Introduction
Human Eyes vs Camera? Well, The human visual system is a far more refined system than a modern camera. It has extremely precise control of focusing, exposure, and white balance. It can respond to far more varying lighting conditions than the most advanced camera.
Table of Contents
Human Visual System
The light and colour information received by our eyes are processed within the visual cortex of the brain.
Our eyes can look around a scene and dynamically adjust based on the subject matter, whereas cameras capture a single still image. For example, our eyes can compensate as we focus on regions of varying brightness, can look around to encompass a broader angle of view, or can alternately focus on objects at a variety of distances. But, what we see is our mind’s reconstruction of objects based on input provided by the eyes — not the actual light received by our eyes.
The main parts of the human eye are the cornea, iris, pupil, aqueous humour, lens, vitreous humour, retina, and optic nerve.
The conjunctiva
It’s a thin, clear membrane that protects our eye.
Cornea
Light enters through the cornea, the transparent outer covering of the eye. The eyeball is rounded, so the cornea acts as a lens. It refracts or bends light.
Sclera
The white outer layer of the eyeball. At the front of the eye, it’s continuous with the cornea.
Aqueous Humour
The fluid beneath the cornea has a composition similar to that of blood plasma. The aqueous humour helps to shape the cornea and provides nourishment to the eye.
Iris and Pupil
Light passes through the cornea and aqueous humour through an opening called the pupil. The size of the pupil is determined by the iris. As the pupil dilates (gets bigger), more light enters the eye.
Lens
While most of the focusing of light is done by the cornea, the biconvex lens allows the eye to focus on either near or distant objects. Constructed primarily of proteins, it has many layers of varying refractive indices. Ciliary muscles surround the lens, relaxing to flatten it to focus on distant objects and contracting to thicken the lens to focus on close-up objects.
Vitreous Humour
A certain distance is required to focus light. The vitreous humour is a transparent watery gel(with a small amount of protein) that supports the eye and allows for this distance.
Retina
The coating on the interior back of the eye is called the retina. It consists of photosensitive and nerve cells. Its job is to encode the incoming light into electrical signals for the brain.
Rods and Cones
When light strikes the retina, two types of cells are activated.
- Rods: Responsible for dim light vision (Scotopic vision). They cannot detect color.
- Cones: Responsible for vision in bright light and color vision (Photopic Vision)
Cone cells are of three different kinds:
- S-cones : For short-wavelengths
- M-cones : For medium wavelengths
- L-cones : For longer-wavelengths
The combination of their responses is responsible for color vision. When the receptors are stimulated equally by an object we tend to see it as white.
When we focus clearly on an object, light strikes a region called the Fovea. The fovea is packed with cones and allows sharp vision. Rods outside the fovea are largely responsible for peripheral vision.
Rods and cones convert light into an electric signal that is carried by the Optic Nerve to the Brain. The brain translates nerve impulses to form an image.
Blind Spot
A small circular area at the back of the retina where the optic nerve enters the eyeball and which is devoid of rods and cones and is not sensitive to light.
Focusing
About 70% of the bending of light takes place as it enters the cornea and the aqueous fluid. This bending is possible because of the curve of the cornea as well as the change in refractive index as light moves from air into the cornea and then into the aqueous fluid between the cornea and the iris.
Behind the aqueous fluid, there is a convex lens that is soft and pliable. The ciliary muscle is a circular ring of muscle that attaches around the lens. This ciliary muscle can change the shape of the crystalline lens by stretching it at the edges. When you are looking at a near object, the lens needs to become more rounded at the center to focus the light rays. This ability to change focus for close-up objects is called accommodation.
Need For An EyeGlass
If the Cornea has too much curvature the optical power of the lens is unable to correct for this, and the image is brought to focus in front of the retina. It’s called myopia. A negative lens can correct this.
If the cornea has too little curvature, the image is brought to focus behind the retina. It’s called hyperopia . A positive lens can correct this.
With aging the lens becomes thicker and stiffer so that the flexibility is reduced. It becomes difficult to focus on near objects. It’s called presbyopia. Weakening of the ciliary muscles adds to this effect. A positive lens can again correct it.
