question: When your iris changes in size from light why does does the area we can see not change?
comment: It's because the retina -- where light is processed into signals for the brain -- it's kind of like a movie screen -- is not immediately adjacent to the iris. If the retina were adjacent to the iris, then yes, a smaller iris aperture would mean a smaller field of vision. But light hits the retina (the curved movie screen) pretty much all over whether the iris aperture is tiny or large, because the retina is a centimeter or so away from the aperture. edit1: I had no idea this thread would be so popular! edit2: I neglected to mention the work of the lens, which is pretty important. I was thinking along the lines of the pinhole camera, in which the hole itself is the lens. edit3: Sorry about the "centimeter or so." How's "centimeter or two"? Heh. My bad.
passages: 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 11.5 mm (0.3 in) in diameter, and 1/2 mm (500 μm) in thickness near its center. The posterior chamber constitutes the remaining five-sixths; its diameter is typically about 24 mm. 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.
   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 4 mm in diameter, although it can range from 2 mm () in a brightly lit place to 8 mm () 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.
   Presbyopia, like other focal imperfections, becomes less noticeable in bright sunlight when the pupil becomes smaller. As with any lens, increasing the focal ratio of the lens increases depth of field by reducing the level of blur of out-of-focus objects (compare the effect of aperture on depth of field in photography). Constricting the aperture may be achieved by forming a tiny hole with one's index finger and peering through it.
   Spherical aberration limits the resolution of a 7 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair. A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans.
   Section::::Near response.:Accommodation of the lens.
  
  Changing the curvature of the lens is carried out by the ciliary muscles surrounding the lens; this process is known as "accommodation". Accommodation narrows the inner diameter of the ciliary body, which actually relaxes the fibers of the suspensory ligament attached to the periphery of the lens, and also allows the lens to relax into a more convex, or globular, shape. A more convex lens refracts light more strongly and focuses divergent light rays from near objects onto the retina, allowing closer objects to be brought into better focus.
  
  Section::::Clinical significance.
  
  Section::::Clinical significance.:Eye care professionals.
   Section::::Binocular interaction.
  
  Apart from binocular summation, the two eyes can influence each other in at least three ways.
  
  BULLET::::- Pupillary diameter. Light falling in one eye affects the diameter of the pupils in both eyes. One can easily see this by looking at a friend's eye while he or she closes the other: when the other eye is open, the pupil of the first eye is small; when the other eye is closed, the pupil of the first eye is large.
   Because different lenses generally require a different camera–subject distance to preserve the size of a subject, changing the angle of view can indirectly distort perspective, changing the apparent relative size of the subject and foreground.
  
  If the subject image size remains the same, then at any given aperture all lenses, wide angle and long lenses, will give the same depth of field.
  
  Section::::Lens types and effects.:Examples.
  
  An example of how lens choice affects angle of view.
  
  Section::::Common lens angles of view.
   the two white pixels closer together than a single photoreceptor.
  
  Pupil Size Inversion
  
  When pupils are narrowed to around 1mm for reading fine print,
  
  the size of the central "Airy" disk increases to a diameter of 10 photoreceptors.
  
  The so-called "blur" is increased for reading.
  
  When pupils are widened for fight/flight response,
  
  the size of the central "Airy" disk decreases to a diameter of about 1.5 photoreceptors.
  
  The so-called "blur" is decreased in anticipation of large movements.
  
  No published neuroanatomical model predicts that discrimination
  
  improves when pupils are narrowed.
  
  Pupil Shape Inversion
  
  Eyes have pupils (apertures) that cause diffraction.
   The effect of viewing distance on perceived size can be observed by first obtaining an afterimage, which can be achieved by viewing a bright light for a short time, or staring at a figure for a longer time. It appears to grow in size when projected to a further distance. However, the increase in perceived size is much less than would be predicted by geometry, which casts some doubt on the geometrical interpretation given above. 
   Retinal image size is determined by many factors. The size and position of the object being viewed affects the characteristics of the light entering the system. Corrective lenses affect these characteristics and are used commonly to correct refractive error. The optics of the eye including its refractive power and axial length also play a major role in retinal image size.
   As in a photographic lens, visual acuity is affected by the size of the pupil. Optical aberrations of the eye that decrease visual acuity are at a maximum when the pupil is largest (about 8 mm), which occurs in low-light conditions. When the pupil is small (1–2 mm), image sharpness may be limited by diffraction of light by the pupil (see diffraction limit). Between these extremes is the pupil diameter that is generally best for visual acuity in normal, healthy eyes; this tends to be around 3 or 4 mm.
   The lens continues to grow after birth, with the new secondary fibers being added as outer layers. New lens fibers are generated from the equatorial cells of the lens epithelium, in a region referred to as the germinative zone. The lens epithelial cells elongate, lose contact with the capsule and epithelium, synthesize crystallin, and then finally lose their nuclei (enucleate) as they become mature lens fibers. From development through early adulthood, the addition of secondary lens fibers results in the lens growing more ellipsoid in shape; after about age 20, however, the lens grows rounder with time and the iris is very important for this development. 
   Section::::Focus on near objects.
  
