What is this thing?

A phoropter is used to measure refractive error.

The device you see pictured here -- from the cover of Justin Timberlake's 2013 album "The 20/20 Experience" -- is called a phoropter.

Used primarily to help determine a patient's glasses/contact lens prescription, it has been around for over 100 years. It contains dozens of different lenses which can correct for hyperopia, myopia, or astigmatism. With you seated comfortably, with both eyes open, and your forehead up against the back of the phoropter, the eye doctor will show you an eye chart and check each eye individually with different combinations of lenses. He or she will ask you which is sharper: #1, #2, or if they are the same. The goal is to find two options that appear the same to you in terms of clarity/sharpness. Based on the results, a prescription for eyeglasses or contact lenses can be issued.

What is astigmatism?

Astigmatism occurs when the focusing structures of the eye -- the cornea and lens -- are not perfectly round.

I get this question a lot, and since it's often misunderstood, I thought it would be a great topic to discuss.

Astigmatism, along with the previously-discussed entities of hyperopia (farsightedness) and myopia (nearsightedness), is one of three common types of refractive error. Refractive errors are problems with the focusing system of the eyes.

A normal eye is round, like the baseball you see here. An eye with astigmatism, by contrast, has a cornea -- the clear, front part of the eye that focuses light -- that is steeper/more curved in one axis and flatter in the axis 90 degrees away, like a football. This means that light in an eye with astigmatism is focused at two different points on the retina, creating a blurry image.

Astigmatism, like all refractive errors, can be corrected with glasses, contact lenses, or laser surgery (e.g. LASIK). It is easy to identify in a routine eye examination.

What does it mean to be "farsighted?"

The anatomy of hyperopia, or farsightedness

In the simplest of terms, people who are farsighted have an easier time seeing far away than up close.

Why is this? Well, a farsighted eye is an eye that is either too short or too weak. Take a look at this picture. The image is focused behind the retina, not right on the retina where it will be sharpest. You can imagine that this could happen either because the eye is too small, or the focusing power is too weak.

Now, here is where hyperopia is a bit different from simply the opposite of myopia. Young people have the ability to accommodate -- to increase the power of their lens focus -- and this can neutralize farsightedness. Hence, a young person who is farsighted may have no trouble at all seeing far away or up close, even if their prescription is high, because there lens can change shape and focus the image. The natural decline of this ability with age is called presbyopia, and is the reason why many people need reading glasses beginning in their 40s and 50s.

If a person's hyperopia is significant enough to warrant correction, the available options are the same as those we discussed last time, for myopia, namely: glasses, contact lenses, and corrective surgery.

What does it mean to be "nearsighted?"

Anatomical diagram illustrating myopia, or nearsightedness

First of all, myopia, or nearsightedness, means that relatively speaking, you see things better up close than far away. This is easy to remember, because the term "nearsightedness" suggests that you are best "sighted" at "near."

Take a look at the drawing here, of a myopic eye. Ideally, the cornea and lens at the front of the eye (left side of the drawing) should focus the light rays, from the image the eye is trying to see, right on the retina, at the back of the eye (right side). But look at this eye -- the image is focused in front of the retina. Nearsightedness! The eye is either too strong in its focusing ability or too long for its focal power.

So how can this be corrected? One way is by moving the object you look at closer to your eye. Why does this work? Simple optics. As the distance from the object to your eye decreases, the distance from the front of the eye to the image created by the eye increases -- the focal plane "moves backward." This means that instead of being focused in front of the retina, the image will be in focus farther back -- ideally, right on the retina. The more nearsighted you are, the closer this distance between the object you are looking at and your eye will need to be for you to see best.

OK, Dr. Weed, holding things close might work for books and whatnot, but it's not so great for sporting events, oncoming cars, other humans, etc. How else can nearsightedness be corrected? There are a variety of medical and surgical options. By wearing corrective lenses -- eyeglasses or contact lenses -- that "push the image back," so to speak, the eye can then focus images on its retina. Alternatively, a variety of surgical options, most commonly laser vision correction (e.g. LASIK), can be pursued.

Why do stars disappear when I look directly at them?

Stars disappear when you look directly at them because of the anatomy of the photoreceptors in your retina.

We all have two types of light-sensing cells in our eyes, the rods and the cones. Cones see fine detail and color. Rods see better in dim light. When you look right at something that is small or far away, the image falls on a part of your retina where there are only cones. This means that if you're in a well-lit environment, you will see this object very well. If however you are in dim light, you'll see the object better out of your peripheral vision (looking just off to the side of your target) because then the image will fall on the part of your retina that has rods, which can see in dim light. This is true of everyone's eyes, but many people have never noticed it. There are a few VERY rare conditions that can exaggerate this phenomenon, but they are like 1 in 10,000 level rare. A dilated eye exam could detect them.