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How UV Light Harms the Eye.

When it comes to the eyes—we only get two! Therefore we need to do our best to take care of the eyes we have.

One of the major culprits of eye damage is the sun. It’s something that is unavoidable, and therefore it is best to understand how to prepare and prevent as much sun damage as possible.


What Exactly Does the Sun Do to the Eye?

The sun emits UV light, specifically UV-A, UV-B, and UV-C.

UV-C light is the most dangerous and is typically blocked by the ozone layer. UV-A and UV-B, however, are ably to freely pass through the atmosphere and can penetrate through the skin and also the eyes.

We all have heard about how dangerous UV light sources are to the skin with the uprise in skin cancer cases. Therefore, we now know how important it is to wear sunscreen when going outdoors.

What is not commonly discussed, however, is that these harmful sunrays can pass into the eye even easier and cause just as harmful damage to the eyes as it does to the skin.

What is it about UV light that is so harmful to our bodies? A process called oxidative stress is the main culprit.

The sun is good in moderation, in general our bodies are able to take the UV light and break it down into useful, non-harmful components through a physiological process called oxidation. In the oxidation process, free radicals are broken down prior to entering cells and causing harm.

However, when we are exposed to too much UV light and the body is no longer able to keep up with the oxidation process, it undergoes oxidative stress and free radicals build up within the cells, causing irreversible damage.

Oxidative stress is one of the causes of cataract development as it causes the normally wellorganized cells of the lens of the eye to become disorganized. When the cells of the lens become disorganized the lens of the eye becomes hazy, cloudy, and yellow in color as the cells begin to break down.

These findings are all characteristics of cataracts.

While cataracts can develop as part of natural aging, we are finding that excessive sun exposure has been linked to early cataract development as well as more dense and advanced cataract development.

Cataracts typically do not begin to cause problems in individuals until they reach the age of 65 or older. However, with more exposure to UV light it is hypothesized that we will begin to see this age of cataract formation begin to become younger and younger.

Other ways UV light can damage the eye include creation of a pinguecula or pterygium (yellow thickened growths on the conjunctiva of the eye), UV keratitis, and macular degeneration (a progressive disease affecting the backmost structure of the eye called the retina).


How Can the Eye Protect Itself Against UV Light?

The eye has several mechanisms to protect itself against UV light. The main mechanisms are that the cornea, lens, and vitreous humor can all absorb UV light to prevent it from reaching the retina.

If too much UV light reaches the retina, it causes irreversible damage and death of light-detecting cells called photoreceptors. When this occurs, an individual can develop macular degeneration (death and destruction of the part of the retina responsible for our 20/20 vision) and even permanent vision loss.

Hence, the eye wants to have as many protective measures in place as possible to prevent UV light from ever even reaching the retina!

The cornea, lens, and vitreous humor, however, all have their breaking points and can only absorb so much at a time before losing the structural integrity.

If too much UV exposure occurs to the cornea, a person can develop UV Keratitis (also known as Welder’s Keratitis or Photokeratitis).

This is a very painful non-infectious inflammation of the cornea (very front structure of the eye) that appears 8-12 hours after excessive exposure to UV light. It is seen most commonly after skiing (UV light is reflected off snow, thus amplifying the amount of UV that reaches the eye), Welding, or spending time on water without sun protection.

Think about UV Keratitis almost as a sunburn on the eye. However, since the cornea has the most concentrated amount of nerves in the human body, it is like an extremely amplified sunburn.

If too much UV is absorbed by the lens, it creates a more chronic problem rather than just “sunburn”. Over time the UV absorption causes oxidative stress and cell denaturing of the lens cells—hence creating cataracts.

Another important protective mechanism the eye has against UV light is that it turns slightly yellow with age due to accumulation of vitamins zeaxanthin and lutein. These two vitamins are known to protect against oxidative stress, and therefore act as extra UV absorbers in the eye.

In short, we want high amounts of zeaxanthin and lutein in the eyes. Unfortunately, we do not reach adult levels of these vitamins until around the age of 20 years old.

This means that those under the age of 20 have a weaker defense mechanism against UV light and therefore are at an even higher risk of saturating the lens of the eye with UV and therefore developing cataracts earlier on in life.

In other words, the younger you are, the more important it is to protect your eyes against UV light to prevent serious damage from occurring to the eyes.

