Author(s): Günther KH Zupanc
Exploring visual acuity requires not only biological experiments, but also some understanding of the underlying physics.
Visual acuity is a measure of how sharp our vision is, particularly how well we can resolve small details. Opticians assess this by asking us to read from a wall chart until the letters become too small for us to make out clearly.
One of the biological factors that determines visual acuity is the density of the photoreceptor cells in the retina (see box ‘How our eyes work’ below). This prompts an intriguing question: could increasing the density of receptors in the retina make our vision sharper? To answer this question, we need to consider both the biology of the visual system and the physics of light. These topics are part of most secondary-level biology and physics curricula. However, they are traditionally taught separately, often to different groups of students. In contrast, the cross-curricular approach proposed here will enable students to gain a deeper understanding of both the biology and physics of visual acuity and will convey an important general message: many problems in modern science can best be solved through teamwork and interdisciplinary collaboration.
This article describes a simple way of assessing visual acuity by creating a wall chart and using it to calculate eye resolution. This method can then be used to estimate the smallest distance on the retina at which the images of two points can be identified as two distinctly separate objects. A follow-up experiment, available in the additional materials,relates this distance to the theoretical limits of visual acuity based on the physical properties of light.
The two activities are most appropriate for students aged 16-19 years and will take about two hours each, including the preparation and data analysis. Another two hours should be allowed for the discussion of the results.
How our eyes work
Light rays reflected by objects enter the eye via the pupil. Four components of the eye – the cornea, lens, aqueous humour and vitreous humour – focus the light rays on the retina, the surface of the back of the eyeball (figure 1). The retina is organised in different layers, one of which is made up of millions of light-sensing photoreceptors that pass signals to the brain via other cells such as the ganglion cells. The photoreceptor cells are specialised nerve cells and come in two types – rods and cones (named for their shape). Cones, found mainly in the centre of the retina, enable sharp colour vision in bright light. Rods, found towards the edges of the retina, help us to detect motion and to see in dim light, and allow peripheral vision. The density of photoreceptors in the retina (which can be over 200 000 cells per square millimetre in the fovea centralis) plays an important role in visual acuity.
Estimating the resolution of the human eye
The angular resolution of the human eye is a measure of the smallest angle between two points that are perceived as distinctly separate and is related in part to the density of photoreceptors on the retina. It is typically around 1 arcminute (1/60th of a degree). In this activity, angular resolution is calculated by determining the ratio of the distance between the two points and the distance between the observer and the points. This simplified mathematical procedure (which replaces the more complicated calculation of the tangent of the angle α in figure 2) is possible because the angular resolution assumes very small values. In mathematics, this shortcut is known as the small-angle approximation. The calculated ratio can then be used to estimate the distance between these two points projected onto the retina, as shown in figure 2.
Figure 2: The reduced eye. This simplified model of the ocular system of the human eye approximately matches the ocular dimensions of the real eye, but combines the refractive bodies of the cornea, lens, aqueous humour and vitreous humour into one, assuming a uniform index of refraction.The students will create a chart with simple black lines separated by gaps of varying width. They will then be asked to state which lines are perceived as separate and which appear to be merged, and to use these results to calculate an estimate of the angular resolution and thus the spacing between the receptors in the eye.
Materials
- Computer with simple drawing software installed
- Printer and white paper
- Tape measure
- Ruler with millimetre scale
Procedure
- Instruct your students to use the drawing software to create a chart of black bars interrupted by small gaps. The width of these gaps should vary between 0.5 mm and 5 mm, and the bars should be arranged on one page, rather like in figure 3. Include one or two complete bars with no gap as a control.
- Ask your students to produce additional charts using the same bar patterns in the individual rows as in step 1 but arranged in a random order.
- Print the charts onto white paper using a high-quality setting.
- Fix the charts to the wall in a well-lit room and mark a viewing point on the floor approximately 7–10 m from the chart. Measure this distance (d) exactly.
- Arrange the students into pairs or small groups, so they will be asked to ‘read’ a different chart to the one they produced. One member of the group is the subject, who waits outside the room, while another, the experimenter, selects the wall chart.
- Ask the subject to come into the room and stand at the viewing point, while the experimenter covers the test chart. Uncover the test chart and ask the subject to ‘read’ the bar patterns by identifying which bars appear uninterrupted, and which ones have visible gaps. Record the results.
- Repeat until all students have served as both subject and experimenter.