Cylindrical power is basically astigmatism.
Angle Of View
Our central angle of view — around 40-60° — is what most impacts our perception.
Subjectively, this would correspond with the angle over which you could recall objects without moving your eyes. Incidentally, this is close to a 50 mm “normal” focal length lens on a full-frame camera.
Resolution
An eye has about 130 million cells, with 6 million sensitive to colors (the cones). In a camera sensor, the pixel density is even. In the eye, there are more cells in the middle of the retina. Away from the center, our visual ability decreases dramatically, such that by just 20°off-center our eyes resolve only one-tenth as much detail. At the periphery, we only detect large-scale contrast and minimal color.
Our mind doesn’t scan images pixel by pixel; it instead records memorable textures, colors, and contrast of an image.
Dynamic range
When looking at a scene, our eye are constantly adjusting to the lighting conditions.
This means that we do not ‘expose’ to the bright or the dark areas of the scene, but perform a kind of dynamic adjustment. This provides our eyes with an apparent dynamic range that is far superior to any camera available.
Dark adaptation
Dark adaptation is the ability of the eye to become more visually sensitive after remaining in darkness for some time. In bright light, the pupil in our eyes constricts to reduce the amount of light entering the eye. When we enter a dark environment it takes time for the pupil to dilate and allow more light in. The adjustment also involves the regeneration of visual pigments in the rod cells. As these pigments regenerate, the sensitivity of the eyes to light increases, allowing us to see better in the dark. The full process can take up to 30 minutes, or even more for older people.
Stereo Vision
The three-dimensional appearance of the image comes from comparing the differences between the images formed by each eye.
When you look straight into distance, your eyes are parallel to each other. The areas seen with the right and with the left eye overlap to a certain extent. With the left eye you see not only what happens at the left side of your body, but also what happens at the centre and partially at the right side and vice versa.
The largest part of the visual field is seen binocularly, in other words with two eyes. Since our eyes are up to 2½ inches apart from each other, we receive two different pictures of our environment from the left and the right eye. The brain “computes” the spatial information from the difference between the two pictures on the retina and creates a joint overall image, which provides extra information about the distance to an object. This process is called stereoscopic vision.
Polarized 3D glasses
The polarized filters allow different images to be presented to each eye. The axis of polarization in polarizing glasses are at right angles relative to each other. If the right eye is vertically polarizing, the left eye will be horizontally polarizing.
The active shutter 3-D display technique
In the latter, two dissimilar images are alternated between at a very high frequency of 60 Hz or greater, meaning that each eye sees 30 images every second. The active shutter technique requires viewers to wear bulkier 3-D glasses with a built-in battery. Since the glasses can turn opaque at the same frequency, when one eye’s image is on the screen, the other eye’s glass turns opaque. By doing so, the brightness and contrast of the movie is preserved and are not lost by polarization, as polarization blocks the majority of light.
Human Eyes vs Camera
Similarities
| Human Eye | Camera |
| Light enters the eye through the pupil. | Light enters the eye through the aperture. |
| The iris regulates the amount of light entering the eye. | The amount of light is also regulated in- camera with the aperture. |
| Light and image are focussed on the retina in the eye. | Light and image are focussed on film/sensor in the camera. |
| The eye contains a lens. | The camera contains a lens. |
| There is a choroid that absorbs light and limits reflection in the eye. | There is also black paint in the camera which absorbs lights and limits reflections. |
Differences
| Human Eye | Camera |
| Human eye can’t change focal length | Camera can change focal length by using different lenses |
| Retina retains the impression of an image for only 1/16th of seconds. | Photographic film or sensor retains the image permanently. |
| Eyes Can’t record image | Camera can record image |
| Image is formed on the retina which is further processed in the brain. | Image is formed on the photographic film/sensor and later processing can be either photochemical or digital. |
| Human eye operates within the visible spectrum | Camera can see beyond the visible spectrum. |
| What we really see is our mind’s reconstruction of objects based on input provided by the eye not the actual light received by our eye | In camera the image is solely based on the light received |
| Eyes have blind spot | Camera has no blind spot |