UV damage is something that accumulates over time to cause big problems—we do not want to accumulate an excessive amount before our eyes have even matured enough to defend against the UV particles!


Other Sources of Harmful UV Light

UV light is emitted from sources besides just the sun, too.

One of the most important ones to note is that electronic screens emit UV-B light. While we do not have enough research to state exactly what UV-B light from electronic screens does to the eyes, it is hypothesized that excessive screen time could result in early onset cataracts.

This is especially true with young children with excessive screen time as their eyes have not yet completely developed their UV protection mechanisms.

Other sources of UV light include tanning beds, black lights, fluorescent lights, lasers, and UV sanitizing lights.

While these sources all emit small amounts of UV light—very minimal in comparison to the sun—excessive use of them can accumulate to be large amounts of UV light absorbed by the body.


What Can I Do to Protect my Eyes from UV Light?

Ultimately, UV light exposure is impossible to avoid. UV light is a good thing in moderation, we just need to limit how much we are getting.

It is always a good thing to wear sunscreen when going outdoors. This includes on the face and eyelids too! Be sure to pick up a sunscreen that is safe for the face as the skin of the face and eyelids is a bit more fragile than on the rest of the body.

Wearing sunglasses when going outdoors is also important—especially for kids! Look for sunglasses with the UV protectant sticker on them.

Sunglasses are particularly important when going outdoors for extended periods of time, and when being around water, sand, or snow as all of these elements are known to reflect light and thus increase the amount of UV light being absorbed by the body.

Another way to protect your eyes from the sun is by wearing a billed hat such as a baseball cap, wide-brimmed sunhat, or bucket hat to keep the eyes shaded from directly overhead sunlight.

Blue light filters on glasses are also a good idea if you or your child use electronics for more than 1 hour a day. This includes computers, tablets, cell phones, and TVs.

The thing that is tricky about UV light and the eyes is that you do not necessarily feel pain from UV accumulation.

UV light causes gradual changes over time that builds up to cause a big issue. Having good prevention methods to protect your eyes against UV damage is the best way to keep your eyes strong and healthy for years to come!


Our eye doctors at Eye Theory in Houston, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrist at 832.831.7386 or schedule an appointment online if you would like to learn more about UV light exposure and the damage it can cause.  Our eye doctor, Dr. Jonathan Tsao, provides the highest quality optometry services and eye exams in the Midtown, Downtown, Museum District, Montrose, East Downtown, and Southside Commons (Southside Place) vicinities of Houston, Texas.

Understanding Soft Contact Lenses

Contact lenses eliminate the need for having to wear glasses, which can be beneficial for sports, cosmetic reasons, and the ability to see outside the limitation of a glasses lens.

In today’s world, with so many different outlets for getting contact lenses, it is more important than ever to understand why you should see your eye doctor yearly and how to properly care for your lenses.


What Does an Optometrist Consider When Prescribing Contacts?

When prescribing corrective lenses—whether they be glasses or contacts—your eye doctor takes many different parameters into consideration including: the distance between the correction and your eye (known as the vertex distance), what your primary working distance is (are you wanting to see best up close, far away, or at an intermediate distance?), the shape of your eye, the health of your eye, and many others.

Contact lenses are a little more complicated than glasses because they fit directly onto the eye and therefore need to sit just right to allow for optimal vision. Your eye doctor will take many different measurements before choosing the best lens for you—sometimes this takes multiple attempts!

Contact lens fittings can be broken down into two major categories—finding a lens that is comfortable and finding a lens that optimally corrects your vision.

At the beginning of a contact lens evaluation, the eye doctor must evaluate the health of your cornea—the very front clear structure of the eye. Your eyes need to be free of any surface disease or infection prior to wearing a contact lens.

Next, your eye doctor will take some measurements to determine what size lens would be best for your eyes, as well as mapping out the curvature of your cornea to make sure the lens is not too flat (resulting in a loose fit) or too steep (resulting in a tight fit) for your eye.

Arguably, the most important part of fitting a contact lens is ensuring that it properly corrects your vision. Since the lens sits directly on your eye, your glasses prescription will need to be converted to a contact lens prescription.

Yes, you read that right. Believe it or not, your glasses prescription and your contact lens prescription are not the same thing!

