Craig Blackwell, MD

Santa Cruz, CA
Diplomate: American Board of Ophthalmology
Fellow: American Academy of Ophthalmology

Welcome to the Website of Craig Blackwell, MD

An Ophthalmology Practice in Santa Cruz, CA

Narrated Eye Exam

The following is a description of an eye exam narrated as though you were looking through the doctor’s eyes.

The style is conversational as would occur during the exam. There is a description, drawings and photographs of each of the eye structures as we look at them.

Following a link will take you to a page with expanded descriptions of exam items and information on a few of the most common eye conditions, like cataract, glaucoma and macular degeneration.
Data from a few landmark studies are included that are interesting in their own right and may be useful in making decisions about treatment choices.

This is not an encyclopedic coverage of all eye diseases. It covers most of the items usually performed in an exam, but there are special techniques and tests that may be added as appropriate. Hopefully this narrative will be helpful in understanding diagnoses you might have. Remember, this in no way replaces the need to consult with your ophthalmologist.

Vision and Glasses

What does the fraction 20/20 mean?

It means that you, the subject, can see at 20 feet what a reference person with “good” vision can see at 20 feet. 20/40 means you can see at 20 feet what the reference person can see at 40 feet.

Rule #1. If vision is less than 20/20 it is our job to figure out why.

The most common reason vision is below 20/20 is the need for glasses correction.

Therefore, the first step in an eye exam is to measure the glasses prescription to determine the “best corrected vision,” which is the number that counts in evaluating the health of the eye.

If vision is less than 20/20 with glasses then we are looking for a medical explanation, such as cloudy cornea, cataract or retinal problem.

Five terms of vision correction:


Myopia – This is the technical term for “near-sightedness,” in which objects up close are clear while objects at a distance are blurred. Optically, the image from a distant object comes into focus in front of the retina.

Near-Sighted, Far-Sighted, or Astigmatism?
See Optics page for explanation of terms.

The above conditions are correctable with an appropriate glasses prescription and/or contact lenses. Some are correctable with refractive surgery.

Refraction and Glasses Prescription.

Refraction is the process of determining the glasses prescription to correct an optical condition; near-sighted, far-sighted, or astigmatism.

Step 1. Autorefractor. This machine uses sophisticated optics to provide an estimate of glasses prescription.

In a new patient this result gives us a place to start, and in a returning patient it gives us an indication of whether or not there has been a change in prescription.

Step 2. Refine the correction. The autorefractor gives us a good place to start, but is not accurate enough to write the finished prescription. We refine the prescription by changing lenses while you view letters on the vision chart, working to reach the sharpest vision we can. Think about this like focusing a camera.

Step 3. Check reading correction. Some time after reaching age 40 most people are faced with the fact that they need help with reading in the form of reading glasses or bifocals.

Step 4. As a final step we compare the old glasses prescription with the new findings and show you the difference to help decide if a change is worthwhile.

Step 5. To get new glasses you take the updated prescription to an optical shop. The optician will help you choose frames, discuss lens choices and do measurements for lens position. Using an experienced optician gives a significant advantage in accuracy, which is particularly important with high power and progressive lenses.

Read more about types of glasses:

  • bifocals, trifocals and progressives
  • sunglasses
  • safety glasses.

A common misconception is that wearing glasses weakens the eyes and makes you need stronger lenses. In adults, wearing glasses does not affect the development of future glasses prescription. There are studies underway to determine if there is such an effect in the developing eye in childhood.

Important point. People often say they don’t come in for an eye exam because their vision is “fine.” Unfortunately, there are a few serious diseases like glaucoma that don’t affect central vision until very late. Even if you have no risk factors age 40 is a good time to have a baseline eye exam.


Eye movement is surprisingly complex, involving six muscles that move each eye.

The brain maintains alignment by actively steering the eyes to keep the images in close enough proximity that the brain can fuse them into a single image. Maintaining “fusion” is the driving force that keeps the eyes aligned.

Depth perception is the ability of the brain that takes the slightly different image from each eye and creates an accurate estimate of distance from an object.

