, but an intermittent esotropia for distance (but rarely vice versa).

Cyclotropia is a form of strabismus in which, compared to the correct positioning of the eyes, there is a of one eye (or both) about the eye's visual axis. Consequently, the visual fields of the two eyes appear tilted relative to each other. The corresponding "latent" condition – a condition in which torsion occurs only in the absence of appropriate visual stimuli – is called cyclophoria.

Cyclotropia is often associated with other disorders of strabism, can result in double vision, and can cause other symptoms, in particular head tilt.

In some cases, subjective and objective cyclodeviation may result from surgery for oblique muscle disorders; if the visual system cannot compensate for it, cyclotropia and rotational double vision (cyclodiplopia) may result. The role of cyclotropia in vision disorders is not always correctly identified. In several cases of double vision, once the underlying cyclotropia was identified, the condition was solved by surgical cyclotropia correction.

Conversely, artificially causing cyclotropia in cats leads to reduced vision acuity, resulting in a defect similar to strabismic amblyopia.

Exotropia is a form of strabismus where the eyes are deviated outward. It is the opposite of esotropia and usually involves more severe axis deviation than exophoria. People with exotropia often experience crossed diplopia. Intermittent exotropia is a fairly common condition. "Sensory exotropia" occurs in the presence of poor vision. Infantile exotropia (sometimes called "congenital exotropia") is seen during the first year of life, and is less common than "essential exotropia" which usually becomes apparent several years later.

The brain's ability to see three-dimensional objects depends on proper alignment of the eyes. When both eyes are properly aligned and aimed at the same target, the visual portion of the brain fuses the forms into a single image. When one eye turns inward, outward, upward, or downward, two different pictures are sent to the brain. This causes loss of depth perception and binocular vision. There have also been some reports of people that can "control" their afflicted eye. The term is from Greek "exo" meaning "outward" and "trope" meaning "a turning".

Strabismus may also be classified based on time of onset, either congenital, acquired, or secondary to another pathological process. Many infants are born with their eyes slightly misaligned, and this is typically outgrown by six to 12 months of age. Acquired and secondary strabismus develop later. The onset of accommodative esotropia, an overconvergence of the eyes due to the effort of accommodation, is mostly in early childhood. Acquired non-accommodative strabismus and secondary strabismus are developed after normal binocular vision has developed. In adults with previously normal alignment, the onset of strabismus usually results in double vision.

Any disease that causes vision loss may also cause strabismus, but it can also result from any severe and/or traumatic injury to the affected eye. Sensory strabismus is strabismus due to vision loss or impairment, leading to horizontal, vertical or torsional misalignment or to a combination thereof, with the eye with poorer vision drifting slightly over time. Most often, the outcome is horizontal misalignment. Its direction depends on the patient age at which the damage occurs: patients whose vision is lost or impaired at birth are more likely to develop esotropia, whereas patients with acquired vision loss or impairment mostly develop exotropia. In the extreme, complete blindness in one eye generally leads to the blind eye reverting to an anatomical position of rest.

Although many possible causes of strabismus are known, among them severe and/or traumatic injuries to the afflicted eye, in many cases no specific cause can be identified. This last is typically the case when strabismus is present since early childhood.

Results of a U.S. cohort study indicate that the incidence of adult-onset strabismus increases with age, especially after the sixth decade of life, and peaks in the eighth decade of life, and that the lifetime risk of being diagnosed with adult-onset strabismus is approximately 4%.

Suppression of an eye is a subconscious adaptation by a person's brain to eliminate the symptoms of disorders of binocular vision such as strabismus, convergence insufficiency and aniseikonia. The brain can eliminate double vision by ignoring all or part of the image of one of the eyes. The area of a person's visual field that is suppressed is called the suppression scotoma (with a scotoma meaning, more generally, an area of partial alteration in the visual field). Suppression can lead to amblyopia.

The signs and symptoms of far-sightedness are blurry vision, headaches, and eye strain. The common symptom is eye strain. Difficulty seeing with both eyes (binocular vision) may occur, as well as difficulty with depth perception.

Concomitant esotropias can arise as an initial problem, in which case they are designated as 'Primary,' as a consequence of loss or impairment of vision, in which case they are designated as 'Secondary,' or following overcorrection of an initial Exotropia in which case they are described as being 'Consecutive'. The vast majority of esotropias are primary.

Nobel-prize winner David H. Hubel described suppression in simple terms as follows:

Suppression is frequent in children with anisometropia or strabismus or both. For instance, children with infantile esotropia may alternate with which eye they look, each time suppressing vision in the other eye.

Heterophoria is an eye condition in which the directions that the eyes are pointing at rest position, when "not" performing binocular fusion, are not the same as each other, or, "not straight". There can be esophoria, where the eyes tend to cross inward in the absence of fusion; exophoria, in which they diverge; or hyperphoria, in which one eye points up or down relative to the other. Phorias are known as 'latent squint' because the tendency of the eyes to deviate is kept latent by fusion. A person with two normal eyes has single vision (usually) because of the combined use of the sensory and motor systems. The motor system acts to point both eyes at the target of interest; any offset is detected visually (and the motor system corrects it). Heterophoria only occurs during dissociation of the left eye and right eye, when fusion of the eyes is absent. If you cover one eye (e.g. with your hand) you remove the sensory information about the eye's position in the orbit. Without this, there is no stimulus to binocular fusion, and the eye will move to a position of "rest". The difference between this position, and where it would be were the eye uncovered, is the heterophoria. The opposite of heterophoria, where the eyes are straight when relaxed and not fusing, is called orthophoria.