Glasses prescriptions and contact lens prescriptions can be converted interchangeably using several different complex equations. However, just because a person’s glasses prescription mathematically computes to a certain contact lens prescription does not necessarily mean that ordering the converted prescription will result in optimal vision.

Once your doctor has made all of these measurements and selected what is believed to be the “perfect” lens for your eye, they need to take a look at the lens on your eye to determine if this is actually the case.

The human eye is like a finger print—no two are the same. Therefore, your doctor will need to take your input into consideration when fitting your lens to make sure your vision is the best it can be. He or she may need to tweak a parameter or two to get your prescription exactly right.

Sometimes everything fits great and you may see great, but the lens is not comfortable. Your doctor may need to switch brands or materials to find a lens that is more comfortable to you.

Or, sometimes a lens fits great and is comfortable, but the contact lens needed for your exact prescription does not exist. Therefore, your doctor may have you try two different sets to determine which lens you see the best out of, and you may have to settle a little on not having “perfect” vision.

But wait, glasses can be made to your exact prescription, so why can’t contact lenses?

As noted above, contacts sit directly on your eye. The eye is a wet, round surface, so lenses tend to move around when blinking, looking to the right or left, up or down, etc. The main point to note here is that contact lenses have some movement whereas glasses lenses do not. This complicates things a little bit.

Contact lenses are also made in bulk by contact lens manufacturers. Since there is an infinite number of different prescriptions possible, contact lens companies have to put some limit to what they make, otherwise some lenses would be bought all the time, and others would sit on shelves for years and eventually go bad and need to get thrown away.

Therefore, unlike glasses that can be made exactly to a very specific prescription, contact lenses have to be estimated and altered to fit what is available. Your eye doctor has to make this decision each and every time your glasses prescription is adjusted.

Here’s an example patient to help explain this system:

Let’s say a person has a glasses prescription of -6.25 -3.25 X 163. After converting this prescription from glasses form to contact lens form, the “optimal” contact lens prescription would be:     -5.81 -3.11 X 163.

Prescriptions are written to the nearest quarter diopter (.00, .25, .50, or .75). For a patient with astigmatism, the available axes in contact lenses are to the nearest 10th, (i.e. X 010, X 100, X 180, X 160, X 170, etc.) Thus, this is where the doctor would need to do some educated trial and error work to find the lens that would best for this example patient.

A good place for the doctor to start would be a lens that is -5.75 -3.00 X 160. However, the doctor could also try a -6.00 -3.00 X 170, or a -5.75 -3.25 X160, or a -6.00 -3.25 X 170, or other combinations of these options.

Then, perhaps after all this calculated decision making, the doctor puts the lens on the patient’s eye and the lens rotates. Now even more calculations have to be considered to make sure the patient’s vision is corrected in the proper manner!

As you can see, it is not quite as straightforward as one might think!


Different Contact Lens Options

There are hundreds of different contact lens options in existence. In fact, every few months an entire book is published to give eye doctors the complete list of contact lenses and their available parameters.

Contact lenses are available as spherical only lenses (i.e. no astigmatism correction), or can correct for astigmatism as well. Astigmatic contact lenses are referred to as “toric” lenses.

Contact lenses that correct for astigmatism tend to be a bit more expensive, so if you have a low amount of astigmatism your eye doctor may try to adjust your prescription to be a spherical contact lens in order to help you save money. This does not work for all patients—only sometimes in those with very small amount of astigmatism (-0.25 to -0.50 diopters)

Contact lenses can also be used to change your eye color! In today’s market, essentially any color is available. Some of the most common colors are blue, green, purple, grey, or hazel.

Another of the most notable differences between contact lenses is their wear schedule. Some lenses are meant to be worn for 1 day only, others may be worn for up to a month, 3 months, etc.

It is very important that you know what your lens wear schedule is and that you discard and change out your lenses as recommended. Over-wearing your lenses can lead to infections and even permanent damage to your eyes.

Daily contact lenses tend to be a little more expensive, but are a great option for children, individuals who have allergies, those with dry eyes, or those who only like to wear lenses every now and again for special occasions.

Monthly lenses must be taken out every night and stored in a contact lens case. They tend to be a little more affordable than daily disposable lenses. It is recommended that these contacts be replaced on the same day every month (for example, the 1st of every month) to help to remember when to change lenses as to not over-wear an old lens.

