Archive for the ‘Eye Facts’ Category

Depth Perception, What Exactly is It?

Have you seen the movie Gravity? Did you see it in 3-D?

If so, you probably found yourself marveling at the amazing technological effect of seeing Sandra Bullock and George Clooney tumble through a very realistic depiction of the depths of outer space.

But you probably never find yourself marveling at the even more amazing technological effect of seeing everything in the world in 3-D – achieved by simply having two eyes, as well as a brain that blends the images your eyes see.

Very simply, this is depth perception, meaning the ability to see things in three dimensions – length, width, and depth – and to perceive how near or far away people and objects are.

One way humans perceive depth is by having eyes that are spaced a little bit apart. This is contrary to certain animals that tend to be prey, and have evolved so that their eyes are on either side of their head to enable them to spot predators. (Predator animals, on the other hand, or paw, usually have eyes positioned closely together, like humans, to enable them to spot, chase, and swoop down upon prey.)

Like those clunky 3-D glasses you wore while watching Gravity, human eyes spaced a little bit apart see two slightly different images, based on each eye’s position on your face. These two slightly different images are called binocular visual cues. They’re called binocular because the images are seen by two eyes.

Here’s a fun way to test your binocular vision. Take a look at the picture of the baseball below:

Now hold up one finger about six inches away from your face, in front of the picture of the baseball. If you focus your eyes on the baseball, you will see two somewhat transparent images of your finger, on either side of the baseball.

Now focus your eyes on your finger. You will see two somewhat transparent images of the baseball, on either side of your finger. That’s depth perception.

Here’s another way to test your depth perception, and to see whether one of your eyes is dominant.

Hold your finger in front of the baseball, just like before. Focus on the baseball and close your left eye. Your finger should appear to the left of the baseball. Now open your left eye and close your right eye. Your finger will seem to jump to the right of the baseball.

Once again, as in the previous test, if this is what you see, you have binocular – also known as stereo – vision. However, you may have faulty depth perception if you experience any of these things:

  • You can see your finger better on one side than the other.
    The view of your finger is larger with one eye than with the other.
  • One eye’s finger image appears right over the baseball, while the other one is far to the left or right.
  • You can only see one finger image.

If you do experience any of these things, it would be a good idea to get your eyes checked by an ophthalmologist or an optometrist.
 

 
But what if you have sight in only one eye? Do you still have depth perception, even if you don’t have binocular (stereo) vision?

Yes! Here is a way to test your one good eye for depth perception.

Let’s say you are doing this indoors. You will need stationary objects in three different distances from you: one object close to you, one a little farther away, and one still farther away from that one.

Let’s try this with three objects you may find indoors: a chair (close to you), a filing cabinet (a little farther away), and a table (farther away).

You can do this outside, too. Try it with, say, a bush (close to you), a tree (a little farther away), and a house (farther away).

Of course you don’t have to use these three indoor and outdoor objects, in these three positions. You can use them in different positions, or you can use three other objects, as long as the one farthest away is large enough so you can see it well.

Focus on the object in the mid-distance range – the table if you are inside, the tree if you are outside, if you are using those objects. Then walk from side to side.

As you focus on the midrange object, perhaps the filing cabinet or tree, you will see the object closer to you (the chair or the bush) move to the opposite direction from where you are walking. You will also see the object farther away (the table or the house) appear to move in the same direction that you are moving.

If you can see this with one eye (if you are sighted in both eyes you can close one eye while you do this), you have depth perception!

Another way either one or both eyes can perceive depth is by perspective – which means how objects appear based on the position of your eyes in relation to the object.

Here’s a very common way we perceive perspective. When we’re standing on the ground and looking straight down at parallel lines, they appear parallel:

If you click the below link it will take you another tab. Click the red “add perspective” ball, and you’ll see the lines converge (and be sure to come back when you are done). It’s like when you look at railroad tracks off in the distance as they approach the horizon. They appear to meet at what is called the “vanishing point”:

http://psych.hanover.edu/krantz/art/linear.html

Just like the way perspective can trick the eye into seeing parallel lines that appear impossibly to meet, artists can use perspective tricks to create other optical illusions. Look at this one, created by the German psychiatrist Franz Muller-Lyer in 1889:

The two parallel lines are the exact same length. You can prove it with your ruler. But the line on the top appears longer, because our brains perceive the inverted angles as lengthening the line.

What causes this? Theorists have advanced various explanations, none proven. If you are interested in exploring this further, you can read the explanations at the link below, and test their validity yourself:

http://www.rit.edu/cla/gssp400/muller/muller.html

Your perspective affects not just your view of distance, but the appearance of colors and hues you perceive as well. Check out this picture:
 

 
The gray bar across the middle of the picture appears lighter on the left and darker on the right. But that’s only because of the contrast between the gray bar and the different shades of gray in the background. The background is darker on the left and lighter on the right. Despite its appearance, the gray bar is actually the same color on the right and left.

In addition to appearing to vary shades of a color, sometimes our visual perspective can cause us to see colors that are not actually there. This is because our brains can perceive color in relation to other colors that are present. Take a look at this optical illusion, follow the simple instructions, and notice if you see green grass:

http://www.illusions.org/dp/color.htm

These optical illusions are all results of perspective – what you perceive as true is determined by your perspective, a principle that can apply not only in vision but in many other aspects of life, as well!

