Vision
How do humans see? Perspective, binocular vision, parallax.
118 flashcards · Physics · by lakesare
What is binocular summation?
Binocular summation refers to the improved visual performance (when compared to monocular vision): Brightness perception. The binocularly perceived brightness is larger than the brightness seen by each individual eye. This helps with detection of dim lights and also provoke pupil to decrease its size, which improves focusing. Flicker perception. Binocular summation can increase the critical flicker fusion rate (CFF) which is the highest perceivable flicker rate before the image appears continuo…
What is vergence?
Vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision. When looking into the distance, the eyes diverge until parallel, effectively fixating the same point at infinity (or very far away).
What is parallax?
Parallax is a displacement in the apparent position of an object viewed along two different lines of sight. A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a needle-style speedometer gauge. When viewed from directly in front, the speed may show exactly 60; but when viewed from the passenger seat the needle may appear to show a slightly different speed, due to the angle of viewing.
How is parallax measured?
By the angle of inclination between two lines of sight.
What is foreshortening?
Foreshortening is the optical illusion that causes an object or distance to appear shorter than it actually is because it is angled toward the viewer. (Here the hand gradually angles towards us, and we see it as shorter)
What are the constructivist VS ecological views of perception?
Constructivist view of perception: Stimulus is inherently ambiguous Stimulus is supplemented with Knowledge, Inference Ecological view of perception: Perceptual system evolved to extract information No inferences, little or not learning aka Direct Perception Constructivist view - states that perception is a cognitive construction based on sensory input plus information retrieved from memory (Helmholtz, Piaget, Vygotsky).Ecological view - states that perception has functional purposes of bringin…
Visible light is usually defined as having wavelengths in the range of 400–700 nm (nanometres). What are these waves?
Electromagnetic radiation.
Both light and sound are waves. What is the difference?
Sound is alternating pressure in the air (or other mechanical medium). Light is an electromagnetic wave. When you bang a cymbal, it vibrates. The air molecules around the tuning fork also start to vibrate. The vibration gets passed through the air as a vibrating sound wave.
Why is the pupil black?
Because it allows the light to pass through it. The iris blocks light from travelling further, while the pupil allows it through, thus appearing black (as the light that goes through is absorbed by the remainder of the visual system).
What does dilated eye exam consist of?
During a comprehensive dilated eye exam, the patient receives special eye drops that dilate the pupils. The pupil dilates, allowing doctor to see the back of the eye clearly.
Name 3 parts on this image:
Where is the blind spot located?
[1] The blind spot, or scotoma, is the place in our eyes where the optic nerve passes through the retina to the brain.
What's a blind spot?
The blind spot (or scotoma) is the area on the retina where optic nerve passes to the brain, and it has no receptors that respond to light. Therefore an image that falls on this region will NOT be seen. Close your left eye and focus your right eye on the dot on the left. Place your eye at a distance from the screen which is approximately twice the distance between the dot and the center of the grid on the screen. Now slowly move your head backwards away from the screen. At a certain point you w…
What is macula?
Area of the retina that provides high-quality focused vision. The center of the macula contains the highest concentration of cone cells, one of the two types of photoreceptor cells in the eye. This small, central pit – the fovea centralis – is located right in the middle of the macula and is responsible for sharp, central vision.
Where is macula?
[2] Macula
Why do humans see everything as grey in the dark?
In the human eye, cone cells are nonfunctional in low light. Our cones therefore play a key part in giving us color vision. We have three types of cones which have maximum sensitivity to either red, blue or green light, respectively, corresponding to the specific wavelengths of daylight. At night we lose the light of these three color wavelengths. As a result, we no longer have access to the color information so only our rods are active – and that's why everything looks grey. Compared to diurna…
What 2 kinds of photoreceptive cells does retina contain?
rods cones
What is fovea?
A pit in the center of macula, with the highest density of cones on the retina.
Where is cornea?
5. Cornea
What is 5, 6, and 7?
5. Cornea 6. Ocular chamber 7. Pupil
Where is iris?
8. Iris
Where is cornea and lens?
5. Cornea 9. Lens
Why do vertebrae have a blind spot, and cephalopods don't?