Child’s Eye Exam

Early pediatric exams assess basic vision function, ocular alignment, clarity of the cornea and lens, and health of the retina. Follow-up at regular intervals during vision development is important to catch any detours as early as possible.

Terms for common children’s eye problems:

  • “Strabismus” is the term for misalignment, usually with the one eye turning either inward or outward, can also be upward.
  • “Amblyopia” or “Lazy Eye,” is the term for reduced vision in a physically normal eye. Lazy eye does not refer to turning in or out, rather it refers to the reduced vision.

Eyelid Exam

Particularly on the first exam the eyelids are checked for position, the presence of growths and for signs of infection or inflammation.

Blepharitis, is a common problem describing inflammation of the eyelids. It can come from skin disease in the proximity of the lids or chronic low grade lid infection. Lid scrubs are often necessary on an ongoing basis to keep the inflammation under control. If that is not enough antibiotics may be needed, usually applied as an ointment, but sometimes orally for stubborn cases.

Microscope Exam of the Front of the Eye

A specialized “slit lamp” microscope is used to examine the fine details of the eye.

Exam at the Slit Lamp Microscope.

View of the front of the eye through the microscope. The slit beam of light is seen as it passes through the clear cornea and illuminates the iris.

The illustration shows the structures of the front of the eye, called the Anterior Segment.

The Cornea is the clear window in the front of the eye.

The Iris is the colored ring surrounding the dark Pupil. Like the diaphragm of a camera it regulates the amount of light entering the eye by changing pupil size.

The Anterior Chamber is the space between the Cornea and the Iris, filled with a clear watery liquid called Aqueous.

The Conjunctiva is an external covering layer (not shown) extending from the edge of the cornea to the inside of the eyelids.

Aqueous Humor
The blue arrows show the circulation of aqueous fluid in the eye. Aqueous is important because it supplies nutrients to the lens and cornea which have no blood vessels of their own.

Exam of the Anterior Segment

Starting at the surface, we check the white part of the eye, the clarity of the cornea, the depth of the anterior chamber and the details of the iris.
For example we might find a pterygium, corneal clouding, or be evaluating conjunctivitis.
Depth estimate of the anterior chamber is important. If the chamber is too shallow there is risk that the normal circulation of fluid within the eye will be blocked and the pressure will rise. This is the mechanism of narrow angle glaucoma.

Eye Pressure

The eye requires a certain amount of pressure to keep it inflated and functioning properly. The amount of pressure within the eye is determined by a balance between fluid being pumped into the eye and fluid leaving through the filtering system. If pressure in the eye gets too high that can damage the optic nerve and result is vision loss, which is what happens in glaucoma.

Measuring Pressure

An instrument that measures pressure is called a tonometer. The most accurate instrument in common use is a Goldmann tonometer. The photo shows the tonometer in position in front of the eye about to measure pressure. After instilling anesthetic drops the lighted blue tip is pressed gently against the cornea. The pressure reading is taken from the dial in the lower center of the picture.

Testing for Glaucoma:

  • Intraocular pressure can vary significantly in a 24 hour period. Since pressure is so variable it is of equal or more importance to examine the optic nerve for signs of damage.
  • Because of the way pressure is measured the thickness of the cornea affects the pressure reading. If there is any suspicion about elevated pressure we measure corneal thickness and apply a correction factor.
  • The amount of pressure required to damage the optic nerve is different for different people.

See section on glaucoma testing for more detail.

See OHTS for review of glaucoma risk and whether taking drops helps lower risk of developing glaucoma.

Dilated Exam

An eye exam is not complete without a look at the inside of the eye. For the first visit of a new patient, even without risk factors or symptoms, we like to dilate the pupils to get a good look at the lens, nerve and entire retina.

Dilation is a must for people who have particular risk factors, like diabetes, or moderate to high myopia that require a careful retinal exam. Dilation is a must for people who have sudden onset of floaters who we consider at risk for a retinal tear.

