The Basics



Visual acuity is how well we see. We think of normal visual acuity as 20/20, or 10/10. This type of acuity notation is actually a fraction, called a Snellen Fraction. It represents the distance at which the test is given over the size of the target.

On VOSH missions we typically use a 10-foot distance because it is very difficult to hear patients or communicate if you get much further apart than that in our crowded, noisy clinics.


20/20 means you can see the 20 size target at 20-feet, which is considered excellent vision. 10/10 is equivalent to 20/20.


20/40 is equivalent to10/20 is considered adequate vision. 20/40 vision allows one to pass the vision tests for the driver’s exam and perform most activities of daily living. 20/40 (10/20) is typically the goal for vision correction on eye care missions in which recycled eyeglasses are used.


Visual acuities should be taken one eye at a time. Always check the acuity in the right eye first (cover the left eye). Then switch the cover and check the acuity in the left eye. AFTER both eyes have been tested separately, then have the patient open both eyes and record the visual acuity for both eyes together.


The line you record should be the smallest line on which the patient can read over half the letters. On a line that has 3 letters, if the patient reads 2 letters, she gets credit for that line. If a line has 5 letters, the patient gets credit if he reads 3 letters.


OD means right eye

OS means left eye

OU means both eyes


Distance visual acuities should be recorded like this:


OD 10/100

OS 10/ 20

OU 10/10


This means that the smallest size letter the patient could read with the right eye was the 100 size letter at 10 feet. The smallest size the patient saw with the left eye was the 20 size letter at 10 feet. With both eyes open the patient could read the 10 size letters at 10 feet.



Retinoscopy and autorefraction give us a fairly good idea of what the prescription is going to be. The doctor will look at the acuities and the retinoscopy or autorefraction, and perhaps do a refraction in order to write the prescription. Retinoscopy and autorefraction look at the reflections coming from the back of the eye. The shape and power of those reflections give an indication of the length of the eye. The length of the eye is a rough indicator of what lens will be needed to bring light to proper focus on the retina (the neural layer) of the eye.




Emmetropia: The condition of the normal eye, no need for eyeglasses to correct vision. An emmetropic eye has no refractive error.


Hyperopia: Farsightedness – person sees better at distance than up close


Myopia: Nearsightedness – person sees better up close than at diatance


Presbyopia: inability or poor ability to see up close, associated with age 30+


Lenses help to focus images in the eye and correct refractive error. Lenses come in two basic types, plus and minus:


Magnifies: If you hold a plus lens a few inches from your eye, you will see that plus lenses make images appear larger, they magnify.

Hyperopia: Plus lenses correct hyperopia (farsightedness)

Presbyopia: Plus lenses also correct presbyobia, the inability to read up close when we get older.

People with hyperopia may not complain about their distance vision, but will complain that they can’t see up close or that they have a lot of eyestrain or headaches.


Minifies: Minus lenses will make images appear smaller (minify) when held a few inches from your eye.

Myopia: Minus lenses correct myopia (nearsightedness). People with myopia often see well up close, but will complain that things are blurry at any distance.

SPHERES: Corrections that are simply plus or minus are called spheres and will have only one number in the prescription, such as:

OD +1.00

OS +1.50



Cylinder (cyl) is a combination of lenses that is used to correct astigmatism. Astigmatism can be thought of as a slight problem with the shape of the eye. The eye is typically round like a basketball, but if a person has astigmatism, the eye is shaped more like a football. Because the eye is not round, it takes more than one correction to give the best focus of light to the eye. Cylinder can be used with either plus or minus lenses to correct for astigmatism. Cylinder corrections will have three numbers in the prescription and will look something like:

OD +1.00 1.00 x 180

OS +1.50-0.75 x 165

The first number is the sphere, the second number is the cyl, and the third number is the axis


Cylinder corrections may be written in two different forms, plus or minus cyl form. Plus cyl form is typically used by ophthalmologists, while minus cyl form is typically used by optometrists. The way they are taught and how they get used to prescribing eyeglasses typically governs how they write the prescriptions. Lenses are all actually cut in minus cylinder form.

+1.00 -1.00 x 180 The second number is (-) which means it is in minus cyl form

+1.00 +1.00 x 090 The second number is (+) which would indicate plus cyl form

Most VOSH glasses are marked in Minus cyl form.


Recycled glasses are cataloged by the lens in the right eye. So if you are looking for a prescription that says:

OD +2.00           remember, OD means right

OS +1.00           OS means left

Then you should go to the box that is marked +2.00. Find a pair that most closely matches the +2.00 in the right lens and +1.00 in the left lens.


We would always like to give the patient a perfect pair of glasses, with a beautiful frame, clear lenses and their exact prescription in those lenses. But when we use recycled eyeglasses that is often not possible. Our goal is to find a pair that is close to the prescription, that is clean with unscratched lenses, and that we can offer the patient to help him/her them see better. The goal for distance vision correction is 20/40, which is vision good enough to pass a drivers test and meet most visual demands. The goal for near vision is the ability to read newspaper size print.


Even though you will not always be able to find the perfect prescription, there are some other options. In particular, for astigmatism, you can often find a lens that is “equivalent” to the cylindrical correction. A spherical equivalent is the average of the two prescriptive powers in the correction of astigmatism.