As you ponder this, here are some other fun optical illusions to look at and share with your family and friends, to get their perspective, too!

Tuesday, December, 10 2013 by

Polarized Sunglasses, What Exactly Are They?

wearing polarized sunglasses, Zenni optical

You’ve probably seen drugstore sunglasses labeled “polarized.” Maybe you looked at those next to sunglasses that weren’t polarized, and wondered what the difference was. Or maybe your eye dr. recommended getting polarized lenses, but you weren’t sure how they worked and what benefit they offered.

Well, we’re going to tell you! Very simply, here’s what polarized lenses do: they reduce glare by blocking horizontal light rays.

Why horizontal? Here’s a little background on how polarization works: light comes from the sun in all directions, and it is reflected in every direction, too. When it reflects from light-colored horizontal surfaces – such as a white sidewalk, a white-sand beach, snow, or sunlight reflecting on water – it is said to be polarized horizontally. Light that is polarized horizontally is responsible for most of the glare that interferes with our vision.

How do polarized lenses work? To understand this, it’s useful to think of window blinds. Let’s start with Venetian blinds, which are horizontal. When Venetian blinds are open, light comes into the room in horizontal stripes, depending on the angle of the sun and the angle the blinds are open.

Polarized lenses are more like vertical blinds. When vertical blinds are open at a certain angle, light comes into the room in vertical stripes. Since horizontally reflected light is responsible for most of the glare we see, the polarized tint is placed on the sunglass lens in vertically angled strips. These vertical strips of tint allow vertically reflected light into the eye but block horizontally reflected light – greatly reducing horizontally polarized glare.

Most polarized lenses are sunglasses. Polarized sunglasses are especially popular with boaters, since water is a very reflective surface on a sunny day. Polarized sunglasses are also great for other outdoor activities, such as golfing and tennis, since they can help to sharpen the focus on the ball. They can also be good for driving, since polarized lenses reduce the glare that reflects from the shiny hood of a car, or the glare from the surface of the road on a hot, sunny day.

However, in some situations there can be drawbacks to polarized lenses. They can be problematic for skiing, since blocking the reflected glare off an icy patch might prevent the skier from noticing and steering away from a potential hazard.

In addition, it can sometimes be difficult to read liquid crystal display (LCD) or light-emitting diode (LED) screens on a boat or plane’s instrument panel while wearing polarized sunglasses. This could interfere with the pilot’s ability to clearly read and quickly respond to the information on the instrument panel. This could also apply to the global positioning system (GPS) and other displays of a car’s dashboard, a smart-phone, an ATM, or a self-service gas pump.

An oddity you may notice while wearing polarized sunglasses is that when you look at your car’s rear or side window from the outside, or perhaps the windows on an office building, you may see splotchy, iridescent spots. When you remove your polarized sunglasses, these spots are invisible.

This effect is created because you are looking at heat-tempered glass. The heat-tempering creates several stress points on the glass, enabling the glass, when broken, to crumble into small, granular chunks, which are safer than splitting the glass into sharp, jagged shards. The stress points also reflect the light in different directions from the parts of the glass that don’t have these stress points. The stress points prevent the polarized lens from filtering out the light evenly across the surface of the glass, creating the splotchy, iridescent effect.

If you are unsure if your sunglasses are polarized, here’s a fun, simple test to see if they are. Hold your sunglasses up to a computer screen, which has an anti-glare coating similar to the anti-glare coating on a polarized lens. Angle your sunglasses about 60 degrees, with one side of the frame at 10 o’clock and the other at 4 o’clock. If the lenses are polarized, they will turn black.

You could also take the test with two pairs of polarized sunglasses – hold one pair at a horizontal (180-degree) angle. Now hold the other pair in front of the first pair, but rotate this second pair of sunglasses a half-turn, till it’s straight up and down, at a vertical (90-degree) angle. You will see that the lenses of both pairs of sunglasses turn considerably darker where the two lenses overlap when they are perpendicular. This is because when you angle one polarized lens to another perpendicularly, they block glare both horizontally (the horizontal pair) and vertically (the vertical pair).

At Zenni Optical, we offer polarized sunglasses in our 1.50 and 1.59 index single-vision and progressive (no-line bifocal) lenses, and in our 1.49 index bifocal lens. All of our detachable sun shades, whether magnetic or clip on, whether standard (the same tint color and shade all the way through) or gradient (the tint is darker at the top, getting progressively lighter toward the bottom) are polarized.

A101204 Sunglasses, Zenni Optical

A note of caution regarding drugstore sunglasses that are not polarized. If the lens is a non-polarized, darkly tinted lens not treated to block UV rays, it could be more dangerous to the eye than wearing clear, un-tinted glasses that have 100-percent UV protection. This is because the dark tint could cause the pupil to dilate, allowing more harmful UV rays into the eye.

You can rest assured that on every pair of glasses Zenni Optical makes, tinted or clear, we include a 100-percent UV-protection coating – for free.

In addition to polarized sunglasses, Zenni Optical also offers a different kind of sunglass lens that sometimes people confuse with polarized: photochromic lenses, which turn dark in the bright sunlight and become clear again in the shade or indoors.

Although you may have heard about a new technology that adds polarization to photochromic lenses, Zenni Optical does not offer these lenses at this time. Our polarized lenses are permanently tinted sunglasses that greatly reduce glare.

Friday, November, 15 2013 by