Instruct: Given a question, retrieve relevant documents that can help answer the question.
Query: What is the diameter of the human eye?
Dimensions typically differ among adults by only one or two millimetres, remarkably consistent across different ethnicities. The vertical measure, generally less than the horizontal, is about 24mm. The transverse size of a human adult eye is approximately 24.2mm and the sagittal size is 23.7mm with no significant difference between sexes and age groups. Strong correlation has been found between the transverse diameter and the width of the orbit (r = 0.88).[4] The typical adult eye has an anterior to posterior diameter of 24 millimetres, a volume of six cubic centimetres (0.4 cu. in.),[5] and a mass of 7.5 grams (weight of 0.25 oz.)..
The idiosoma, or body, of the "A. tgifi" larva measures . Its anterior eyes are 11 μm in diameter while its posterior eyes have a diameter of 6 μm.
The adults of the star's most intelligent species, called cheela (no flexion for gender or number), have about the same mass as an adult human. However, the extreme gravity of Dragon's Egg compresses the cheela to the volume of a sesame seed, but with a flattened shape about high and about in diameter. Their eyes are wide. Such minute eyes can see clearly only in ultraviolet and, in good light, the longest wavelengths of the X-ray band.
The eye is not shaped like a perfect sphere, rather it is a fused two-piece unit, composed of the anterior segment and the posterior segment. The anterior segment is made up of the cornea, iris and lens. The cornea is transparent and more curved, and is linked to the larger posterior segment, composed of the vitreous, retina, choroid and the outer white shell called the sclera. The cornea is typically about 11.5mm (0.3in) in diameter, and 1/2mm (500μm) in thickness near its center. The posterior chamber constitutes the remaining five-sixths; its diameter is typically about 24mm. The cornea and sclera are connected by an area termed the limbus. The iris is the pigmented circular structure concentrically surrounding the center of the eye, the pupil, which appears to be black. The size of the pupil, which controls the amount of light entering the eye, is adjusted by the iris' dilator and sphincter muscles.
Much of the basic design considerations and related parameters are the same as for binoculars and are covered in that entry, but some expanded comments have been added where appropriate:
Exit pupil is defined as the diameter of the objective lens divided by the magnification and expressed in mm. (e.g. a 8x40 will give an exit pupil diameter of 5mm). For a given situation, the greater the exit pupil, the better the light transmission into the eye. Hence a large objective lens with a low magnification will give good light admission, especially important in deteriorating light conditions. The classic 7x50 marine binocular or monocular is ideally suited to low light conditions with its relatively large exit pupil diameter of 7.1mm and a realistic magnification which is practical on a moving boat. However, the exit pupil should be considered in relationship with the human eye pupil diameter. If the exit pupil of the chosen instrument is greater than the human eye pupil then there will be no benefit, as the eye will be the limiting factor in light admission. In effect, the extra light gathering potential is wasted. This is a consideration as one ages, because human eye pupil dilation range diminishes with age, as shown as an approximate guide in the table below.
The macula is an oval-shaped pigmented area near the center of the retina of the human eye and some other animalian eyes. The macula in humans has a diameter of around and is subdivided into the umbo, foveola, foveal avascular zone, fovea, parafovea, and perifovea areas. The anatomical macula at is much larger than the clinical macula which, at , corresponds to the anatomical fovea.
The theory of operation behind the aperture sight is often stated that the human eye will automatically center the front sight when looking through the rear aperture, thus ensuring accuracy. However, aperture sights are accurate even if the front sight is not centered in the rear aperture due to a phenomenon called parallax suppression. This is because, when the aperture is smaller than the eye's pupil diameter, the aperture itself becomes the entrance pupil for the entire optical system of target, front sight post, rear aperture, and eye. As long as the aperture's diameter is completely contained within the eye's pupil diameter, the exact visual location of the front sight post within the rear aperture ring does not affect the accuracy, and accuracy only starts to degrade slightly due to parallax shift as the aperture's diameter begins to encroach on the outside of the eye's pupil diameter. An additional benefit to aperture sights is that smaller apertures provide greater depth of field, making the target less blurry when focusing on the front sight. 
Four major cognitive systems are involved in eye movement during reading: language processing, attention, vision, and oculomotor control. Eye trackers bounce near infrared light off the interior of the eyeball, and monitor the reflection on the eye to determine gaze location. With this technique, the position of a fixation on a screen can be precisely determined. Wang (2011) mentioned that a video-based eye-tracker which uses video cameras to record the eye position of human subjects—recording pupil dilation and eye movement—can be used to examine how fixations, saccades, and pupil dilation responses are related to the information on the screen and behavioral choices. According to Wang (2011:185), "understanding the relationship between these observables can help us to understand how human behavior in the economy can be affected by what information people acquire, where their attention is focused, what emotional state they are in, and even what brain activity they are engaged in. This is because fixations and saccades (matched with information shown on screen) indicate how people acquire information (and what they see), time lengths of fixations indicate attention, and pupil dilation responses indicate emotion, arousal, stress, pain, or cognitive load." 