The doctor will often write a prescription in cyl form, then also write a spherical equivalent, so that you have a choice of glasses to give the patient. It will help you a great deal if you are also able to calculate a spherical equivalent. It can usually be done in your head easily. But you may want to take a calculator with you, for when you get busy and maybe a little tired.

To calculate the spherical equivalent, you add half of the cylinder power to the sphere power.

The first number is the sphere, the second number is the cyl, and the third number is the axis

Using Minus Cyl

To calculate the spherical equivalent for:’

+1.00 – 3.00 x 90

  1. Ignore the third number, the axis
  2. Divide the second number (the cyl) by 2.
    1. -3.00/2 = -1.50
  3. Add the cyl you just calculated to the sphere (the first number)
    1. +1.00 +(-1.50) = -0.50

The spherical equivalent of +1.00-3.00×090 is


Using Plus Cyl

To calculate the spherical equivalent for:

+1.00 + 3.00 x 90

  1. Ignore the third number, the axis
  2. Divide the second number (the cyl) by 2.
    1. +3.00/2 =+1.50
  3. Add the cyl you just calculated to the sphere (the first number)
    1. +1.00 +(+1.50) = +2.50

The spherical equivalent of +1.00 + 3.00 x 90 is


You start with three numbers and end up with one number when you calculate a spherical equivalent.


You may need to transpose a prescription written in plus cyl form to minus cyl form so that you can match up the prescription with the recycled glasses. Remember, the prescription in the recycled glasses is most often recorded in munus cyl form.

Transposing means changing from plus cyl to minus cyl form or vice versa. Transposing is a little more complicated than calculating spherical equivalents.

When you transpose you will still have three numbers in the prescription:


  1. Add the cyl (the second number) to the sphere (the first number) to get the new sphere (the new first number)
  2. Change the sign of the cyl, the second number (don’t change the power of the second number, just the sign)
  3. Add or subtract 90 from the axis (the third number). You add or subtract to keep the number between 1 and 180. A simple explanation is that you only need ½ the circle to describe the whole circle. Unless you are an engineer and want a better explanation, simply accept it and stay between 1 and 180.




To transpose a prescription written in plus cyl form


+1.00 + 3.00 x 090


1)  +1.00 + 3.00 = +4.00

2)  +1.00 changes to – 1.00

3)  90 + 90 = 180


=   +4.00 – 3.00 x 180





+2.00 – 2.50 x 180


1)  +2.00 – 2.50 = – .50

2)  -2.50 changes to +2.50

3)  180 – 90 = 90


=   -0.50 +2.50 x 90







  1. Determine whether the patient is male or female. A female patient will likely reluctantly (or not at all) accept a male frame, and vice versa.


  1. Determine the size of the person’s face. A pair of huge frames for a small face is impractical. A small pair cannot be comfortably worn on a large face.


  1. Know whether the glasses are for distance, near, or both (bifocal)


  1. Take a look at the lenses before you present the glasses to the patient. Scratched lenses will not make vision better. Clean the lenses before you present them to the patient, if necessary.


  1. Present the glasses with the closest prescription to the patient. Smile and look proud of yourself for doing such a good job in finding them the right pair of glasses.


  1. If the glasses are for distance, ask the patient if he can see faces across the room. If he nods and smiles, chances are these glasses will work well.


  1. If the glasses are for reading, hand the patient something to read and see if she can read it. Again, if she nods and smiles, you did a good job for her.


  1. If you are dispensing bifocals that are for distance and near, you must explain to the patient that he must look through the top of the lens for distance and the bottom of the lens for reading.


  1. Adjust the glasses so that they fit well on the nose and wrap appropriately behind the ears. Your clinic leader will teach you how to do this.


  1. If the patient is having difficulty with the glasses,


  1. First check to make sure the lenses are not scratched.
  2. Next check again to make sure you picked up + if the prescription said +, and – if the prescription said -. When we get in a hurry or tired, errors will happen.


  1. If the + or – power is correct, check the cylinder power.


  1. If the cyl power (the second number) in the glasses is higher than that in the prescription, the patient will have difficulty with the glasses. Find another pair of glasses with lower cylinder or calculate a spherical equivalent and find the equivalent pair of glasses


  1. If the cyl power is correct, check the axis.
    1. If the cyl in the prescription or in the glasses is more than minus 1.00, you have to choose a pair of glasses with the correct or slightly less cyl power in the lenses,
    2. The axis of the cyl in the glasses must match or be within 5 to 10 degrees of the specified axis.
      1. If the axis of the prescription is 90, the axis in the lens must be between 80 and 110.
      2. If the axis is 180, the axis in the lens must be between 170 and 10.
    3. This is difficult, so if you have problems, check with the doctor or the clinic leader.



  1. If you are sure you matched the prescription with the glasses properly, try to determine if the patient simply does not like the frame you brought to them. If you retrieve another pair with the same or very similar prescription and the patient accepts it, you may assume that was the problem.


  1. If the patient does not accept the second pair of glasses, ask the doctor to look at the prescription again. Take the prescription and the pair of glasses to the doctor.


  1. When you have successfully dispensed the glasses, smile and shake hands with the patient as you dismiss them. They will likely thank you, and it is important that you let them know that it is your pleasure to help them. You are the last person they will see at they leave the clinic after typically standing in lines many hours for your services. They are as tired as you are, perhaps more so, by the time they get to see you. Reward them with your smile. They will remember your smile and your face just as you will remember theirs!

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