There are also lenses that can be approved to sleep in, but there are not quite as many prescriptions options for these lenses as other types, as it is not recommended to sleep in contact lenses.


The Do’s and Don’t of Contact Lenses

Do NOT swim or take showers in your contact lenses. Bacteria and fungi within the water system are known to latch onto contact lenses. When these sit on your eyes all day long every day it can lead to some very serious infections.

Do replace your contact lens case every month. Bacteria like dark, warm, moist environments—like contact lens cases! You might be cleaning your lenses properly, but putting clean contacts into an old, contaminated contact lens case will only transfer the bacteria to your lenses, which will be put into your eyes, and will result in a nasty infection.

Do replace your contact lens solution each night. Letting solution sit in a case all day is asking for bacteria to move in and proliferate in the solution. Just as mentioned above, this can result in bad infections!

Do NOT sleep in your contact lenses. When we sleep, our eyes get less oxygen and dry out a little bit. When this happens, the contact lens shrinks up slightly and will tighten onto your eye—allowing even less oxygen to get to your eye. This is very dangerous!

Do replace your lenses as directed. If your lens is a monthly lens, it must be replaced after 1 month—even if you only wore it a couple of times! Lenses only stay “good” for a given amount of time. If you have a 2-week lens, it will go “bad” 2 weeks after it was removed from its foil package. Not properly disposing lenses after the time indicated by the manufacturer increaes your risk for infection.

Do discard contact lenses after they have expired. All boxes of contact lenses should be marked with an expiration date (if not, you shouldn’t be wearing that brand!). Just like medications, contact lenses expire. New contact lenses are stored in air-tight foil packets filled with a sterile solution. After the expiration date, the sterile solution has lost its effect and the lenses could be contaminated with bacteria. It is also possible that the expired lenses have lost their integrity and may no longer properly correct your prescription. Either way, it is not safe to wear expired lenses (or use expired solutions!).

Do NOT overwear your lenses. This is super important. Overwearing your lenses increases risk for infection, results in poorer optical quality, and can damage the structural integrity of the eye. One of the most common findings in contact lens over-wearers are corneal ulcers. These are very painful and can result in permanent vision loss, sometimes even blindness, if they occur on the central cornea.

Do clean your contact lenses every night. There are different types of solutions, but be sure yours has a disinfectant property to it and manually rub your lenses to ensure bacteria, debris, makeup, dirt, etc. are removed from the lenses prior to inserting them into the case for storage each night.

Do NOT fib to your eye doctor when they ask you questions about your contact lens care. Contact lenses are a wonderful tool for those who have prescriptions but do not want to be hindered by the glasses lifestyle. They are safe when used as directed. It is of utmost importance that you tell your eye doctor the truth about how you handle your contact lenses—they will not judge you, they only want what is best for your eye health. If you are not honest with your doctor, they may not be able to help you properly when problems arise. Who knows, there might be a better options to suit your lifestyle and preferred care regimen! Open conversations with your doctor are key.


Our eye doctors at Eye Theory in Houston, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrist at 832.831.7386 or schedule an appointment online if you would like to learn more about soft contact lenses.  Our eye doctor, Dr. Jonathan Tsao, provides the highest quality optometry services and eye exams in the Midtown, Downtown, West University, Museum District, Montrose, East Downtown, and Southside Commons (Southside Place) vicinities of Houston, Texas.

Is LASIK or Refractive Surgery an Option For Me?

Refractive Surgery

Are you tired of wearing glasses or contacts and considering corrective surgery? This article will briefly discuss the different options and how corrective surgery works!

Anatomy of the Eye

To understand how corrective surgery works, a basic understanding of the eye’s anatomy will be beneficial.

The eye has 3 major parts to it—the cornea, the lens, and the retina.

The cornea is the front structure of your eye. It is clear and overlays the pupil and colored part of the eye (iris). It consists of 5 layers—from the most outside layer to inside layer: epithelium, Bowman’s layer, Stroma, Descemet’s membrane, and the endothelium.

The lens is the middle part of the eye. It is a flexible structure that can change shape based on where you want to focus—far away or up close.

The retina is the very back of the eye. It is responsible for actually detecting light and transmitting it to the brain to form images.

Light has to be able to pass easily and undeviated through the cornea and lens to land properly on the retina. If the cornea and/or lens are misshaped, the light will not focus correctly on the retina and vision will appear blurry—hence the need for glasses or contact lenses.