Binoculars concentrate the light gathered by the objective into a beam whose diameter, the exit pupil, is the objective diameter divided by the magnifying power. For maximum effective light-gathering and brightest image and to maximize the sharpness, the exit pupil should at least equal the diameter of the pupil of the human eye — about 7 mm at night and about 3 mm daytime, reducing with age. If the cone of light streaming out of the binoculars is "larger" than the pupil it is going into, any light larger than the pupil is wasted. In daytime use the human pupil is typically dilated about 3 mm, which is about the exit pupil of a 7×21 binocular. Much larger 7×50 binoculars will produce a cone of light bigger than the pupil it is entering, and this light will, in the daytime, be wasted. An exit pupil that is too "small" will also present an observer with a dimmer view since only a small portion of the light gathering surface of the retina is used. For applications where equipment has to be carried (birdwatching, hunting), users opt for much smaller (lighter) binoculars with an exit pupil that matches their expected iris diameter so they will have maximum resolution and are not carrying the weight of wasted aperture. 
The eye's fundus is the only part of the human body where the microcirculation can be observed directly. The diameter of the blood vessels around the optic disc is about 150 μm, and an ophthalmoscope allows observation of blood vessels with diameters as small as 10 μm.
The ciliary ganglion is a parasympathetic ganglion located just behind the eye in the posterior orbit. It measures 1–2 millimeters in diameter and in humans contains approximately 2,500 neurons. The oculomotor nerve coming into the ganglion contains "preganglionic" axons from the Edinger-Westphal nucleus (a part of the brainstem) which form synapses with the ciliary neurons. The "postganglionic" axons run in the short ciliary nerves and innervate two eye muscles:
The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart. However, in any given moment of time, the eye can only sense a contrast ratio of 1,000. What enables the wider reach is that the eye adapts its definition of what is black.
The diameter of regular contact lenses that are sold in the United States are on average 14mm–16mm. Similar to the diameter of regular contact lenses, circle lenses have no more than 15mm diameter since larger sizes would be harmful to the eyes at daily wear. When the diameter of the circle lens is described as 16mm or 18mm, it is only the provided visual effect of the circle lens. The difference between the two types of lenses is that circle lenses are tinted not only in areas that cover the iris of the eye, but also prominently in the extra-wide outer rim of the lens. The result is the appearance of a bigger, wider iris with a limbal ring, creating an illusion of large eyes.
Dazzlers emit infrared or invisible light against various electronic sensors, and visible light against humans, when they are intended to cause no long-term damage to eyes. The emitters are usually lasers, making what is termed a "laser dazzler". Most of the contemporary systems are man-portable, and operate in either the red (a laser diode) or green (a diode-pumped solid-state laser, DPSS) areas of the electromagnetic spectrum. The green laser is chosen for its unique ability to react with the human eye. The green laser is less harmful to human eyes.
In humans and most mammals and birds, the iris (plural: irides or irises) is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupil and thus the amount of light reaching the retina. Eye color is defined by that of the iris. In optical terms, the pupil is the eye's aperture, while the iris is the diaphragm.
Steavenson studied how the human eye operates, particularly at the low-light levels encountered in visual astronomy. He measured the diameter of the pupil of a dark-adapted eye to be 1/3 inch (8mm), which was larger than the figure that was believed at that time.
The giant and colossal squids have the largest recorded eyes of any living animal, with a maximum diameter of at least and a pupil (Nilsson "et al.", 2012:683). This is three times the size of the largest fish eyes—up to in swordfish—and more than twice the diameter of the largest whale eyes—up to , , and in blue, humpback, and sperm whales, respectively—which are the largest among vertebrates (Nilsson "et al.", 2012:683). A large colossal squid caught in 2014 and dissected at the Museum of New Zealand Te Papa Tongarewa reportedly had eyes across (Farquhar, 2014). There are unconfirmed reports from the 19th century of giant squid eyes up to across (Land & Nilsson, 2012:86). Only the extinct ichthyosaurs are known to have approached these dimensions (Motani "et al.", 1999; Humphries & Ruxton, 2002), with some species having eyes up to in diameter (Nilsson "et al.", 2012:687).
To use an optical instrument, the entrance pupil of the viewer's eye must be aligned with and be of similar size to the instrument's exit pupil. This properly couples the optical system to the eye and avoids vignetting. (The entrance pupil of the eye is the image of the anatomical pupil as seen through the cornea.) The location of the exit pupil thus determines the eye relief of an eyepiece. Good eyepiece designs produce an exit pupil of diameter approximating the eye's apparent pupil diameter, and located about 20 mm away from the last surface of the eyepiece for the viewer's comfort. If the disc is larger than the eye's pupil, light will be lost instead of entering the eye; if smaller, the view will be vignetted. If the disc is too close to the last surface of the eyepiece, the eye will have to be uncomfortably close for viewing; if too far away, the observer will have difficulty maintaining the eye's alignment with the disc.