- Using the ruler, determine the width x of the smallest gap on the charts that each student was able to correctly identify.
- For each student, use the values for distance from chart, d, and smallest perceived gap, x, to calculate the angular resolution α (in arcminutes), which is given by equation 1. Make sure that the units of d and x are identical.
α = (180x/dπ)60 Equation 1 - What is the range of values of α for different students? What is the mean angular resolution of this group of students? Give an error estimate for your measurements by taking into account the accuracy when you measuredwith the tape measure andxwith the ruler.
- Now calculate the distanceybetween the images of two points when projected onto the retina. For this calculation, assume a simplified model of the eye’s optic system with a single refractory surface and a uniform index of refraction. The focal lengthfof this reduced eye is 20.1 mm. The distanceybetween the two points on the retina is given by rearranging equation 1 to give equation 2:
y= (απf)/ (180 ⋅ 60)Equation 2 - Ask your students what, theoretically, would be the minimum number of retinal photoreceptors required to resolve these two points. What would the centre-to-centre distance between these photoreceptors be?
What is happening?
The angular resolution of the human eye typically ranges between 40 arcseconds and 1 arcminute. To perceive two separate points, at least three photoreceptors arranged in a row are required: one to receive light from each of the points, and one for the gap in between the points. For an angular resolution of 1 arcminute (which corresponds to 0.3 m at a distance of 1 km), the images on the retina are separated by approximately 6 μm, meaning the centre-to-centre distance between two neighbouring receptors is 3 μm. At an angular resolution of 40 arcseconds, the distance between the imaged points is approximately 4 μm.
The actual resolution of the eye is affected not only by photoreceptor spacing, but also by the diffraction of light as it passes through the pupil. You can explore this further by downloading experiment 2.
So, could visual acuity be improved by increasing the density of the cones in the retina?
An angular resolution of 40 arcseconds to 1 arcminute is achieved only when we look fixedly at an object. The image of the object is then projected onto a specific part in the centre of the retina, the fovea centralis, which contains only cone photoreceptors. The density of cones in the fovea is much higher than anywhere else in the retina, and the cones here have a diameter of only 3 μm (compared to up to 10 μm in other areas of the retina). Allowing for some extracellular space around each cone (e.g. for transport of nutrients), the centre-to-centre distance between the cones in the fovea is about 4 μm. Thus the density of the cones in the fovea is already very close to the maximum packing density possible.
As can be explored in the follow-up experiment, light diffraction in the eye means the minimum resolvable distance between two points of light is about 5 μm, giving a minimum predicted distance between cones of approximately 2.5 μm. Allowing for some extracellular space between the cones, this theoretically predicted distance is in excellent agreement with the actual distance between the cone receptors in the fovea centralis of approximately 4 μm. Therefore a further increase in the density of cones might not be possible on biological grounds and, based on physical limitations determined by the properties of light, would not lead to any significant gain in visual acuity.
References
- Across the USA, huge test charts – similar to the wall charts employed in this experiment – are used to calibrate ‘flying cameras’.
Resources
- A follow-up experiment, exploring the theoretical limits of visual acuity based on the physical properties of light, can be downloaded from the additional materialsection.
Author(s)
Günther KH Zupanc is a professor of biology at Northeastern University in Boston, Massachusetts, USA. He holds degrees in biology, physics and neuroscience. Over the past 25 years, he has taught a wide range of biology classes to numerous students at German, British and US universities. His book Behavioral Neurobiology: An Integrative Approach (Oxford University Press) is the most frequently adopted text for teaching this subject around the world. He would like to thank his son Frederick B Zupanc and his wife Dr Marianne M Zupanc for their helpful comments on this article.
Review
This article describes two experiments related to visual acuity, an example of the numerous links between biology and physics that can be found in nature. Cross-curricular activities can make science more appealing and can offer a great opportunity to collaborate with other teachers.
All the materials required for the experiments are readily available and the instructions are easy to follow, making the activities suitable for students to perform in small groups.
The text could be used as a starting point for discussing the importance of the teamwork and interdisciplinary collaboration to solve many problems in modern science and also in other subjects.
Mireia Güell, Spain
License
CC-BY-NC-ND
Supporting materials
Follow-up experiment (Word)
Follow-up experiment (Pdf)
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FAQs
How sharp can the human eye see? ›
The visual field of the human eye spans approximately 120 degrees of arc.
What is the max resolution our eyes can see? ›According to scientist and photographer Dr. Roger Clark, the resolution of the human eye is 576 megapixels.
