Question 1

What does OD and OS mean on a prescription?

Accepted Answer

OD (Oculus Dexter) means right eye, OS (Oculus Sinister) means left eye. OU (Oculus Uterque) means both eyes. These Latin abbreviations are standard in optical prescriptions.

Question 2

What is the difference between sphere, cylinder, and axis?

Accepted Answer

Sphere (SPH) corrects nearsightedness (-) or farsightedness (+). Cylinder (CYL) corrects astigmatism. Axis (in degrees 1-180) indicates the angle/orientation of the astigmatism correction.

Question 3

How do I transpose a prescription from plus to minus cylinder?

Accepted Answer

Three steps: 1) Add the sphere and cylinder algebraically to get new sphere. 2) Change the sign of the cylinder. 3) Add or subtract 90° from the axis (keep between 1-180).

Question 4

What does Add power mean on a prescription?

Accepted Answer

Add (Addition) is the extra magnifying power added to the distance prescription for reading/near vision in progressive or bifocal lenses. It's always a positive number, typically ranging from +0.75 to +3.50.

Question 5

How do I neutralize a lens on the lensometer?

Accepted Answer

Place the lens against the lens stop with the back surface facing you. Focus the eyepiece first. Move the lens until one set of lines is clear - read sphere. Then focus until the perpendicular lines are clear - the difference is cylinder.

Optician FAQ

What does OD and OS mean on a prescription?

What is the difference between sphere, cylinder, and axis?

How do I transpose a prescription from plus to minus cylinder?

What does "Add" power mean on a prescription?

What is prism and base direction?

How long is an eyeglass prescription valid?

How do I neutralize a lens on the lensometer?

What is the difference between measuring in plus and minus cylinder?

How do I find the optical center of a lens?

How do I measure progressive lenses on a lensometer?

How do I read prism on the lensometer?

Why are my lensometer readings inconsistent?

What does a keratometer measure?

How do I use a manual keratometer?

What do K-readings tell me about astigmatism?

What is "with-the-rule" vs "against-the-rule" astigmatism?

How do K-readings help with contact lens fitting?

What is a slit lamp used for?

What are the main illumination techniques?

How do I evaluate a contact lens fit with slit lamp?

What corneal conditions can I see with slit lamp?

How do I use fluorescein with the slit lamp?

How do I measure pupillary distance (PD)?

What measurements do I need for progressive lenses?

How do I adjust a frame that's sitting crooked?

How do I select a base curve for best cosmetics?

What's the minimum fitting height for progressive lenses?

Patient says their new glasses make them dizzy - what do I check?

Patient complains about blurry vision in new progressives - what's wrong?

Why does the patient see better with their old, weaker prescription?

How do I handle a patient who insists on a specific frame that's wrong for their prescription?

What's the difference between CR-39, polycarbonate, and high-index lenses?

When should I recommend anti-reflective coating?

What causes lenses to craze or crack?

What is chromatic aberration and which lenses have it?

How do I convert a glasses prescription to contact lenses?

What base curve should I select for soft contact lenses?

How do I troubleshoot a tight contact lens fit?

What's the difference between ABO and NCLE certification?

Is licensing required to be an optician?

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