Since the eye's pupil varies in diameter with viewing conditions, the ideal exit pupil diameter depends on the application. An astronomical telescope requires a large pupil because it is designed to be used for looking at dim objects at night, while a microscope will require a much smaller pupil since the object will be brightly illuminated. A set of 7×50 binoculars has an exit pupil just over 7.1 mm, which corresponds to the average pupil size of a youthful dark-adapted human eye in circumstances with no extraneous light. The emergent light at the eyepiece then fills the eye's pupil, meaning no loss of brightness at night due to using such binoculars (assuming perfect transmission). In daylight, when the eye's pupil is only 4 mm in diameter, over half the light will be blocked by the iris and will not reach the retina. However, the loss of light in the daytime is generally not significant since there is so much light to start with. By contrast, 8×32 binoculars, often sold with emphasis on their compactness, have an exit pupil of only 4 mm. That is sufficient to fill a typical daytime eye pupil, making these binoculars better suited to daytime than night-time use. The maximum pupil size of a human eye is typically 5–9 mm for individuals below 25 years old, and decreases slowly with age as shown as an approximate guide in the table below.
Kino-Eye (Anglophonic: Cine-Eye) is a film technique developed in Soviet Russia by Dziga Vertov. It was also the name of the movement and group that was defined by this technique. Kino-Eye was Vertov's means of capturing what he believed to be "inaccessible to the human eye"; that is, Kino-Eye films would not attempt to imitate how the human eye saw things. Rather, by assembling film fragments and editing them together in a form of montage, Kino-Eye hoped to activate a new type of perception by creating "a new filmic, i.e., media shaped, reality and a message or an illusion of a message - a semantic field." Distinct from narrative entertainment cinema forms or otherwise "acted" films, Kino-Eye sought to capture "life unawares" and edit it together in such a way that it would form a new, previously unseen truth.
The fastest f-number for the human eye is about 2.1, corresponding to a diffraction-limited point spread function with approximately 1 μm diameter. However, at this f-number, spherical aberration limits visual acuity, while a 3 mm pupil diameter (f/5.7) approximates the resolution achieved by the human eye. The maximum density of cones in the human fovea is approximately 170,000 per square millimeter, which implies that the cone spacing in the human eye is about 2.5 μm, approximately the diameter of the point spread function at f/5.
This technique is mostly used for Dyneema ropes. The principle of a Dyneema eye is a core-to-core splice, in which a length of at least 60 times the diameter of the rope is taken back into itself. DSM advises using 60 times the diameter for coated Dyneema, and 100 times the diameter for uncoated Dyneema. For 6mm coated rope, this would mean 36 cm. Under tension the rope will pull into itself tightly, which produces a strong eye. One can pull out the eye when the rope is not under tension, unless one makes a lock-splice (also called brummel splice).
The ovum is one of the largest cells in the human body, typically visible to the naked eye without the aid of a microscope or other magnification device. The human ovum measures approximately 0.1 mm in diameter.
 Howard's research areas included human spatial orientation (how we tell whether we are upright or lying down), stereopsis (how we sense the distance of objects from our eyes from the differences in the images in the eyes), eye movements, and perceptual ambiguity. Howard was reluctant to use computer-simulated stimuli for his studies; Howard's approach was to study visual perception in realistic settings with real objects, unlike many others in the field, who studied it using pictures of objects. Realistic settings Howard constructed included a full-size rotating room and a 3-m-diameter sphere. He also researched human orientation in microgravity using the "vomit comet" and the space shuttle.
The eye includes a lens similar to lenses found in optical instruments such as cameras and the same physics principles can be applied. The pupil of the human eye is its aperture; the iris is the diaphragm that serves as the aperture stop. Refraction in the cornea causes the effective aperture (the entrance pupil) to differ slightly from the physical pupil diameter. The entrance pupil is typically about 4mm in diameter, although it can range from 2mm (f/8.3) in a brightly lit place to 8mm (f/2.1) in the dark. The latter value decreases slowly with age; older people's eyes sometimes dilate to not more than 5-6mm in the dark, and may be as small as 1mm in the light.[23][24]
The light-gathering power of an optical telescope, also referred to as light grasp or aperture gain, is the ability of a telescope to collect a lot more light than the human eye. Its light-gathering power is probably its most important feature. The telescope acts as a "light bucket", collecting all of the photons that come down on it from a far away object, where a larger bucket catches more photons resulting in more received light in a given time period, effectively brightening the image. This is why the pupils of your eyes enlarge at night so that more light reaches the retinas. The gathering power formula_27 compared against a human eye is the squared result of the division of the aperture formula_2 over the observer's pupil diameter formula_29, with an average adult having a pupil diameter of 7mm. Younger persons host larger diameters, typically said to be 9mm, as the diameter of the pupil decreases with age.