  The refractive index of the eye’s lens system allows the eye to produce sharply focused images. For example, geometrical optics show that as a distant object is brought closer to the eye, the focus of the object becomes blurrier in the plane behind the retina; however, as a result of the increase in the refractive power of the eye, this image becomes clear. The refractive power mainly resides in the cornea, but the overall refractive power is achieved by the actual lens changing its shape.
   Section::::Ambient light response.
  
  A minor mechanism of adaptation is the pupillary light reflex, adjusting the amount of light that reaches the retina very quickly by about a factor of ten. Since it constributes only a tiny fraction of the overall adaptation to light it is not further considered here.
  
  In response to varying ambient light levels, rods and cones of eye function both in isolation and in tandem to adjust the visual system. Changes in the sensitivity of rods and cones in the eye are the major contributors to dark adaptation.
   In addition, all bodies have the same coefficient of dilatation so every body shrinks and expands in similar proportion as they move about the sphere. To finish the story, Poincaré states that the index of refraction will also vary with the distance "r", in inverse proportion to formula_1.
  
  How will this world look to inhabitants of this sphere? 
   The distance of the exit pupil from the sensor plane determines the range of angles of incidence that light will make with the sensor. Digital image sensors often have a limited range of angles over which they will efficiently accept light, especially those that use microlenses to increase their sensitivity. The closer the exit pupil to the focal plane, the higher the angles of incidence at the extreme edges of the field. This can lead to pixel vignetting. For this reason, many small digital cameras (such as those found in cell phones) are image-space telecentric.
  
  Section::::See also.
  
  BULLET::::- Transmittance
   Section::::Accelerating dark adaptation.:Anthocyanins.:Effect on dark adaptation.
   Knapp's rule
  
  Knapp's Rule states that lenses placed at the anterior focal point of the eye, generally 15 mm in front of the eye, will create similarly sized images on the retina, whenever the disparity between the two eyes is due to a difference in axial length of the eyes.
   Section::::Neural mechanisms.
   Retinal eccentricity is when objects projected directly onto the fovea are the sharpest, while those further out in the periphery of the retina become steadily less focused. This is due to the distribution of cones and rods throughout the retina. When we view an image, light passes through the cornea, and through the pupil, which is controlled by the iris. It then passes through the lens and is projected onto the retina at the back of the eye. In the middle of the retina is the fovea, which contains a high concentration of cones. Cones are responsible for colour vision and have high spatial acuity, whereas rods are not. Rods are instead responsible for vision at lower light. Cones are much more sparsely located in the periphery of the retina, where there is instead a greater concentration of rods. When a central image is projected directly onto the fovea it is sharper and processed with greater speed and accuracy due to these cones, whereas images further in the periphery (greater eccentricity) are processed more slowly and less accurately.
   The iris contains two groups of smooth muscles; a circular group called the sphincter pupillae, and a radial group called the dilator pupillae. When the sphincter pupillae contract, the iris decreases or constricts the size of the pupil. The dilator pupillae, innervated by sympathetic nerves from the superior cervical ganglion, cause the pupil to dilate when they contract. These muscles are sometimes referred to as intrinsic eye muscles.
   Increased corneal curvature is yet another change known to arise from long-term contact lens wear; this increase in corneal curvature can be as much as 0.5 diopters greater than normal. Corneal surface irregularity and asymmetry are also caused by long-term contact lens wear; these problems are sometimes correlated with astigmatism in contact lens wearers and are thought to be caused by hypoxia, surface molding, and chronic and mild trauma to the cornea from contact lens use.
  
  Long-term use of PMMA or thick hydrogel contact lenses have been found to cause corneal warpage (shape distortion).
   Section::::Facial discrimination.:Cross species.
   Section::::Dark adaptation.
   Section::::Efficiency.
  
  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.
  
presuppositions: The iris determines how much an eye sees. A bigger iris aperture should let us see more.
corrections: The retina determines how much an eye can see, not the iris. Light is processed into signals when it hits the retina, which does not change with the iris.
raw_presuppositions: The iris determines how much an eye sees. A bigger iris aperture should let us see more. A bigger iris aperture should let us see more.
raw_corrections: The retina determines how much an eye can see, not the iris. Light is processed into signals when it hits the retina, which does not change with the iris. Light is processed into signals when it hits the retina, which does not change with the iris.