Vertebrate eyes: the nerve fibers route before the retina, creating a blind spot where the fibers pass through the retina. Cephalopod eyes: the nerve fibers route behind the retina, and do not block light or disrupt the retina. 1 - retina2 - nerve fibers3 - optic nerve4 - vertebrate blind spot. Links: https://en.wikipedia.org/wiki/Cephalopod_eye
What is optical power?
Optical power is the degree to which a lens, mirror, or other optical system converges or diverges light. The SI unit for optical power is the inverse metre (m−1), which is commonly called the dioptre.Converging lenses have positive optical power, while diverging lenses have negative power. An eye that has too much or too little refractive power to focus light onto the retina has a refractive error. A myopic eye has too much power so light is focused in front of the retina. Conversely, a hypero…
One theory accounting for some empirical observations is known as the opponent theory of colour vision. The observation that it most clearly explains is the commonly-expressed view that some colours are opposites of each other: red and green are opposites, and so are yellow and blue. By ‘opposite’ here is meant that there is no such thing as a greenish-red colour or a bluish-yellow; whereas other colours are perceived as mixtures of these opponent primaries. The proposed mechanism for this oppo…
The physiological effects of opposed colours was investigated in Theory of Colours by Goethe;4 more recent psychological theories use the mechanism of opposition to explain not only colour vision but also other sensations such as emotion and addiction. However, even in the case of colour vision, there is experimental data that suggests that the opponent theory is not precisely true: under certain conditions, it is possible to cause people to perceive a colour which is described as reddish-green.
Show cones and rods.
Are there more cones or rods in humans?
There are a lot more rods! The human retina contains about 120 million rod cells, and 6 million cone cells. The number and ratio of rods to cones varies among species, dependent on whether an animal is primarily diurnal or nocturnal. Certain owls, such as the nocturnal tawny owl, have a tremendous number of rods in their retinae.
What lies in the center of the retina (a point directly behind the lense)?
Fovea (inside the Macula).
How many cones and rods are there in: Fovea (center of Macula) Blind spot (optic nerve)
Fovea:::::::::::: 0 rods, highest cones Blind spot:::: 0 rods, 0 cones
What is affected during the LASIC surgery?
Cornea. The LASIK surgery is performed by an ophthalmologist who uses a laser to reshape the eye's cornea in order to improve visual acuity.
How is signal by photoreceptors ambiguous?
Photoreceptors do not signal color; they only signal the presence of light in the visual field. A given photoreceptor responds to both the wavelength and intensity of a light source. For example, red light at a certain intensity can produce the same exact response in a photoreceptor as the green light of different intensity. Therefore, the response of a single photoreceptor is ambiguous when it comes to color.
Name 3 frequencies on the electromagnetic spectrum (from smallest to largest frequency)
Radio waves Microwave Infrared Visible Ultraviolet X-ray Gamma ray
What is scotopic vision?
Scotopic vision is the vision of the eye under low-light levels. Simulated examples of vision under low light. Top: human; bottom: cat The term comes from Greek:skotos - "darkness",opia - "a condition of sight".
What does the Purkinje effect consist of? Simulated appearance of a red geranium and foliage in normal bright-light (photopic) vision, dusk (mesopic) vision, and night (scotopic) vision.
Purkinje effect consists of a difference in color contrast under different levels of illumination. Reds will appear darker relative to other colors as light levels decrease. In bright sunlight: geranium flowers appear bright red against the dull green of their leaves, At dusk: the contrast is reversed, with the red petals appearing a dark red or black, and the leaves and blue petals appearing relatively bright.
What is the cause of the Purkinje effect?
In dark conditions, rods take precedence for vision, and rods are completely insensitive to light of wavelengths above 640nm (a shade of red). The rods have a single kind of pigment within them, which absorbs light in the wavelength range of about 380–640nm. This is the cause of the Purkinje effect, the name given to the observation that blue objects appear brighter than red ones under darker illumination conditions. Apart from this, rods play little part in colour vision, as they cannot discri…
What is binocular vision?
Vision using two eyes with overlapping fields of view, allowing good perception of depth.
What photoreceptor cells are more sensitive to light - rods or cones?