How good can your eyes see? ›While 20/20 vision is considered to be perfect, it's not actually what we think of as perfect. Many people, mostly children, have better than 20/20 vision. Healthy eyes can easily go down to the 20/15 level with some accuracy.
What is the sharpest human eye in the world? ›It seems that the best eyesight ever reported in a human was in an Aborigine man with 20/5 vision! To give you an idea of how clear and far he could see, his vision measurement compares to the natural sight of eagles. From 20 feet, he could perceive the fine details that most people can only see from 5 feet away!
Where is the sharpest vision? ›The fovea (centre of eye's sharpest vision) contains only cones and is the point in the retina where the sharpest vision is possible.
Can the human eye see 4K resolution? ›Most humans can see only a very minor improvement in picture quality between 1080 and 4K screens. This is because a 4K screen has about 8.3 million pixels but the human eye has only about 6 million "cones" which see color.
Can the human eye see 8K resolution? ›At four times the horizontal and vertical resolution of 1080p and sixteen times the overall pixels, 8K images — named for the approximate number of pixels along the horizontal axis — are likely the clearest digital pictures the human eye will ever see.
Can the human eye see 120 fps? ›Human eyes cannot see things beyond 60Hz. So why are the 120Hz/144Hz monitors better? The brain, not the eye, does the seeing. The eye transmits information to the brain, but some characteristics of the signal are lost or altered in the process.
Is one eye sharper than the other? ›The simple answer is yes, it's relatively normal. In fact, both eyes having the exact same visual acuity is probably a little unusual. It's entirely possible, for example, to be short-sighted in one eye, and long-sighted in the other.
How tough is your eye? ›Eyes are incredibly complex, highly productive, and resilient organs that can adjust to different conditions and environments immediately. The muscles that move your eyes are the fastest and strongest muscles in your body, relative to their function. They're 100 times more powerful than necessary.
Do they touch your eye during an eye exam? ›
Answer: Although the optometrist will get very close to your eye, they will not touch your eye directly, except for some specialised tests which are not routinely performed. The optometrist will touch your eyelids, though, in a contact lens check up.
What is actually perfect vision? ›Having 20/20 vision means that your vision is normal. If you don't have 20/20 vision, your eye care professional will help you find out what condition may be affecting your vision. They'll also help you find a way to improve your vision, such as wearing glasses or having eye surgery.
What is the strongest eye vision? ›Mantis shrimps probably have the most sophisticated vision in the animal kingdom. Their compound eyes move independently and they have 12 to 16 visual pigments compared to our three. They are the only animals known to be able to see circular polarised light.
Who has the best vision in the world? ›Perhaps, the most sophisticated eyes belong to the mantis shrimp, which have 12 to 16 photo-receptors and can see both polarised and ultraviolet light.
What ethnicity has the best eyesight? ›As a group, the Aborigines have significantly better visual acuity than the Europeans. This was true for both monocular and binocular vision. Some Aborigines have acuities below the previous postulated threshold levels. Aborigines as a group also have the previous postulated threshold levels.
Who is the rarest eye? ›Green Eyes
Green is considered by some to be the actual rarest eye color in the world, though others would say it's been dethroned by red, violet, and grey eyes.
We see our world in a huge variety of colour. However, there are other “colours” that our eyes can't see, beyond red and violet, they are: infrared and ultraviolet. Comparing these pictures, taken in these three “types of light”, the rainbow appears to extend far beyond the visible light.
What is the weakest eye sight? ›"Partially sighted": the person has visual acuity between 20/70 and 20/200 with conventional prescription lenses. "Legally blind": the person has visual acuity no better than 20/200 with conventional correction and/or a restricted field of vision less than 20 degrees wide.
What time of day is your vision the sharpest? ›Furthermore, research suggests your vision might peak at certain times of day. A study by neuroscientists at Goethe University Frankfurt indicated 8 a.m. and 8 p.m. are the prime times for vision, and eyesight is at its worst at 2 p.m.
What is the weakest vision? ›20/500 to 20/1000, this is considered profound visual impairment or profound low vision. Less than 20/1000, this is considered near-total visual impairment or near-total low vision. No light perception, this is considered total visual impairment, or total blindness.
Why is 8K pointless? ›
On a 32-inch monitor, a single pixel on a 4K monitor is about 0.007 inches small. A 32-inch 8K monitor's pixels are a quarter of that size. No human can reliably distinguish between 0.007in from 0.0035in with their eyes, much less so when you're focused on the game and not on the pixels.
