Eyes are complex organs that allow us to see the world around us. They come in different shapes, sizes, and colors across the animal kingdom. But how can we quantify and describe the anatomy of eyes using numbers? Here we will explore some key figures and measurements related to the structure and function of eyes.
The Parts of the Eye
Let’s start by looking at the main components of the human eye:
- Cornea – The clear, protective outer layer of the eye.
- Iris – The colored part of the eye that controls pupil size.
- Pupil – The opening at the center of the iris that allows light in.
- Lens – Focuses light onto the back of the eye.
- Retina – Light-sensitive tissue lining the back of the eye.
- Optic nerve – Carries signals from the retina to the brain.
Size of the Eye
The average adult human eyeball is about 1 inch (24 millimeters) in diameter and weighs around 0.25 ounces (7 grams). The eye is roughly spherical in shape.
Here are some key measurements for the anatomical structures of the human eye:
- Cornea thickness – 0.5-0.6 mm
- Cornea diameter – 11.5 mm
- Iris diameter – 12 mm
- Pupil diameter – 2-8 mm (varies based on light levels)
- Lens thickness – 4 mm
- Lens diameter – 9 mm
- Retina thickness – 0.5 mm
- Retina area – 1024 mm2
The size of the eye structures varies somewhat between individuals. Women tend to have slightly shorter and narrower eyes than men on average.
Field of View
The field of view describes the total area that can be seen at a given moment. For human eyes, our binocular field of view (with two eyes) is around 114-120 degrees horizontal by 100-108 degrees vertical.
This allows us to see objects within about 60 degrees to the left and right of the point we’re looking straight ahead at any moment. Our peripheral vision extends farther, up to 90 degrees left/right.
Here are some key field of view measurements for human eyes:
- Binocular field of view – 114-120 x 100-108 degrees
- Monocular field of view (one eye) – Around 114 degrees horizontal
- Vertical range – Around 60 degrees up and down
- Total visual field (monocular) – Around 150-160 degrees horizontal, 120 degrees vertical
Other animals have different fields of view depending on the placement and abilities of their eyes. For example, rabbits have a field of view of around 340 degrees to help detect predators.
Eye Movement Range
Although our eyes have a wide field of view, they can only focus sharply on a small point at any given time. The fovea centralis region of the retina provides the sharpest vision.
To build up a complex visual perception, our eyes constantly make small rapid movements called saccades to scan objects and scenes. During each saccade, the eyes traverse an angle of about 1–10 degrees very rapidly. Between saccades, the eyes remain relatively stable for about 200–500 ms in what are known as fixation periods.
Humans can make voluntary, larger eye movements as well. Here are some ranges of motion:
- Horizontal tracking range – Around 110 degrees total movement
- Vertical range – Around 60 degrees up and down
- Torsional range (rotational) – Around 25 degrees total
Having this range of motion allows our eyes to shift gaze rapidly and align both eyes on objects of interest in order to examine them closely with central vision.
Vision Capabilities
Let’s explore some measurements related to what our eyes can actually see and perceive.
Visual Acuity
Visual acuity refers to the sharpness or clarity of vision. It is measured by the smallest detail that can be resolved by the eye at a given distance.
Normal human visual acuity is considered 20/20 vision. This means a person can resolve details as fine as 1 arcminute, which allows distinguishing two points 0.02 inches (0.5 mm) apart from 20 feet (6 meters) away.
Here are some common visual acuity benchmarks:
- 20/15 vision – Better than normal, able to distinguish details 1 arcminute apart
- 20/20 vision – Normal, able to distinguish details 1 arcminute apart
- 20/40 vision – Able to distinguish details at 2 arcminutes
- 20/200 vision – Legal blindness, able to distinguish details at 10 arcminutes
Visual acuity varies between individuals and declines with age as the lenses lose flexibility and the retina deteriorates.
Color Vision
The human eye can normally detect around 10 million different colors thanks to special light-sensitive cells called cones. There are three types of cones that are most sensitive to long (red), medium (green), and short (blue) wavelengths of light.
Color vision varies between species based on their cone types. For example:
- Humans – trichromatic, 3 cone types (red, green, blue)
- Cats & dogs – dichromatic, 2 cone types (yellow & blue)
- Bulls – dichromatic (yellow & green), but can’t distinguish red
- Eagles – tetrachromatic, 4 cone types
Around 8% of men and 0.5% of women have some type of color vision deficiency like color blindness. This is usually due to the loss of function of the red or green cone systems.
Dynamic Range
The human eye has an impressive dynamic range, meaning it can function well across a wide range of light conditions. Here are some key figures:
- Luminance range – 10-6 to 108 cd/m2
- Pupil diameter range – 2-8 mm
- Adaptation time – Around 5 minutes to adjust from dark to light conditions
The retina adapts to brighter conditions by constricting the pupil to limit light intake. In dim conditions, the pupil can dilate to up to 8 mm to allow as much light in as possible.
The eye achieves a remarkable dynamic range of around 1:100,000,000. This allows us to see well in both faint starlight and bright sunlight.