Rods. Because of the high sensitivity of rods to light, they are the primary photoreceptors used in dark conditions, such as at night. In addition, the rods are largely absent from the centre of the retina (completely absent from the fovea) and are common on the outside; thus they are used for peripheral vision. The rods are sensitive to motion, but have poor spatial discriminating power, which explains why peripheral vision often gives the information that something is moving, but not any deta…
Explain 3 types of cone cells: S-cones, M-cones, and L-cones
Each cone is therefore sensitive to wavelengths of light - S-cones: short-wavelength light, M-cones: medium-wavelength light, and L-cones: long-wavelength light. Respectively, their peak sensitivities are at wavelengths around 570nm, 540nm and 430nm. The cones are able to respond quickly to a stimulus, and have high concentration in the fovea, giving high-detail central vision. However, they only operate under conditions of bright illumination.
Name 2 effects tetrachromacy may introduce.
Tetrachromacy is when organism's retina contains four types of cones with different absorption spectra. This means that: Organism may see wavelengths beyond those of a typical human's vision, and may be able to distinguish between colors that, to a normal human, appear to be identical.
What is a photopic vision?
Vision in normal bright light.
Why do airplane cockpits use red lights?
Under the red light both photopic and scotopic visions are active! Because: rods: are insensitive to the red light, thus they don't get tired of the red light - they remain night-adjusted cones: get enough light to provide high-acuity vision required for reading Airplane cockpits use red lights so pilots can read their instruments and maps while maintaining night vision to see outside the aircraft.
There are 3 types of color blindness: monochromacy dichromacy anomalous trichromacy. Explain each type!
monochromacy - when 2 or all 3 cones are absent. dichromacy - when 1 cone is absent. anomalous trichromacy - when 1 of the cone pigments is altered in its spectral sensitivity. Monochromacy is also known as "total color blindness", is the lack of ability to distinguish colors (and thus the person views everything as if it were on a black and white television); caused by cone defect or absence. Monochromacy occurs when two or all three of the cone pigments are missing and color and lightness vis…
How is color blindness diagnosed?
Using The Ishihara test.
Why are males more likely to be color blind that females?
Genes responsible for the most common forms of color blindness are on the X chromosome. As females have 2 X chromosomes, a defect in one is typically compensated for by the other, while males only have one X chromosome. About 8% of males, and 0.6% of females, are color blind in some way or another, whether it is one color, a color combination, or another mutation. Males are at a greater risk of inheriting an X-linked mutation because males only have one X chromosome (XY, with the Y chromosome c…
Name 5 separate ways in which binocular vision is advantageous. It gives a creature a spare eye in case one is damaged. It gives a wider field of view. 4. 5. - ?
Gives depth perception. It allows to see more of an object behind an obstacle. It gives binocular summation in which the ability to detect faint objects is enhanced. Field of view: For example, humans have a maximum horizontal field of view of approximately 190 degrees with two eyes, approximately 120 degrees of which makes up the binocular field of view (seen by both eyes) flanked by two uniocular fields (seen by only one eye) of approximately 40 degrees.Depth perception: It can give stereopsi…
What is Greek for first second?
prōtos (e.g. prototype) deuteros (e.g. deuteranopia) Deutero- is pronounced as /ˈdjuːtərəʊ/, дьютеро
Name 3 types of dichromacy.
protanopia - insensitivity to red light. deuteranopia - insensitivity to green light. tritanopia - insensitivity to blue light. Protanopia - Early 20th century from proto-‘original’ (red being regarded as the first component of colour vision) + an-‘lacking’ + -opia.Deuteranopia - Early 20th century from deutero-‘second’ (the colour green being regarded as the second component of colour vision) + an-+ -opia.Tritanopia - Early 20th century from trito-‘third’ (referring to blue as the third colour…
What is positive afterimage VS negative afterimage?
An afterimage is an image that continues to appear in the eyes after a period of exposure to the original image. positive - appear the same color as the original image. negative - opposite colors. Better physical explanation, TODO https://en.wikipedia.org/wiki/Afterimage
What photopigments are active in mesopic vision?
mesopic - both cones and rods photopic - only cones scotopic - only rods In a relatively bright environment, the cones alone function to give what is called photopic vision.In a relatively dark environment, the rods alone function to realize what is called scotopic vision.In environments having an intermediate brightness between photopic vision and scotopic vision, both the cones and the rods function to provide what is called mesopic vision. In photopic vision, the photochemical reaction in th…
Explain what's going on on this picture: Progress of dark adaptation
This is a graph of minimum luminance sensed as light. for white light for red light, as times goes by during the dark adaptation. For white light, - we change from curve A to curve B as the rods take over from the cones upon transfer from light adaptation to dark adaptation. For red light, - rods never take over, because they are not sensitive to red light! In the initial stage of dark adaptation (about 10 min), cones function to give curve A but, in the later stages, rods with their higher res…
How much time does it take to adapt to: bright environment VS dark environment?
