What are the limits of human vision?

We'll explain these visual thresholds initially through the lens – pun intended – of what many of us first think of when we consider vision: colour.
Why we perceive violet versus vermillion depends on the energy, or wavelengths, of the photons impinging on our retinas, located at the back of our eyeballs. There, we have two types of photoreceptor cells, known as rods and cones. Cone cells deal in colour, while rod cells allow us to see in grayscale in low-light conditions, for example at night.

Opsins, or pigment molecules, in retinal cells absorb the electromagnetic energy from impacting photons, generating an electrical impulse. That signal travels via the optic nerve to the brain, where the conscious perception of colour and imagery is created. We have three types of cone cells and corresponding opsins, and each peaks in sensitivity to photons of particular wavelengths. These cone cells are dubbed S, M, and L, for short, medium and long wavelengths. Shorter wavelengths we perceive as bluer, while longer wavelengths are redder. All wavelengths in between (and combinations of them) serve up the full kaleidoscopic rainbow. "All lights we see – except those made artificially with a prism or some fancy device like a laser – are mixtures of multiple wavelengths," says Landy.

Of all the possible photon wavelengths out there, our cone cells detect but a small sliver, typically in the range of about 380 to 720 nanometres – what we call the visible spectrum. Below our narrow perceptual band is the infrared and radio spectrum, with the latter's longer, less energetic wavelengths ranging from a millimetre to kilometres in length. Above our visible spectrum into higher energies and shorter wavelength we find the ultraviolet band, then X-rays, topping off with the gamma ray spectrum, whose wavelengths are in the mere trillionths-of-a-metre range.

While most of us are limited to the visible spectrum, people with a condition called aphakia possess ultraviolet vision. Aphakia is the lack of a lens, due to surgical removal for cataracts or congenital defects. The lens normally blocks ultraviolet light, so without it, people are able to see beyond the visible spectrum and perceive wavelengths up to about 300 nanometres as having a blue-white colour.

A study in 2014 pointed out that, in a manner of speaking, we all can see infrared photons, too. If two infrared photons smack into a retinal cell nearly simultaneously, their energy can combine, converting them from an invisible wavelength of, say, 1000 nanometres to a visible 500 nanometres (a cool green to most eyes).

How many colours can we see?

A healthy human eye has three types of cone cells, each of which can register about 100 different colour shades, therefore most researchers ballpark the number of colours we can distinguish at around a million. Still, perception of colour is a highly subjective ability that varies from person to person, thus making any hard-and-fast figure difficult to pinpoint.

"You'd be hard-pressed to put a number on it," says Kimberly Jameson, an associate project scientist at the University of California, Irvine. "What might be possible with one person is only a fraction of the colours that another person sees." Jameson knows what she's talking about, given her work with "tetrachromats", people who possess apparent superhuman vision. These rare individuals, mostly women, have a genetic mutation granting them an extra, fourth cone cell. As a rough approximation based on the number of these extra cones, tetrachromats might see 100 million colours. (People who are colour-blind, or dichromats, have only two cones and see perhaps 10,000 colours.)

What's the smallest number of photons we need to see?

To yield colour vision, cone cells typically need a lot more light to work with than their cousins, the rods. That's why in low-light situations, colour diminishes as the monochromatic rods take over visual duties. In ideal lab conditions and in places on the retina where rod cells are largely absent, cone cells can be activated when struck by only a handful of photons. Rod cells, though, do even better at picking up whatever ambient light is available. As experiments first conducted in the 1940s show, just one quanta of light can be enough to trigger our awareness. "People can respond to a single photon," says Brian Wandell, professor of psychology and electrical engineering at Stanford. "There is no point in being any more sensitive."

In 1941, Columbia University researchers led subjects into a darkened room and gave their eyes some time to adjust. Rod cells take several minutes to achieve full sensitivity – which is why we have trouble seeing when the lights first go out. The researchers then flashed a blue-green light in front of the subjects’ face. At a rate better than chance, participants could detect the flash when as few as 54 photons reached their eyes. After compensating for the loss of photons through absorption by other components in the eye, researchers found that as few as five photons activating five separate rods triggered an awareness of light by the participants.

What is the smallest and farthest we can see?

Now here’s a fact that may surprise you: There is no intrinsic limit to the smallest or farthest thing we can see. So long as an object of whatever size, distance or brevity transfers a photon to a retinal cell, we can spy it. "All the eye cares about for vision is the amount of light that lands on the eye," says Landy. "It's just the total number of photons. So you can make [a light source] ridiculously tiny and ridiculously brief, but if it's really strong in photons, you can still see it."

Psychology textbooks, for instance, routinely state that on a clear, dark night, a candle flame can be spotted from as far away as 48 kilometres. In practice, of course, our eyes are routinely inundated by photons, so stray quanta of light from great distances get lost in the wash. "When you increase the background intensity, the amount of extra light you need to see something increases," says Landy.

The night sky, with its dark background pricked by stars, offers some startling examples of long-distance vision. Stars are huge; many we see in the night sky are millions of kilometres in diameter. Even the nearest stars, however, are more than 24 trillion miles away, and are therefore so diminished in size our eye cannot resolve them. Lo and behold, we can still see stars as intense, gleaming "point sources" of light because their photons cross the cosmic expanse and hit our retinas.

All the individual stars we see in the night sky are in our galaxy – the Milky Way. The absolute farthest object we can see with our naked eye is outside of our galaxy: the Andromeda Galaxy, located 2.5 million light years from us, or a cool 23 quintillion miles. (Well, controversially, some keen-sighted folks have claimed to have glimpsed the Triangulum Galaxy in extraordinarily dark night sky conditions, which is about three million light-years distant, but we’ll have to take their word for it.)

The trillion stars in the Andromeda Galaxy, on account of their extreme distance, add up to just a fuzzily luminous patch in the sky. That said, the Andromeda Galaxy is colossal. In terms of its apparent size, even quintillions of miles away, the galaxy is six times the width of the full Moon. But so few of its photons reach our eyes that this celestial behemoth is rendered faint.

How clearly can we see?

Nevertheless, why is it that we can't pick out individual stars in the Andromeda Galaxy? The limits of our visual resolution, or acuity, come into play here. Visual acuity is the ability to discern a detail such as a point or line as separate from another without them blurring together. You might therefore think of acuity's limits as the number of "pixels" we can discern.

Several factors set the boundaries for visual acuity, such as the spacing between the cones and rods packed onto the retina. The optics of the eyeball itself, which as we mentioned before prevent every available photon from alighting upon a photoreceptor cell, are important as well. Theoretically, studies have shown, the best we can do is about 120 pixels per degree of arc, a unit of angular measurement. That works out to about a fingernail held at arm's length with 60 horizontal and 60 vertical lines on it, alternating in black and white, creating a checkerboard pattern. "That's about the finest pattern you could ever see," says Landy.

Vision tests, like the popular Snellen eye chart at your optician's with the progressively smaller letters on it, operate on the same principle. The chart gauges at what point someone can no longer separate out a white gap in a black letter, distinguishing a capital F from a capital P, for instance. These acuity limits help explain why we cannot discern and focus on a single, dim, biological cell that's mere micrometres across.

But let's not sell ourselves short. A million colours; single photons; galactic realms quintillions of miles distant – not bad for the blobs of jelly in our eye sockets, wired to a 1.4 kilogram sponge in our skulls.

Adam Hadhazy