A prescription is usually valid for two years, but your optometrist may recommend that you have your eyes tested more frequently than this, depending on your particular circumstances.
Remember, an eye test does not only check that your prescription is right for you; it is also a check on the health of your eyes and visual system. Many eye diseases can go unnoticed until it is too late to treat them effectively so it is important to have regular eye tests even if you feel you can still see well.
What does it all mean?
SPH stands for sphere and details the type and strength of lens need to correct your vision. The bigger the number, the stronger the lens required. Stronger lenses tend to be thicker than weaker ones. A + in front means you are long sighted (hyperopic) and a – means you are short sighted (myopic).
CYL is for cylinder and represents the amount of astigmatism that is present. Astigmatism is when the eye is not completely spherical, like a football, and is closer in shape to a rugby ball. Your vision will be distorted for both distance and near objects with astigmatism. The cylinder amount may be plus or minus regardless of whether the sphere is positive or negative.
AXIS defines the orientation of the cylinder (from 0-180 degrees) and is the angle at which the lens is to be set into the frame. A cylinder always has an axis.
PRISM is the correction needed (if any) to align the eyes, so that they are looking straight and working well together. A prism is a lens that bends the path of light without altering its focus.
ADD represents the amount of correction that must be added to your distance prescription for reading glasses, varifocals or bi-focals. Most people won’t have an ADD until they are between the age of 40 – 45 years old. If you are over 45, you may have a number here. This is your reading addition and relates to the amount of additional correction needed to focus at close distances. A measurement will mean you have different prescriptions for distance and reading.