From dark to bright: 1 minute From bright to dark: 30 minutes When one enters a bright environment from a dark one, one’s vision changes from scotopic to photopic via mesopic. This change is completed in about 1 min, and the eye readily adapts to the bright environment.On the contrary, when one enters a dark environment from a bright one, vision changes from photopic to scotopic much more slowly. It takes about 30 min to completely accomplish the adaptation.
How many photons need to be incident on the rods VS cones before they respond?
Rods: 5-14 photons Cones: 100-1000 TIMES more photons Taking into account absorption and scattering of light inside the eye and the absorption efficiency of the retina, this limit corresponds to about 5–14 photons incident on the rods. Cones on the other hand require about 100–1000 as many photons before they respond.
What is [11]?
Vitreous body Or vitreous humour
How is accommodation achieved in the eye? Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies.
When the ciliary muscle contracts, the lens becomes more spherical – and has increased focusing power. When the ciliary muscles relax, these fibres become taut – pulling the lens out into a flatter shape, which has less focusing power. The effect of ciliary muscle contraction or relaxation on the focussing power of the lens. When viewing a far object, the circularly arranged ciliary muscle relaxes allowing the lens zonules and suspensory ligaments to pull on the lens, flattening it. The source…
What is [7]?
Ciliary body/ ciliary muscles
What changes the shape of the pupil VS the lens?
Shape of pupil: iris. Shape of lens: ciliary muscle.
What is this?
Sclera The sclera, also known as the white of the eye, is the opaque, fibrous, protective, outer layer of the human eye containing mainly collagen and some elastic fiber.
What is latin for eyelash?
Cilium (pl. cilia) 1. BiologyA short microscopic hairlike vibrating structure found in large numbers on the surface of certain cells, either causing currents in the surrounding fluid, or, in some protozoans and other small organisms, providing propulsion.2. AnatomyAn eyelash. Early 18th century (in the sense ‘eyelash’): from Latin.
What is the problem of univariance?
The problem of univariance is the fact that an infinite set of different wavelength–intensity combinations can elicit exactly the same response from a single type of photoreceptor. In general, the problem of univariance is if you have one variable, you will only be able to represent the one type of information.
Name 2 ways in which a response from the single cone is ambiguous.
1. Light of 2 different wavelength can produce the same response 2. Light of different wavelengths and different power can product the same response Young's theory of colour vision may now be stated in terms of cone pigments. 'There are three classes of cone each containing a different visual pigment. The output of each cone is univariant, depending simply upon the quantum catch of its pigment. Our sensation of colour depends upon the ratios of these three cone outputs'.
How is this 'normalized absorbance' measured for cones?
Since cones respond to both the wavelength and intensity of light, the cone's sensitivity to wavelength is measured in terms of its relative rate of response if the intensity of a stimulus is held fixed, while the wavelength is varied. From this, in turn, is inferred the absorbance. The graph normalizes the degree of absorbance on a hundred-point scale. For example, the S cone's relative response peaks around 420 nm (nanometers, a measure of wavelength). This tells us that an S cone is more lik…
Is trichromatic theory or opponent color theory right?
Both theories have solid empirical evidence. The conundrum was resolved by the discovery of color-opponent ganglion cells in the retina and lateral geniculate nucleus. We now know that the human eye possesses three types of color-sensitive receptors which then combine their signals in three color-opponent channels as proposed by Hering.Thus, both the Hering and Young-Helmholtz theories are correct.
Where does the opponent process happen biologically?
In the brain, in parvocellular cells (P cells) of the lateral geniculate nucleus. The information from cones is then passed to ganglion cells, of which there are two major classes: magnocellular, parvocellular - handle the majority of information about color and fall into two groups: one that processes information about differences between the firing of L and M cones, and one that processes differences between S cones and a combined signal from both L and M cones. P-type retinal ganglion cells…
What is inverse problem in optics?