The dilating drops not only open the pupil, they also relax the focusing muscles. After dilation vision is both bright and blurry for 4 to 6 hours. It may be longer in some people who are sensitive to the medication. If it is your first visit, or you are returning and know you will be dilated, consider arranging for a friend to drive you home.

The Lens

Once the pupil is dilated we get a good look at the lens, checking for clarity, opacity and whether there are any deposits on the lens.

Slit lamp view through a dilated pupil. The slip beam, coming at an angle from the left, first cuts through the cornea (broken arrows), followed by a dark gap for the anterior chamber, then it lights up a large section through the lens (solid arrows). This lens is average clarity for a person of 50 years of age.

When the lens becomes cloudy we call it a cataract. In a young person the lens is clear like glass, with no significant opacity or color tint. As time goes by, in everyone’s life, the lens becomes increasingly hazy and takes on a yellow-green tint (see photo above) that progresses to orange and brown.
Some things that are associated with accelerated cataract development are extended UV exposure, Diabetes, trauma and Prednisone use.
Objectively we measure the cataract by how much it reduces visual acuity.

Decision About Cataract Surgery

Rule #2. Everyone gets some degree of cataract with time, but just because you have a cataract does not mean you need to have surgery.

As the lens gets cloudier you will likely notice increased glare along with reduced vision. When reduced vision interferes with things that are important to you, like driving, reading and job performance, then it is time to consider cataract surgery.

Modern cataract surgery is a technologic marvel. The cloudy natural lens is removed and replaced with a new artificial lens. Done under local anesthetic, the procedure usually takes under half an hour and you are back to your normal activities in a few days. Newer intraocular lens designs are promising less dependence on glasses.

Through the dilated pupil a lens implant (IOL) is visible. It is tricky to see because it is clear. The arrows point to the edge of the circular optic of the IOL. At the 6:30 position one can see the end of one of the stabilizing arms embedded in the clear lens. The other end of the arm is behind the iris.

Rule #3. It is important to remember that while the complication rate of cataract surgery is low, it is not zero, and the new lenses have their downsides. Consider your decision for surgery and lens choice carefully.

Retina and Vitreous

The Vitreous

The eye is an inflated hollow structure like a basketball. The large space behind the iris and lens is filled with a clear jelly-like material called Vitreous.
Looking into the living eye with the microscope we see thin strings and gossamer veils suspended in a clear liquid. As the eye moves the gel shifts, looking very much like the kelp forest swaying with the action of the waves.

Flashes and Floaters

In youth the gel is optically quite clear. Over time opacities gradually accumulate in the vitreous which cast shadows we see as “floaters.”

Besides accumulation of the floaters the body of vitreous gradually shrinks which causes it to pull on the places inside the eye where it is attached. As the gel pulls on the retina it mechanically triggers a nerve impulse which you see as a brief flash of light. For most people the attachment of the vitreous to the retina is not particularly strong and the vitreous pulls free. You may notice a few new floaters, which are a nuisance, but no harm done.

Retinal Tear and Detachment
In a small percentage of people the vitreous gel is firmly attached to the retina and when it pulls it causes a tear in the retina. This is usually accompanied by the sudden appearance of many new floaters.

Once there is a tear in the retina then the liquid portion of the vitreous, shown by the blue arrow, can get through the hole and create a separation of the retina from the wall of the eye, referred to as a detachment. As the detachment grows the retina loses function and vision is lost in that area. To the person involved it looks like a gray curtain or window-shade blocking vision.

If a retinal tear is caught early then it is a relatively simple thing to repair it with a laser. If the tear is not treated early, then as time goes by, the area of detachment spreads and it requires an increasingly complicated procedure to repair it.

Recommendation. If you see the sudden appearance of new floaters there is no way for you to tell if there has been a retinal tear and it is worthwhile having the retina checked.

The Optic Nerve

There are approximately 1 million individual nerve fibers that are bundled together to form the optic nerve. It is like a telephone cable carrying information from the eye→ to the brain.

The most common causes of optic nerve damage would be glaucoma and interruption in circulation, followed by rarer things like inflammation and compression.