Why does 1080p look better than 4K? ›Full HD is just another term for 1080p or 1920x1080, and those are all ways of referring to the same resolution. By contrast, 4K has a resolution of 3840x2160. That's a lot more pixels in the overall image — totaling over 8 million pixels. As always, the higher the resolution, the sharper the picture will be.
What resolution is real life? ›It turns out, someone smart used some pretty complex math and (assuming 20/20 vision) got to 576 megapixels. 576 megapixels is roughly 576,000,000 individual pixels, so at first glance, it would seem that we could see way more than an 8K TV has to offer.
What do dogs see? ›Dogs can see color, but only in shades of blue and yellow. Because dogs can only see two colors, they have dichromatic vision. They can also see shades of gray. Colors such as red, orange, and green are out of a dog's color spectrum, so these colors are not visible to dogs.
Why does 8K look better than real life? ›8K resolution has four times the number of pixels as a 4K display while 8K TV has a resolution of 7680 x 4320, so an 8K screen will be able to show images with much more detail and clarity than a 4K TV.
How many megapixels is the human eye compared to a camera? ›Most current digital cameras have 5-20 megapixels, which is often cited as falling far short of our own visual system. This is based on the fact that at 20/20 vision, the human eye is able to resolve the equivalent of a 52 megapixel camera (assuming a 60° angle of view).
Can eyes see 240 FPS? ›The human eye can see at around 60 FPS and potentially a little more. Some humans believe they can see up to 240 FPS, and some testing has been done to prove this. Getting humans to see the difference between something that is 60 FPS and 240 FPS should be rather easy.
Can the human eye see 500 FPS? ›Some experts will tell you that the human eye can see between 30 and 60 frames per second. Some maintain that it's not really possible for the human eye to perceive more than 60 frames per second.
How many FPS do dogs see? ›While people have an image frame rate of around 15-20 images per second to make moving pictures appear seamless, canine vision means that dogs need a frame rate of about 70 images per second to perceive a moving image clearly.
How rare is it to not have a dominant eye? ›Interestingly, 17 percent of people have no identifiable dominant eye. To establish which eye is dominant, extend one arm forward at shoulder height and form a small circle with your thumb and forefinger. Pick an object in the distance and center it in the circle with both eyes open.
Which eye is stronger than the other? ›
Most people have a dominant eye that corresponds to their dominant hand. For example, if you are left-handed, you are more likely to have a dominant left eye. Right-handed people can also have a dominant left eye, but it is not as common.
Can you train your non dominant eye? ›You can actively change eye dominance by suppressing the dominant eye such as using an eye patch, or, in more extreme cases, opt for laser eye surgery.
Can the human eye feel pain? ›Dr. Van Gelder clarified that "the retina has no pain fibers. The cornea, in the front of the eye, has more pain receptors per square inch than anywhere else in the body. But those don't provide sensation to the back of the eye."
What happens if you push your eyes hard? ›By applying extra pressure, the blood flow to the back of the eye is disrupted. This can result in nerve damage or, in the most extreme cases, loss of vision.
How much force can a human eye take? ›The purpose of this paper is to determine the static and dynamic rupture pressures of 20 human and 20 porcine eyes. This study found the static test results show an average rupture pressure for porcine eyes of 1.00 ± 0.18 MPa while the average rupture pressure for human eyes was 0.36 ± 0.20 MPa.
Why do eye doctors make you follow their finger? ›As part of the exam and testing, the doctor will be looking at your eye movements. These tests require you to follow the doctor's finger while it moves and/or follow a target on a screen while you wear VNG goggles. These tests allow the doctors to evaluate how your eyes move.
Why do doctors look in your eyes with a light? ›You've seen it on television: A doctor shines a bright light into an unconscious patient's eye to check for brain death. If the pupil constricts, the brain is OK, because in mammals, the brain controls the pupil.
Why does a doctor ask you to follow his finger with your eyes? ›To examine smooth pursuit, they ask a patient to follow their finger as it moves to the left or right and back again, or from top to bottom and back again. Pursuit is a slow, smooth eye movement to capture the target with the fovea (i.e., central vision), and it can be assessed relatively reliably at the bedside.