The fact that we can't reverse engineer illumination, reflectance, etc. Fundamentally ambiguous mapping between sources of retinal stimulation and the retinal images that are caused by those sources. For example, the size of an object, the orientation of the object, and its distance from the observer are conflated in the retinal image. For any given projection on the retina there are an infinite number of pairings of object size, orientation and distance that could have given rise to that proje…
How does the inverse optics problem apply to geometry? inverse problem in optics (or the inverse optics problem)
Name 4 layers in the retina.
Ganglion cell layer Bipolar, amacrine, horizontal cells Cones&rods layer Sclera
What is retinal summation?
Retinal summation describes how many photoreceptor converge on a single bipolar cell. With high retinal summation, a large number of photoreceptor cells converge on a smaller number of bipolar cells in transferring their signals to ganglion cells.Zero summation occurs when each cone photoreceptor cell contacts a single ganglion cell via a single bipolar cell.
What's high retinal summation, and what effect does it have?
When large number of photoreceptors converge on 1 bipolar cell in transferring their signals to ganglion cells. High summation: increases sensitivity to light reduces visual acuity. Low retinal summation results in high visual acuity, with individual photoreceptor cells sending their own signals. High retinal summation yields high sensitivity to low light levels, where the signal is summed before reaching the brain—presumably advantageous when the signals reaching individual photoreceptor cells…
Name 3 steps in the primary visual pathway.
Retina Lateral Geniculate Nucleus (LGN) Primary visual cortex
What's optic chiasma?
What's retinotopy?
Retinotopy is the mapping of visual input from the retina to visual cortex. Retinotopic maps in cortical areas other than V1 are typically more complex, in the sense that adjacent points of the visual field are not always represented in adjacent regions of the same area.
What does LGN stand for?
Lateral geniculate nucleus
What's lateral geniculate nucleus (LGN)?
LGN is a relay center in the thalamus for the visual pathway. Receives input from the retina, leads it into the primary visual cortex (in the occipital lobe). Both the left and right hemisphere of the brain have a lateral geniculate nucleus, named after its resemblance to a bent knee (genu is Latin for "knee").
Name 2 types of layers in lateral geniculate nucleus (LGN).
Layer with parvocellular cells - cones Layer with magnocellular cells - rods In humans as well as in many other primates, the LGN has layers of magnocellular cells and parvocellular cells that are interleaved with layers of koniocellular cells. In humans the LGN is normally described as having six distinctive layers. The inner two layers, (1 and 2) are magnocellular layers, while the outer four layers, (3,4,5 and 6), are parvocellular layers.
In the lateral geniculate nucleus (LGN) - where do the P-cells and M-cells get their input from?
From the retinal ganglion cells. P-cells: from ganglion P-cells (then from cones) M-cells: from ganglion M-cells (then from both cones and rods)
Why is vision in the fovea more detailed?
In the fovea: 5 photoreceptors to 1 gangion cell. In the periphery: 100 photoreceptors to 1 ganglion cell. 100 million photoreceptors, to 1 million retinal ganglion cells! This is like a data compression system.
Luminance VS lightness?
How is it possible to see anything when we have blood vessels in front of our photoreceptors?
Photoreceptors adapt. When they fire for too long, they get tired. They adapt to the vessels as well.
What substance is used to activate cells in monkeys and observe retinopy upon dissection?
Radioactive sugar.
Explain why this grey looks like it has a different lightness.
Links: http://courses.washington.edu/psy333/lecture_pdfs/Week2_Day3.pdf
What's Mach bands?
Illusory brightness maxima and minima perceived at the initiation and termination of luminance gradients. Mach bands is an optical illusion named after the physicist Ernst Mach. It exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system. Up until 50 years ago the dominant explanation of Mach Bands was that they were caused by lateral inhibition among retinal neurons.More recentl…
What's the semantic origin of the word 'parvocellular cell'? What's the semantic origin of the word 'magnocellular cell'?
'Parvus' is Latin for 'small', and the name "parvocellular" refers to the small size of the cell compared to the larger magnocellular cells. 'Magnus' is Latin for 'great'.
What's lightness constancy?
