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

Refraction and Glasses Prescription

Refraction is the process of determining the glasses prescription to correct the near-sighted or far-sighted condition.

See Optics for an explanation of the terms near-sighted, far-sighted, astigmatism and presbyopia.

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 based on the distance vision. By changing lenses while you view letters on the vision chart we work to reach the sharpest vision we can. Think about this like focusing a camera, we are trying to get the image in sharp focus on the retina.
Sometimes people want to see better and ask if we can’t just make their glasses stronger. Once you have the image in focus that is the best you can do with a single glasses lens. When the vision cannot reach 20/20 despite the best glasses correction that means other problems are present in the eye, such as cataract or macular degeneration.

Readers, Bifocals and Progressives

The above graph shows the decrease in accommodation (focusing power) that occurs with age.

Step 3. Check reading correction.

Some time after reaching age 40 most people are forced to face up to the fact that they need help with reading in the form of reading glasses, or bifocals.
The exception is if you are nearsighted by an appropriate amount then this is the payback for all those years wearing glasses. You may be able to read without glasses. Enjoy your advantage. Show off. With your glasses off help your friends read a menu in dim light.

Reading correction is magnification added to whatever distance prescription you have.

A single vision reader is set to be in focus for the particular distance you prefer to read, with a small range around that. A photographer would call this range the “depth of field.” If you look up while wearing a reader everything beyond arm’s length is out of focus.
Generic readers, (Dean Edells, cheaters) are single vision readers that do not account for distance prescription. If your distance prescription is small then these can work well. If you choose these then at the display rack pick the lowest power that gets you in focus at the distance you want to read. You cannot damage your eyes with the wrong glasses. You just may not see as well or be as comfortable as you might with prescription glasses.

Bifocal vs Progressive

For many people the inconvenience of having to take glasses on and off is a reason to get a bifocal or a progressive lens.

  • The traditional bifocal (Lens #2 in the chart below) has distance prescription on the top half of the lens and reading in a half moon segment on the bottom. This gives vision at far and near, but not middle distance.
  • A modern progressive lens (Lens #5) is so named because the reading power increases gradually as the eye travels downward in the lens. Functionally, this is the most versatile choice because it allows you to be in focus at any distance by finding the right part of the lens to look through. Most people choose this lens type because of that versatility. There are a few people who find the progressive lenses difficult to adjust to and get regular bifocals or separate pairs of glasses.
  • For people at 50 years and beyond, middle distance, like a computer monitor and piano music, eventually becomes limited by continuation of the same process that created the need for reading glasses. If this is an issue it is important to bring that up during the vision part of the exam. To solve this requires either a trifocal (Lens #4), a progressive lens (Lens #5) or dedicated computer/piano glasses.
  • Another option for people who do not want to wear glasses is monovision contacts. That is, wear a contact lens in the dominant eye set for distance, and a contact in the other eye in focus at near. Yes, surprisingly, most people can adjust to using one eye for distance and the other for near. This works best if visual demands are not too great. A person who reads or works at near for prolonged periods may find it tiring.

Step 4. Compare.

The last step is to compare the correction in your old glasses with the new findings. We show you as best we can the difference between the old and new prescriptions to help you decide if a change is worthwhile. In the end we give you an updated prescription.
If you want new glasses take the updated prescription to an optician where you choose frames, discuss lens choices and get measured. While you are wearing the frame the optician will measure the position of your pupils relative to the frame so the optical centers of the lenses will be accurately aligned. Using an experienced optician gives a significant advantage in accuracy, which is particularly important with high power and progressive lenses which are very sensitive to alignment.

Choice of Lenses

Choice of lens type is simple before age 40 because it is almost always a single power. After that things  become more complicated because the lens choice must take into account what activities are important to you and the distance from and position of the objects you need to see. The main distance categories are shown above, with the colors matching the diagrammatic glasses shown next.