When we talk about the optic nerve in an eye exam we are discussing the part we can see, the end of the nerve as it enters the eye, the optic nerve head. We cannot view the rest directly, and if we need to check the rest of the nerve it requires imaging with a CT or MRI scan.

Evaluation for Glaucoma Requires

  1. Measurement of pressure and corneal thickness
  2. Evaluation of optic nerve health

We already discussed pressure measurement and corneal thickness.
If pressure is elevated or the nerve has a suspicious appearance then we check nerve health by looking at both structure and function.

First, we inspect the nerve and using a scanning laser (Heidelberg Retinal Tomograph) we can obtain a contour map which compares specific characteristics to normal.

Second, we test the functioning of the optic nerve with a visual field.
There are specific changes in both of these measures that are characteristic of glaucoma.

See the Testing for Glaucoma page for a more detailed look at how we do an evaluation of the optic nerve looking for glaucoma, with an example.

The Retina

The retina is a layer of nerve tissue lining the inside of the eye that receives light and makes a picture, like film in a camera.

Rods and Cones

The Rods and Cones are the sensory cells within the retina that receive light and generate a nerve impulse. Other retinal cells perform basic processing on those impulses and then carry that information back to the brain.

See the Retina section for more details on retinal structure and function.

The Macula

The Macula is the center part of the retina used to see moderate detail. The Fovea is the very center where the finest detail is seen, like reading and recognizing faces. This is the retina of a young person as evidenced by the reflection from the surface.

This is a view of the central retina looking in through a dilated pupil.

Diabetic Retinopathy

Diabetes is a disease in which abnormally high blood sugar content damages fine capillary vessels throughout the body. The retina is a privileged location where we can directly observe the health of the capillary vessels, both to monitor the status of the retina, and to get an idea of the circulation to other organs, like the heart, kidneys and brain.

Diabetic Retinopathy. Moderate severity.

The white arrow is pointing at a ring of “exudates.” The yellow spots are the lipid part of the leaking serum and mark the edge of an area of edema.
The black arrow is pointing at a medium size hemorrhage.

Principle. Keys to maintaining good vision:

  • Maintain the best, most consistent, control possible over blood sugar and blood pressure.
  • Have regular retina checks. If there are complications they are much more successfully treated if caught early.

Macular Degeneration: Dry vs. Wet

The biggest unsolved problem in ophthalmology is macular degeneration. It describes deterioration, related to aging, of the center part of the retina. Thus it reduces the part of vision that allows you to read and recognize people.
Macular degeneration comes in two categories, dry and wet.
Progress of the “dry” form usually takes a very slow and protracted time course.

Macular Degeneration. Moderate. Dry.
The yellow spots, called “drusen,” are the accumulation of waste material in the pigment layer under the retina.

Macular Degeneration. “Wet” Form. The hemorrhage seen here is under the retina.

As the dry form advances the chance of developing leakage from underlying blood vessels increases. That leakage represents a dramatic change in the course of events leading to a large and rapid loss of central vision unless there is prompt intervention.
If the leakage can be caught in the very early stages there is the potential to treat it with laser or new injected medications. The new injected medications (Lucentis or Avastin) aim to make the leaking vessels regress without damaging any retina. It has been successful beyond initial expectations and is fast becoming the mainstay of treatment. Since it is new there are still protocols to be worked out, like how often it must be administered, and for how long.


The landmark Age-Related Eye Disease Study showed us that the progress of macular degeneration may be slowed somewhat by taking a supplement containing antioxidant vitamins and the mineral zinc. A graph showing the ten year results is shown in the section describing the AREDS results.

Vision Monitoring

Because early detection of change from dry to wet AMD is the key determining factor in visual outcome it is important for people with macular degeneration to monitor their vision on a regular basis. This can be done with the Amsler grid or simply with a page of print. Cover each eye and look for an increase in distortion or missing areas.

If there is a progressive change you should contact your ophthalmologist promptly.

There are more detailed descriptions of each of the above subjects found by following the links.

The above descriptions are intended to help you better understand several of the most common eye conditions you might have or be curious about. They do not take the place of consultation with your ophthalmologist.