How Does Refractive Surgery Work?

Refractive surgery works by a surgeon (ophthalmologist) restructuring your cornea. This can be accomplished a few different ways, but ultimately the surgeon will use a laser to cut and remove part of your cornea to make its’ shape optimal for your refractive error.

If you are farsighted (i.e. hyperopic) the surgeon will want to steepen the central part of your cornea, this can be accomplished by flattening (i.e. removing part of) the outer edges of your cornea.

If you are nearsighted (i.e. myopia) the surgeon will want to flatten (i.e. remove part of) the central portion of your cornea.


Am I a Candidate for Refractive Surgery?

There are many aspects of your eyes that must be considered prior to deciding if you are a candidate for refractive surgery.

Your cornea needs to be a certain thickness to allow the surgeon to manipulate the tissue without making it too thin.

You need to have a stable prescription—once you undergo refractive surgery it is much more difficult to perform a second surgery.

You must be at least over the age of 18 years old.

Your prescription must fall within certain parameters—parameters very upon different techniques.

Your eyes must be healthy and free of inflammation. Certain ocular diseases will disqualify you from this procedure.

Your eye doctor will evaluate you closely and weigh the pros and cons of the procedure.


Different Types of Refractive Surgery

There are three major types of refractive surgery to date. Over the years these techniques have been improved and perfected, whereas others have been discontinued due to complications.



PRK, or Photorefractive Keratectomy, is a procedure in which the cornea epithelium and Bowman’s membrane are removed with a laser, and the stroma is “shaved down” to create the optimal surface to correct for your prescription.

PRK has a longer post-operative recovery time (about 2 weeks) since the epithelium will need to regrow completely.

PRK is recommended for individuals who live especially active lifestyles or are in the military.



LASIK, or Laser-Assisted In Situ Keratomileusis, is also a procedure in which a laser is used to correct a patient’s vision by reshaping the cornea.

LASIK differs from PRK in the fact that a laser is used to create an epithelium and bowman’s layer “flap”. The surgeon lifts the flap to access the stroma and “shave it down” to create the optimal surface to correct for your prescription.

Since LASIK does not actually remove the epithelium and Bowman’s layer, it has a quicker recovery time of around 24 hours.



SMILE, or Small Incision Lenticule Extraction surgery, is the newest form of refractive surgery.

SMILE creates a very small incision—no flap is created. The epithelium and Bowman’s layer remain in tact and the stroma is shaved down to reshape the cornea to the optimal shape to correct for your prescription.


Common Questions About Refractive Surgery

Can I get refractive surgery if I am farsighted (hyperopic)? Yes! In the past hyperopes may have been excluded, but today techniques have improved greatly to include our hyperope friends as well.

Can I get refractive surgery if I have astigmatism? Yes! It’s a bit more complicated, and there is a limit to how much can be corrected, but astigmatism does NOT exclude you from refractive surgery.

Is it true that if I get refractive surgery I’ll never need glasses again? No! While the majority of people post-surgery will not require correction, a select few number of patients will still require glasses post-opt. Also, you will still need reading glasses with age as cataracts/the need for reading glasses are an issue with the lens, not the cornea. So, almost everyone will need glasses eventually (typically decades down the road) post-opt.

Is it true that I will be awake during surgery? Yes. You will be given a topical numbing drop so you will not feel anything, but you will be asked to stare at a target for a few seconds. The laser system will tract your eyes so that if you move slightly the system will shut down until proper focus is regained—this prevents the surgery from continuing in case of needing to sneeze or another reason in which focus is lost.

Since a laser is being applied to my eye, does the surgery burn? No! You won’t feel much of anything. The laser is perfectly focused so that it cannot hurt anything besides just reshaping your cornea.

After surgery I won’t need to see my eye doctor yearly, right? Wrong. You will need a few follow-ups with your eye doctor after the surgery, and then it is recommended that you continue to see your eye doctor yearly. Refractive surgery has the tendency to cause an increase in dry eye symptoms, which your eye doctor will be able to help you manage, if needed.

It is also very important to continue to have yearly eye exams to check the health of your eyes as your cornea is being altered during surgery and complications (while uncommon) do occur. The sooner these complications are caught, the better off the prognosis.


