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Understanding Your Sight Test Prescription

Your optician has a legal obligation to provide you with a written sight test prescription following a sight test. We recommend you ask for your written sight test before leaving the premises, as some opticians will not offer to give you your prescription as a matter of course.

 As you can see from the diagram below a sight test prescription consists of a number of measurements for your right and left eyes, although the layout can look different depending on your optician.

 sight_test_prescription

The sphere (or SPH) measurement is the measurement of nearsightedness or farsightedness (i.e. the measure of the correction or focusing power of the lens. It is measured in dioptres). A positive sphere measurement indicates farsightedness (able to see in distance but not near), and a negative sphere measurement indicates nearsightedness (able to see near but not in distance). The higher the sphere number the greater amount of correction required to see correctly, and the thicker the lenses will be. Note that the plus and negative signs are often written above the numbers. A person with no sphere correction will have either a blank, 0, DS or the ∞ symbol.

The cylinder (or CYL) measurement is a measurement of the degree of astigmatism. A person’s eye is naturally shaped like a sphere i.e. like the shape of a football. However the eye of a person with astigmatism is shaped like a rugby ball. The higher the number, measured in dioptres, the higher the degree of astigmatism. Note that the cylinder measurement can be written either with a plus sign or a minus sign. A person with no astigmatism will have either a blank, 0, DS or the ∞ symbol as a cylinder measurement.

The axis measurement is a measurement of the orientation of the astigmatism, and is measured in degrees from 1 to 180. The axis measurement is sometimes written in half degree increments.

The prism measurement is the amount of prismatic power, measured in dioptres, required to correct alignment problems. Only a small percentage of prescriptions include prism powers. The direction of the prism is indicated by the base – either base up (BU), base down (BD), base in (BIN) or base out (BOUT). Base up and down are sometimes denoted by arrows pointing up or down.

As we get older changes occur to the lenses within our eyes, making them harder and less elastic. This is known as presbyopia. People with presbyopia usually require bifocal or varifocal (progressive lenses), and their sight test prescription includes correction for both distance and near vision. The sight test prescription is either written with separate distance and near prescriptions or with and ADD measurement. The near ADD measurement is the additional sphere correction (in dioptres) which when applied to a distance prescription gives the person’s near prescription. A near prescription is the amount of correction required for objects around 30cm away (e.g. reading a book or newspaper). An ADD can also be applied to the sphere correction of a distance prescription to give the person’s intermediate (INT) prescription which is the amount of correction required for objects around 1 metre away (e.g. a computer screen).

Measurement of Pupillary Distance

A measurement often missing from a sight test prescription, but still required to make a pair of glasses, is the Pupillary Distance (PD). Your PD is the distance in millimetres, between the centres of your pupils and is required to align the lenses correctly in the glasses frame. A ruler is usually used to measure a PD and for accuracy purposes is taken from the inside edge of the pupil of one eye to the outside edge of the pupil of the other eye, as in the diagram below. The PD in the diagram is 56mm.

pdmeasurement

Here is an alternative way:

  1. Wear your glasses. (or any glasses if you don't have your own)
  2. Have a felt-tip marker handy.
  3. Focus on a single object in the far distance (anything farther than 10 metres works, but the farther is better).
  4. Raise the marker to your right lens and precisely put a dot on it directly over the distant object.
  5. Repeat for your left eye. If done correctly, with both eyes open the two dots should overlap into a single dot over the distant object. If not repeat making the markings until they do form a single dot.
  6.  Measure the distance between the two dots on your lenses with a millimeter ruler.
  7. That's your distance PD.
  8. If you need a near PD for reading or computer glasses, just do the same procedure but focus instead on the object you will be looking at, either reading material or computer monitor.
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