What does 20 10 vision look like? ›If you have 20/10 vision you are above average! You are better than the “normal” person and you have better than what is considered to be standard or normal, vision. If you have 20/10 vision, you can see at 20 feet, what a normal person can see at 10 feet from an eye chart.
Is perfect vision genetic? ›Eyesight can be heavily influenced by genetics, while there are also environmental factors that can determine how good our eyesight will be. We may not be able to challenge our genetic predispositions, but we can follow a healthier lifestyle to ensure we prevent certain eye diseases as much as possible.
What does 20 50 vision look like? ›
For example, if you have 20/50 vision, it means that your vision is worse than average and that at twenty feet away, you can read the letters that most people can read from 50 feet away.
Who has the most beautiful eyes in the world? ›- Olivia Wilde ...
- Mila Kunis. ...
- Megan Fox ...
- Emma Stone ...
- Charlize Theron ...
- Angelina Jolie ...
- Ian Somerhalder ...
- Zooey Deschanel Her big, bright blue eyes are a hallmark of her quirky style.
Shout at an elephant, stare out a lion but NEVER make eye contact with a leopard: How to survive attacks from the world's most dangerous animals.
What animal has the worst vision? ›- Golden moles (family Chrysochloridae)
- Blind mole-rats (subfamily Spalacinae)
- Blind worm lizard (Amphisbaena caeca)
- Tauredophidium hextii (spiny blind brotulid)
- Typhlopseudothelphusa (blind cave Pseudothelphusid crabs)
- Sessile animals.
- Typhliasina pearsei.
- Dorylus worker ants.
Huskies are known for their captivating eyes, but this pup is especially striking. As you might have already guessed, this doggo has heterochromia, a condition in which the two irises of the eyes can be totally different colors.
What is the closest lens to the human eye? ›Understanding Human Field of View
We often hear that a 50mm lens on a full frame camera is the closest to the human field of view. We call the 50mm a standard lens because the focal length is equal to the diagonal size of its sensor. Our eyes' focal length is approximately 22mm.
576 megapixels is roughly 576,000,000 individual pixels, so at first glance, it would seem that we could see way more than an 8K TV has to offer. But it's not that simple.
Can humans see 180 degrees? ›We humans are largely binocular beings. Each eye alone gives us roughly a 130-degree field of vision. With two eyes, we can see nearly 180 degrees. Most of that field is what's called a Cyclopean image -- the single mental picture that a Cyclops might see.
Can you see 50 miles away? ›Ever stare out at the ocean? The farthest point you can see is about 3 miles out. 6 miles: The average 747 passenger plane flies at about 6.6 miles up in the air. 50 miles: On clear days, city buildings can be seen from 50 miles away (if you're standing on the ground).
Can human eye lens be replaced? ›During a lens replacement procedure, the eye's natural lens is removed and replaced with an artificial intraocular lens (IOL) in order to reduce refractive error and improve focus. In doing so, the patient should have significantly reduced the need for glasses or no longer need them at all.
Which animal has highest eye megapixel? ›
Mantis shrimps probably have the most sophisticated vision in the animal kingdom. Their compound eyes move independently and they have 12 to 16 visual pigments compared to our three.
What resolution is 1000000 pixels? ›Pixel count is in the form of megapixels. One megapixel (MP) is one million pixels.
Can humans see infinite? ›The human range of vision is infinite. However, several factors affect the distance you can see. The human eye can see objects at any distance provided they are bright enough and there are no obstructions in between.
Can the human eye see infinitely far? ›The range of vision for a person is infinite. You can see for miles and miles. On a clear day, you can see for up to 3 miles before the horizon due to the curvature of the earth. Yet you can see skyscrapers in a further distance than 3 miles due to no horizon obstruction.
How many degrees can dogs see? ›Because dogs' eyes live on either side of their heads, they can see an impressive 250 degrees. This is 60 degrees wider than their human friends, who max out at 190 degrees.
Can you see 1 mile away? ›The Earth curves about 8 inches per mile. As a result, on a flat surface with your eyes 5 feet or so off the ground, the farthest edge that you can see is about 3 miles away.
HOW far CAN military binoculars see? ›Its maximum effective range is 4,000 meters. In the day mode, the binoculars are direct-view optical devices. For night missions, the binoculars offer interchangeable Image Intensifier GEN III eyepieces. Soldiers can tripod mount or hand hold the binoculars.
How far until you can't see land? ›The furthest distance to the horizon that the human eye can see varies slightly depending on a person's height. In miles, the horizon is approximately 3.1 miles away.