Our eye doctors at Eye Theory in Houston, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrist at 832.831.7386 or schedule an appointment online if you would like to learn more about LASIK and the types of refractive surgery.  Our eye doctor, Dr. Jonathan Tsao, provides the highest quality optometry services and eye exams in the Midtown, Downtown, Museum District, Montrose, East Downtown, and Southside Commons (Southside Place) vicinities of Houston, Texas.

Why You Should Try Multifocal Contact Lenses

Presbyopia, the inability to focus on objects up close, is a process that often begins to become more noticeable in an individual’s late thirties to mid forties. Multifocal lenses, usually being progressive lenses, are a great option that provide improved vision at various distances.

However, many also prefer to be free of glasses and look for a contact lens option. Today, there are a variety of lenses that are designed to provide the same multifocal ability as progressive or bifocal glasses lenses.

Our optometrist will find you the lens that fits your needs and eye anatomy, but it is a specific process that can take some time to fine tune. Below are some of the major components that we look for when deciding on a lens.

Your contact lens prescription

Each lens comes in a variety of powers, with the options varying between companies and product lines. After finalizing your prescription, our optometrist will make the necessary calculations and decide on the right lens for your specific needs and prescription.

In the case of high plus or minus lenses, only a few lines will have available options and the right lens will be selected for and trialled by you.

Contact lenses are chosen based on your eye’s anatomy

Each contact lens product line will come in specific sizes and curvatures. Your individual eye also has specific sizes and parameters.

It is important that our optometrist selects the correct parameters for you that allow for a good fit of the lens. A lens that is too tight or too loose can cause not only discomfort but also affect the amount of oxygen that gets to the eye.

In addition, our optometrist may also determine if you have a preference for one eye over the other. This is important because of the way some of these lenses are designed.

Select multifocal contact lenses are designed such that the center provides the correction necessary to see up close and the surrounding is for distance vision. The other eye will have the opposite correction in that the center provides proper distance vision and the surrounding area provides near vision.

Depending on your preference, our optometrist may select one eye over the other for primarily distance vision and vice versa.

Multifocal contact lenses can meet your visual needs

With the trial lens on, your vision will be assessed. It is important to keep in mind that multifocal contact lenses, while great for allowing viewing of varying distances, will often not provide vision as optimal as glasses.

There is often a compromise that must be made between the clarity of distance and near vision, as it is difficult to achieve perfect clarity at both ends. There is a happy medium that will be achieved that will allow for independence from glasses during regular daily tasks.

Depending on an individual’s visual needs, a prescription may be altered to allow for more near or distance clarity. It is common for our doctor to provide a variety of options for you to take home and test out in your daily life to see which ones work best for you.


Our eye doctors at Eye Theory in Houston, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrist at 832.831.7386 or schedule an appointment online if you would like to try multifocal contact lenses.  Our eye doctor, Dr. Jonathan Tsao, provides the highest quality optometry services and eye exams in the Midtown, Downtown, Museum District, Montrose, East Downtown, and Southside Commons (Southside Place) vicinities of Houston, Texas.

Understanding Your Glasses Prescription

When you receive an eye examination, you often leave with a copy of your prescription, which includes numbers for the exact specifications of the glasses you need. These numbers include the sphere, the cylinder, and the axis.

Sphere, cylinder, and axis can also all be broken down into different categories. Read on to learn about these specifications.


Sphere is the first number on your glasses prescription

The sphere power is the first number in the prescription and specifies the overall type and amount of power in the lens. It can be positive, indicating a hyperopic or farsighted prescription, negative, indicating a myopic or nearsighted prescription, or it can be zero.

If you have a farsighted prescription, your eyes have an easier time focusing far away and need some extra power for seeing things up close.

If you have a small to moderate number, you may have gone most of your life without needing glasses and only start requiring them when the focusing system inside your eyes begins to noticeably decrease in power (presbyopia).

For a nearsighted (ie: myopic) prescription, the eyes are naturally adapted to see at near without spectacles but require diverging power to make distant objects clear.

For low myopic prescriptions, you might only want your glasses or contact lenses to drive or see a movie, while for higher prescriptions, you pretty much need your glasses for everything you do.


The cylinder power is used to correct astigmatism

The second number is the magnitude of the cylinder power, which is used to correct astigmatism. It is usually specified in minus form, but can be converted to plus form by changing the entire prescription to an equivalent format.