The amount of light coming to the eye from an object depends on the amount of light striking the surface, and on the proportion of light that is reflected. If a visual system only made a single measurement of luminance, acting as a photometer, then there would be no way to distinguish a white surface in dim light from a black surface in bright light. Yet humans can usually do so, and this skill is known as lightness constancy.
What does Cornsweet illusion consist of?
Left part of the picture seems to be darker than the right one. In fact they have the same brightness.
How is the Cornsweet illusion best explained?
Bayesian theories of perception describe how sensory data (that have a particular likelihood) are combined with prior beliefs (a prior distribution) to create a percept (a posterior distribution). The main point made by this work is that illusory percepts are optimal in the sense of explaining sensations in terms of their most likely cause. In brief, illusions occur when the experimenter generates stimuli in an implausible or unlikely way. From the subject’s perspective, these stimuli are ambig…
Depth VS distance?
depth - sense of 3-dimensionality, mediated binocularly distance - sense of how far an object is, mediated monocularly
List 3 monocular cues into what distance the object is in.
perspective occlusion (object is covered up by more close object) aerial perspective (fuzzier/bluer) Aerial perspective - Due to light scattering by the atmosphere, objects that are a great distance away have lower luminance contrast and lower color saturation. Due to this, images seem hazy the farther they are away from a person's point of view. In computer graphics, this is often called "distance fog".
What's stereopsis?
Stereopsis is a term that is most often used to refer to the perception of depth and 3-dimensional structure obtained on the basis of visual information deriving from two eyes by individuals with normally developed binocular vision. We can more accurately touch one pen point to another when using both eyes because of stereopsis.
What's horopter?
Horopter - all the points that seem to be equidistant from you when you look at the fixation point. From the Greek words horos (= boundary) and opter (observer).
What's retinal disparity?
What's correspondence problem?
The correspondence problem refers to the problem of ascertaining which parts of one image correspond to which parts of another image, where differences are due to movement of the camera, the elapse of time, and/or movement of objects in the photos. It can be achieved by comparing the left and right retinal images, an ability known as stereopsis. To do this the brain must first determine for each point in one retinal image which point in the other originated from the same part of the same object…
What's spherical aberration?
The fact that if the lense is not perfectly 'thin', - it's focal point will slightly vary.
What's chromatic aberration?
Diffraction happens. Red gets bent a little less than other colors.
What's this phenomenon called?
Binocular rivalry. When one image is presented to one eye and a very different image is presented to the other (also known as dichoptic presentation), instead of the two images being seen superimposed, one image is seen for a few moments, then the other, then the first, and so on, randomly for as long as one cares to look.
What's Van Hare effect?
Two identical images of a historic photo of a flight of RAF Hawker Hurricanes are placed side-by-side to demonstrate the Van Hare Effect, where a 2D image can be artificially perceived as if it is stereoscopic. Seeing Artworks in 3D -- Painting and flat image artworks of most types can be seen as if in artificial 3D, even if the artist did not paint any significant clues as to the dimensions of the subjects. Using the Van Hare Effect, the human brain simply fills in an artificial dimensional de…
What's cortical blindness?
Cortical blindness is the total or partial loss of vision in a normal-appearing eye caused by damage to the brain's occipital cortex.
What's blindsight?
Blindsight is the ability of people who are cortically blind due to lesions in their primary visual cortex (or V1), to respond to visual stimuli that they do not consciously see.
What's striate cortex?
V1 in visual cortex. The striate cortex is the part of the visual cortex that is involved in processing visual information. The striate cortex is the first cortical visual area that receives input from the lateral geniculate nucleus in the thalamus.
Ecological (James J. Gibson) VS cognitivist approach to perception (Hermann von Helmholtz)?
ecological approach - for Gibson, perception is a bottom-up process, whereby the agent accesses information about the environment directly from the ambient optic array, rather than recovering it by means of complex cognitive processes. cognitivist approach - sensations require 'enriching', which is the role of the mental model. Assumes there is a poverty of stimulus. This (with reference to perception) is the claim that sensations are, by themselves, unable to provide a unique description of th…
What's near point?
The point nearest to the eye at which an object is accurately focused on the retina when the maximum degree of accommodation is employed. A normal eye is considered to have a near point at 25 cm. Сlosest point at which an object can be placed and still form a focused image on the retina, within the eye's accommodation range.