  1. Reading and close work are usually done around 14 inches. Some people prefer 12 or 16, so think about your preferred distance before your visit because you will be asked that question. Note that reading is usually done below eye level, which is why the bifocal is set in the lower part of the glasses lens.
  2. Think of intermediate as arm’s length, usually 18 to 24 inches, where a lot of life’s activities occur. It is the distance for desk work, the computer monitor, the dashboard, shopping, cooking, reading sheet music, painting, playing cards, and doing shop work.
  3. The computer. Consider that several of the activities on this list, particularly the computer, occur at eye level. Which means that if you need to look through the bifocal part of your glasses then you must tilt your head up. It is uncomfortable to maintain that posture if you are working on a computer for hours. So you can make glasses designed for that situation. For example, you can make a bifocal with intermediate on the top, where distance usually goes, and still have reading in the bifocal segment (Lens #3). This is a time honored solution for desk work, but if you forget to remove them and get up and walk around things will be blurred beyond a few feet. There is a nice workplace progressive lens which gives vision to 10 feet in the top (Lens #6).
  4. Room distance. That brings us to that awkward space between desk distance and wall distance, say up to 10 feet, which falls between reading and distance. For example, if you are in an art gallery and get close enough to see the brush strokes through your bifocal you will most likely also get a visit from the nice, but firm docent. This distance is covered by the progressive lens (Lens #5).

Sun Glasses

Sun glasses are mostly for comfort in the bright sun. There is also some protection from ultraviolet light. Lets start by getting a few things straight.
UV Protection. Exposure to ultraviolet light can cause skin cancer, pterygia and accelerates the development of cataracts. UV and blue wavelength light has a theoretic basis for potential retinal damage. However, multiple large studies (from three continents) have not shown that wearing sunglasses affects the development of macular degeneration. If you want UV protection wear glasses with UV coating and a hat. Any lens material filters out about half the UV light. An added UV coating can filter out over 90% of the UV.
Polarization. This kind of lens blocks out reflected glare. I find it the most effective for providing comfort, particularly while driving, on the water or skiing.
Color is a matter of preference. Gray and green are considered color neutral. Amber highlights contrast.
Darkness. The lenses can be dyed to different levels of darkness according to your preference.
Photochromic. These lenses darken with exposure to UV light. For example, the B&L brand is “Transitions.” They are not polarized. They do not fully darken up in a car because the glass windshield filters out 50% of the UV, which is required for activation. Current lenses darken and lighten more quickly than early lenses. Over time the level of darkening gradually decreases.
Corning has a new photochromic lens that is polarized and will darken in a car, but it is new, and still pricey.

Safety Glasses

Regular glasses give limited protection against injury. If you are in an occupation where there is risk of eye injury there are specialized safety glasses made to an industry standard. The lenses are made of an impact resistant material, polycarbonate or Trivex, and are set into the frames so they are much more difficult to dislodge than regular lenses.
Children’s glasses are usually done in impact resistant material for safety.

Additional Comments

If you have only one good eye it is worth wearing glasses to protect the good eye, even if there is no prescription.

Often patients tell me their eyes are getting worse; they are thinking this means the health of the eye is declining. On testing we find a change in glasses brings vision back to 20/20, or the level it was previously. In that case we explain the prescription has changed, but the corrected vision is still the same. That is relatively reassuring that eye health is stable. But good central vision does not rule out the possibility of disease like glaucoma.

A common misconception is that wearing glasses correction weakens the eyes and makes you need stronger glasses. As far as we know, in adults, wearing glasses does not affect the development of future glasses prescription. You may become accustomed to and like the sharper image, noticing the contrast with the blurrier uncorrected image.
The distance prescription is a result of a mismatch between the length of the eye and the focusing power of the cornea and lens. There is no known Lamarkian feedback mechanism to change those structures in an adult. The only way to change those structures is by surgery, like Lasik.
However, there are studies underway to see if such a feedback mechanism exists in the developing eye.
Change in reading prescription happens at the same rate whether you wear glasses or not. Adults we see in Guatemala, who have never had glasses, need the same reading strength at the same age as people here.

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 when a large amount of peripheral vision is already lost. To be intentionally redundant, it is worthwhile to get a comprehensive eye exam at regular intervals, the spacing depending on age and risk factors.