They can see clearly about eight times as far as humans can, allowing them to spot and focus in on a rabbit or other animal at a distance of about two miles. While we can see a candle flame at that distance, a small animal camouflaged in its surroundings would be hidden from us.
They also can see a wider range of colors than we can, allowing them to differentiate small changes in coloration in their prey, as well as see UV light.
As far as daytime vision goes, eagles, hawks, and falcons reign supreme. These nighttime predators take the torch from eagles as soon as the sun goes down. Unlike many birds with eyes that sit at an angle, owl eyes face directly forward, giving them incredible binocular vision.
Their large, tube-shaped eyes contain many more rods than human eyes, which allow them to be more sensitive to light. Their irises widen to allow more light to reach their retina at night. Because the iris adjusts, owls can also see during the day unlike other nocturnal animals that can only see well at night , but their vision is slightly blurry and they cannot see colors well.
Owls and other animals with excellent night vision have a reflective surface behind their retina known as the tapetum lucidum. Eagles may be able to spot a rabbit from the sky, but mantis shrimp might have the most complex eyes in the entire animal kingdom.
Common causes of sudden vision loss include eye trauma, blockage of blood flow to or from the retina retinal artery occlusion or retinal vein occlusion , and pulling of the retina away from its usual position at the back of the eye retinal detachment.
Recent research has shown that some birds are capable of seeing ultraviolet light, wavelengths that are too short for humans to see. We have some remarkable anecdotes for Old World birds. In the middle of the retina is a small dimple called the fovea or fovea centralis. The fovea is the region with the most acute vision in the retina. It is easily identified by the depression within the retina. The fovea is on the visual axis of the cornea. With enough time, our eyes can adapt and see the low levels of light present in partial darkness.
Human eyes take several hours to fully adapt to darkness and reach their optimal sensitivity to low light conditions. The quickest gains in vision sensitivity are made in the first few minutes after exposure to darkness. For this reason, many people think that after only a few minutes, their eyes have reached their peak sensitivity. But several hours into darkness exposure, the human eyes continue to adapt and make small gains in sensitivity.
There are several factors that contribute to our eyes adapting to darkness. As described in the textbook Optometry: Science Techniques and Clinical Management , edited by Mark Rosenfield and Nicola Logan, the three main players in dark adaptation are the pupil, the cone cells, and the rod cells. The pupil is the dark hole near the front of your eye that lets the light into your eye so that the light can form an image on the back the retina.
The iris that surrounds the pupil contains muscles that control the size of the pupil. When confronted with low light conditions, the iris expands the pupil as wide as possible. This dilation lets as much light as possible into the eye so that sensitivity is enhanced. The pupil's contribution to dark adaptation takes only a few seconds to a minute to be completed. The cones cells along the retina are responsible for color vision. Similar to a grid of pixels in a digital camera, a vast spatial array of cone cells along the retina detect the different bits of colored light that make up the image we are seeing.
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