The iris and the pupil
The most obvious response the eye makes to changes in brightness is to change the size of the pupil. This is the little opening in the center of the iris (the colored part of your eye). As the iris contracts or dilates, the pupil gets smaller or larger. At its largest, the pupil can admit up to thirty times more light than when it's at its smallest. But this is only part of the story.
At the back of your eye the retina contains millions of light-sensitive cells. There are two basic types of cell, the rods and the cones. They both send information to the brain by initiating electric pulses in the optic nerve.
The rods and the cones
Human eyes can see over a wide range of light intensities. For example, full daylight is a million times brighter than the light of a half moon. The characteristics of the rods and the cones make this possible.
We can thank the rods for our night vision and our peripheral vision. They are stimulated by quite dim light and are over a thousand times more sensitive than the cones. The rods are spread all around the retina, except in two places. There are only cones right in the region known as the fovea centralis, and there aren't any light-sensitive cells at the blind spot where the optic nerve joins the retina.
The rods outnumber the cones by about twenty to one, and it takes much brighter light to activate the cones. However they let us see fine detail and colors. Three types of cone have maximum sensitivities to red, blue or green. The brain combines the information they send into an image with all the shades of color that we see.
In contrast to cones, rods don't resolve detail or convey color information. In dim light we see indistinctly and objects look greyish. However rods are more sensitive than cones to the green-blue part of the spectrum. At twilight, when both rods and cones are involved in vision, the color of greener objects is enhanced and that of redder objects subdued. You may have noticed how different the colors of a garden seem at dusk.
When people first see the aurora borealis (northern lights) or its southern equivalent, they are often surprised that it's not the bright green seen in photographs. The green is there and a digital camera captures it. But unless the aurora is quite bright, its light is too dim to activate the cones that let us see its colors.
Averted vision
When we want to see something clearly, we look straight at it. That's because the cones are concentrated in the center of the retina. In fact, we're so used to this that it seems odd that astronomers find faint objects by not looking at them directly.
Under a dark sky, looking for deep-sky objects, the dark-adapted vision relies on the rods. Since there aren't any in the fovea – the center of vision – that's effectively a second blind spot. Yet since there's a concentration of rods around the periphery of the fovea, this is where you're most likely to detect a faint nebula or star cluster. Novices can start by looking 12 degrees off center, but the angle varies from person to person. This technique is called averted vision, and those who have mastered it can see objects at least twenty times fainter than without it.
Unless the objects you see in the telescope are bright enough to activate the cone cells, you'll see them in black and white.
Light and dark adaptation
Going into a dark movie theater or outside into the bright sunshine, the iris responds. However the rods and cones have a greater effect. Both types of cell contain pigments that react to light: rhodopsin for the rods and photopsin for the cones. When light stimulates the cell a change occurs that sends an electrical pulse along the optic nerve. Since it also removes the pigment from the cell, it's called bleaching. In eyes completely adapted to bright light, the rods are completely bleached and unable to detect faint light until they're recharged.
Cones adjust rapidly to light or dark. But not rods. Dark adaptation can be undone in an instant if there's a camera flash or any bright white or blue light. But it takes half an hour or more to restore it. Interestingly, each eye adapts separately, so some astronomers keep one eye covered in order to protect their dark vision.
But if people want to look at star maps, make notes or adjust equipment, how do they do it in the dark? A red LED flashlight does the trick. You can see, but there isn't enough energy in the light to affect the rhodopsin. But there's a slight downside to this insensitivity to red light. Many nebulae are emission nebulae that shine because energized hydrogen releases red light. These nebulae look stunning in photographs, but we can't see this light with our own eyes.