Astigmatism is when the eye requires different powers in different orientations to see clearly, owing to its bending light like a football rather than a soccer ball: one direction with much more curvature than the other.

This can also be imagined in that someone with uncorrected astigmatism might see the vertical part of a capital letter L as perfectly clear while the lower horizontal part of it is blurry, or vice versa in any direction.

Everyone usually has a little bit of astigmatism, but some people may have none at all. Astigmatism is also not directly related to the amount of sphere power: you can have zero sphere power and a lot of cylinder power which still requires you to wear your spectacles or contact lenses.


The axis determines where to put the astigmatism in your glasses

The axis is the final number and is written as an angle made of three digits without the extra degree sign usually put in geometric angles.

This specifies the direction of the astigmatism correction, as it is a line that can be any degree or orientation from 180 degrees (a straight line left and right) to 90 degrees (a straight line up and down) and anywhere in between these.

The axis can be specified as with the rule (close to a flat horizontal line), against the rule (close to a straight vertical line), or oblique (somewhere in between these, close to a 45 or 135 degree angle).

As we age, we tend to move from with the rule astigmatism as children to against the rule as elderly.


Our eye doctors at Eye Theory in Houston, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrist at 832.831.7386 or schedule an appointment online if you would like to learn more about your glasses prescription is made and interpreted.  Our eye doctor, Dr. Jonathan Tsao, provides the highest quality optometry services and eye exams in the Midtown, Downtown, Museum District, Montrose, East Downtown, and Southside Commons (Southside Place) vicinities of Houston, Texas.

Why are there Floaters in My Vision?

Depending on your age and eye medical history, you may have noticed a few or many floaters in your vision. Floaters are black dots or squiggles that you may see moving around in your vision, more obvious when you look at a light background or are somewhere with high illumination. When you move your eyes, they move with your gaze. These form in the vitreous humour, the gelatinous filling near the back of the eye. Often, we have a few floaters that our visual system gets used to with time, but there are also reasons why new floaters might appear later on in life. Some of these are normal, and some can be signs of something wrong in the eyes. Read on to learn about some common causes of floaters.

Posterior Vitreous Detachments (PVD)

The most common, and most harmless, reason to develop a new floater is the change in composition of the vitreous with age. As we grow older, this gel becomes more liquid-like and shrinks, pulling back from the retina and forming pockets of material that become a floater, or a posterior vitreous detachment. You may have some floaters even when you are young, but as we grow older you will always acquire more. Gradually seeing some more floaters in your vision through the years is normal, but there are other symptoms that can accompany them that signal something wrong with your eyes, and these will be explained below.


Vitreous and Preretinal Hemorrhage

When the vitreous pulls away from the retina with age, as explained above, sometimes it can tear blood vessels in the retina and lead to some bleeding that will show up as floaters in your vision. This bleeding can also result from trauma or other disease processes going on in the body, such as diabetes, sickle cell disease, or blood vessel blockages. These diseases can cause new, fragile blood vessels to grow from the retina into the vitreous, and these can then easily bleed to block vision.

Any bleeding like these described will cause a sudden and dramatic increase in the number and size of floaters in your vision and can completely block your vision if a full vitreous hemorrhage occurs. This can last for 2-3 months while the blood is cleared from the eyes. This shows the need to protect your eyes from trauma and to get your eyes checked regularly with one of our eye doctors, especially if you have underlying medical conditions that can manifest in the eyes.

Retinal Breaks and Floaters

A retinal break, tear, or detachment (retina peeling off) is the most urgent cause for sudden formation of floaters. Other symptoms often accompany this, such as lightning flashes of light in one’s vision, a foggy film appearance to vision in one eye, or the appearance of a dark curtain over one’s vision. If you are experiencing these symptoms, see one of our eye doctors immediately, as you may require urgent surgery in the coming days to prevent you from losing vision in that eye.


Our eye doctors at Eye Theory in Houston, TX excel in the prescription of contact lenses, glasses and various eye diseases.  Call our optometrist at 832.831.7386 or schedule an appointment online if you would like to learn more about floaters and vit.  Our eye doctor, Dr. Jonathan Tsao, provides the highest quality optometry services and eye exams in the Midtown, Downtown, Museum District, and Southside Commons (Southside Place) vicinities of Houston, Texas.