In contrast, fixation disparity is a very small deviation of the pointing directions of the eyes that is present while performing binocular fusion.

Heterophoria is usually asymptomatic. This is when it is said to be "compensated". When fusional reserve is used to compensate for heterophoria, it is known as compensating vergence. In severe cases, when the heterophoria is not overcome by fusional vergence, sign and symptoms appear. This is called decompensated heterophoria.

Heterophoria may lead to squint or also known as strabismus.

Far-sightedness can have rare complications such as strabismus and amblyopia. At a young age, severe far-sightedness can cause the child to have double vision as a result of "over-focusing".

The symptoms and signs associated with convergence insufficiency are related to prolonged, visually demanding, near-centered tasks. They may include, but are not limited to, diplopia (double vision), asthenopia (eye strain), transient blurred vision, difficulty sustaining near-visual function, abnormal fatigue,

headache, and abnormal postural adaptation, among others. In some cases, difficulty with making eye contact have been noted as a complaint amongst sufferers.

Note that some Internet resources confuse convergence and divergence dysfunction, reversing them.

Hypertropia is a condition of misalignment of the eyes (strabismus), whereby the visual axis of one eye is higher than the fellow fixating eye.

Hypotropia is the similar condition, focus being on the eye with the visual axis lower than the fellow fixating eye.

Dissociated Vertical Deviation is a special type of hypertropia leading to slow upward drift of one or rarely both eyes, usually when the patient is inattentive.

Convergence insufficiency or convergence disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence.

Refractive errors such as hyperopia and Anisometropia may be associated abnormalities found in patients with vertical strabismus.

The vertical miscoordination between the two eyes may lead to

- Strabismic amblyopia, (due to deprivation / suppression of the deviating eye)

- cosmetic defect (most noticed by parents of a young child and in photographs)

- Face turn, depending on presence of binocular vision in a particular gaze

- diplopia or double vision - more seen in adults (maturity / plasticity of neural pathways) and suppression mechanisms of the brain in sorting out the images from the two eyes.

- cyclotropia, a cyclotorsional deviation of the eyes (rotation around the visual axis), particularly when the root cause is an oblique muscle paresis causing the hypertropia.

"Cross-fixation congenital esotropia", also called "Cianci's syndrome" is a particular type of large-angle infantile esotropia associated with tight medius rectus muscles. With the tight muscles, which hinder adduction, there is a constant inward eye turn. The patient cross-fixates, that is, to fixate objects on the left, the patient looks across the nose with the right eye, and vice versa. The patient tends to adopt a head turn, turning the head to the right to better see objects in the left visual field and turning the head to the left to see those in the right visual field. Binasal occlusion can be used to discourage cross-fixation. However, the management of cross-fixation congenital esotropia usually involves surgery.

Stereoblindness (also stereo blindness) is the inability to see in 3D using stereopsis, or stereo vision, resulting in an inability to perceive stereoscopic depth by combining and comparing images from the two eyes.

Individuals with only one functioning eye always have this condition; the condition also results when two eyes do not function together properly.

Stereoblind persons with two healthy eyes do employ binocular vision to some extent, albeit less than persons with normally developed eyesight. This was shown in a study in which stereoblind subjects were posed with the task of judging the direction of rotation of a simulated transparent cylinder: the subjects performed better when using two eyes than when using their preferred eye. They appeared to judge the direction of rotation from the images in each eye separately and then to combine these judgments, rather than relying on differences between the images in the two eyes. Also, purely binocular motion stimuli appear to influence stereoblind persons' sensation of self-motion. Furthermore, in some cases each eye can contribute to peripheral vision for one side of the field of view (see also monofixation syndrome).

Fixation disparity exists when there is a small misalignment of the eyes when viewing with binocular vision. The misaligment may be vertical, horizontal or both. The misalignment (a few minutes of arc) is much smaller than that of strabismus, which prevents binocular vision, although it may reduce a patient's level of stereopsis. A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near.

When the fusional vergence system can no longer hold back heterophoria, the phoria manifests. In this condition, the eyes deviate from the fixating position.

Anisometropia is the condition in which the two eyes have unequal refractive power. Each eye can be nearsighted (myopia), farsighted (hyperopia) or a combination of both, which is called antimetropia. Generally a difference in power of two diopters or more is the accepted threshold to label the condition anisometropia.

In certain types of anisometropia, the visual cortex of the brain will not use both eyes together (binocular vision), and will instead suppress the central vision of one of the eyes. If this occurs often enough during the first 10 years of life while the visual cortex is developing, it can result in amblyopia, a condition where even when correcting the refractive error properly, the person's vision in the affected eye is still not correctable to 20/20.

The name is from four Greek components: "an-" "not," "iso-" "same," "metr-" "measure," "ops" "eye."

An estimated 6% of subjects aged 6 to 18 have anisometropia.