Spectacle Skills



This page is part of the Ophthalmic Assistant Basic Training Course.
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Pupilary Distance (PD)

The distance between the pupillary centers, or the interpupillary distance (PD), is an important measurement to know when choosing glasses frames, aligning optical centers, evaluating induced prism, and adjusting binocular optical instruments.


The pupillometer has been proven to be the most accurate method for measuring the PD.  Use one if it is handy.  If not, the light reflex method is simple, sufficiently accurate, and inexpensive (mm ruler + penlight).

Procedure for the light reflex method

1) Face the patient, arms length away.

2) Hold your mm ruler across the bridge of the patient’s nose with the "0" mark approximately lined up with the patient’s right pupil.

3) Close your right eye and fix the patient’s right eye with your left eye. Instruct the patient to look at your left eye.

4) Hold the penlight directly under your left eye and shine it at the patient’s right eye.

5) Align the "0" mark of the mm ruler directly under the light reflex on the cornea of the patient’s right eye. The reflex should appear to be in the patient’s pupil.



6) Without moving the ruler, shift the penlight to a position directly under your right eye, and shine it at the patient’s left eye. Instruct the patient to look into your right eye.

7) Open your right eye and close your left eye. Observe the position of the light reflex in the patient’s left pupil. Read the mm mark directly below the light reflex. This is the PD reading.



The procedure will become smooth and quick with a little practice.

If the patient’s face is asymmetrical it may be necessary to measure monocular PDs. The light reflex method is still used, but the measurement for each eye is taken from the center of the nasal bridge to the light reflex.


Lens Decentration

When a patient is fit with glasses, if the patient's pupil does not line up very near the center of the lens, then the glasses lens must be decentered to line up the optical center of the lens with the patient's pupil.  This results in an unwanted increase in the lens thickness. 

There are two formulas used to figure lens decentration:

Frame PD = A-box measurement + Distance Between Lenses (DBL)



Lens Decentration =  Frame PD - Patient PD 

The take home point is that lens decentration should be as small as possible. When trying to choose between two frames, choose the frame that has a frame PD closest to the patient's PD.



Spherical Equivalent

The spherical equivalent is an important optical concept for the technician to understand. It is used in the refractometric process, in figuring corrections for visual field exams, and in contact lens power calculations.

Mathematically, the spherical equivalent is computed by algebraically adding half the cylinder power to the sphere power of a spherocylinder. It represents the average of the two powers that make up the spherocylinder.

Minus cylinder example:


The spherical equivalent is—

+1.00+(-1.50) = -0.50

or +1.00-1.50 = -0.50

Plus cylinder example:


The spherical equivalent is—

+1.00+1.50 = +2.50

Optically,  the spherical equivalent represents the Circle of Least Confusion, or the Circle of Most Confusion as I like to call it, because it is not an easy concept to grasp. The Circle of Least Confusion is part of the Conoid of Sturm

You can read about the Conoid of Sturm, if you have trouble going to sleep, in any book on optics. For our purposes, a simplified explanation will do.


An image that falls on the retina can be thought of as being made up of many dots, just like a photo in the newspaper.

If an ametropic eye (needs glasses to see well) is not optically corrected, then the image will consist of many blur circles instead of sharp dots. The more out of focus the image is, the larger the circles are. If an astigmatic eye is not optically corrected, the blur circles will be distorted into ellipses.

For example, if an emmetropic person is looking at a cross, it may be represented as an image with sharp dots, like this—



A minus 2D myope, without correction, may see the image like this—



If a person with an Rx of -2.00-2.00x90, without his glasses, may see the same image like this—



Sometimes we do not supply the patient with his full astigmatic correction: such as patients with 1.00D or less cylinder correction when performing a visual field exam, or perhaps the soft contact lens patient with 1.00D or less astigmatism in one eye. In these situations we use the spherical equivalent.

These patients will not see the sharply focused dot image. The spherical equivalent, representing the "Circle" of Least Confusion, provides a blur circle instead of a blur ellipse. The basic idea is that if the image is going to be a little blurry, it is better that it not be distorted also.




Transposing a glasses prescription is simply converting the prescription from minus cylinder notation to plus cylinder notation. The optical properties of the prescription remain the same.


1) Algebraically add the cylinder power to the sphere power to arrive at the new sphere power.
2) Change the sign of the cylinder power.
3) Add or subtract 90 from the axis.

Example 1— Transpose the following prescription 

+2.00 – 2.50 x 105

1)  +2.00 – 2.50 = - .50 (new sphere)

2)  -2.50 changes to +2.50 (new cyl.)

3)  105 – 90 = 15 (new axis)

-.50 +2.50 x 15

Example 2 —

 +1.00 + 3.00 x 35

1)  +1.00 + 3.00 = +4.00

2)  +3.00 changes to – 3.00

3)  35 + 90 = 125

+4.00 – 3.00 x 125

Why are there plus and minus cylinders? Plus cylinder lenses exist only in phoropters and trial lenses. Glasses lenses are made in minus cylinder. Plus cylinder phoropters are popular because it is easier to teach retinoscopy in plus cylinder. Ophthalmologists typically learn retinoscopy and refraction in plus cylinder. Optometrists typically use minus cylinder.



Vertex Distance

The distance from the back surface of the glasses lens to the front surface of the eye (the vertex distance) can affect the effective power of the lens, especially in higher powered prescriptions.



The vertex distance changes the effect of plus lenses and minus lenses in opposite directions, as shown below:

1) Increasing the vertex distance of plus lens will increase the effective power of the lens.

2) Decreasing the vertex distance of a plus lens will decrease the effective power of the lens.

3) Increasing the vertex distance of a minus lens will decrease the effective power of the lens.

4)Decreasing the vertex distance of a minus lens will increase the effective power of the lens.

In order for the glasses prescription to have exactly the same effective power as the refraction, the vertex distance of the phoroptor or trial frame must match the vertex distance of the lenses in the frames that the patient will wear.  A difference in the two vertex distances only becomes significant if the diopter power of the prescription exceeds 6 diopters.

  The Distometer
  The distometer is a device used to measure the vertex distance.  It is usually used to measure the distance between the closed the eyelid and the back surface of the spectacle lens.  It can also be used with a trial frame and with the phoropter.
  Part B touches the closed eyelid.  Part A moves when the plunger (D) is pushed.  Part A is moved using Part D until Part A is touching the back of the lens.  At this point the vertex distance is read from the scale at C.  The distometer scale accounts for the thickness of the eyelid.  Your office or clinic should have one of these.  The operation will become obvious once you have used one. 

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