Temporal Resolution
The eye and visual system can process visual stimuli at high speeds. Key measurements include:
- Critical flicker fusion threshold – Around 50-60 Hz, the point where a flashing light appears steady to the eye.
- Saccadic suppression – Info is blocked during eye movements to avoid blurring.
- Bloch’s law – Shorter flashes need higher brightness than longer flashes to reach visual threshold.
Experiments show the human visual system can differentiate flashes just 13 milliseconds apart under ideal conditions. This temporal resolution helps us interpret motion smoothly.
Eye Focusing
The eye uses various mechanisms to focus on objects at different distances:
- Accommodation – Adjusting the shape of the elastic lens to focus.
- Convergence – Turning the eyes inward when looking at close objects.
- Pupil constriction – Increases depth of field.
The nearest point the eye can focus on is called the near point of accommodation. This is around 7-10 inches for young adults but declines with age as the lenses become stiffer.
Perceptual Abilities
In addition to basic visual measurements, researchers have quantified various perceptual abilities related to how we interpret the visual world.
Contrast Sensitivity
Contrast sensitivity refers to the ability to distinguish an object from its background based on contrast between the two. It varies with the size and spatial frequency of the pattern.
Here are some key contrast sensitivity benchmarks:
- Peak sensitivity – Around 2-5 cycles/degree
- Low sensitivity – Below 1 cycle/degree and above 60 cycles/degree
- Minimum detectable contrast – As low as 1-2% under optimal conditions
Smaller visual detail requires greater contrast to detect. Different visual disorders can impact contrast sensitivity in specific ways.
Visual Memory
Experiments have revealed some metrics around our visual sensory memory and working memory capacities:
- Iconic memory – Ultra-short visual memory lasts for about 250-500 ms.
- Visual short term memory – Around 3-4 objects can be actively held.
- Long term memory – Virtually unlimited capacity.
Selective attention allows us to selectively hold some visual information in working memory while filtering out distractors.
Face Recognition
Recognizing faces is an important perceptual ability mediated by specific brain areas. Key facts about facial recognition include:
- holistic processing – Faces perceived distinctly from components.
- 6 facial vectors – Eyes, nose, mouth, cheeks, head outline, and eyebrows.
- age decline – Recognition declines after age 60.
- super-recognizers – Some people have elite face recognition abilities.
The fusiform face area of the brain shows increased activity when processing faces. Damage can lead to an impairment called prosopagnosia.
Depth Perception
Having two front-facing eyes allows for stereopsis, the perception of depth based on slight differences between the two retinal images. This helps with judging distances. Other depth cues include:
- Motion parallax – closer objects move faster across field of view.
- Overlap – closer objects block more distant objects.
- Relative size – farther objects appear smaller.
- Light & shadows
Around 5-10% of people have impaired stereopsis and cannot perceive depth from stereopsis alone.
Color Constancy
The visual system displays color constancy – the ability to determine the actual color of objects regardless of changing light conditions. Mechanisms include:
- Chromatic adaptation – Adjusting to ambient color.
- Memory colors – Knowing expected color for objects.
- Comparing adjacent colors.
This allows someone to recognize a white shirt as white even if viewed indoors under yellow incandescent lighting.
Eye Diseases and Disorders
Here are some key statistics on common eye diseases and vision disorders:
- Refractive errors – Around 158 million people globally have uncorrected refractive errors like near-sightedness and far-sightedness.
- Cataracts – Around 65 million people suffer from age-related cataracts worldwide.
- Glaucoma – Affects around 6.7 million Americans age 40 and older.
- Age-related macular degeneration (AMD) – Around 8 million Americans have some vision impairment from AMD.
- Diabetic retinopathy – Around 7.7 million American adults with diabetes have diabetic retinopathy.
These along with other conditions like amblyopia and strabismus can result in mild to profound vision loss.
World Blindness
The World Health Organization estimates there are around 1.1 billion people globally who have some form of vision impairment. Key statistics include:
- Distance vision impairment – Around 596 million people
- Near vision impairment – Around 510 million people
- Blindness – Around 36 million people are completely blind worldwide
- Over 80% of blindness is preventable or treatable
Around 90% of those with vision impairment live in low-income settings. Increased access to eye care services could prevent much of this.
Color Blindness Prevalence
Here are statistics on the prevalence of color vision deficiencies worldwide:
- Total affected – Around 1 in 12 men (8%) and 1 in 200 women (0.5%).
- Red-green color blindness – Around 99% of cases.
- Complete color blindness (extremely rare) – Achromatopsia, around 1 in 30,000 people
- Blue-yellow color blindness – Around 1% of cases.
The most common type is a red-green deficiency making it hard to distinguish certain shades of red and green.
Conclusion
Our eyes and visual system are complex and can be described by many quantitative measures. These include the anatomy of the eyeball, field of view, visual acuity, contrast sensitivity, color vision, temporal resolution, depth perception, and more. Statistics also shed light on the prevalence of various eye diseases and disorders that can impair vision. Overall, the human eye and visual system are remarkable in their abilities to sense and perceive the vast amount of visual stimuli around us.
