How to Measure ADD Power With a Lensometer

A lensometer measures the power of prescription eyeglass lenses. Also known as a lensmeter or focimeter, opticians use this device to check lenses against the optometrist's or ophthalmologist's prescription before they dispense a new pair of glasses to a customer. Bifocal lenses, which enable you to use the same pair of glasses for reading and distance, include a separate segment in the bottom of the lens that magnifies at close distance. The power of that segment is known as "ADD," and you can measure it yourself with a lensometer.

Instructions

• 1

  Set the power drum of the lensometer to the zero, or plano, position. Look into the eyepiece of the viewfinder, which is known as the ocular. Turn the ocular to the highest plus number position and then slowly turn it toward the negative position until the focusing images that you see in the viewfinder look clear to you.

• 2

  Rotate the diopter dial on the side of the lensometer, which is also known as the power wheel, until all focusing lines and spirals, called mires, are clearly focused. Make sure the power dial is lined up at the "0" position once you reach the point where the mires are in focus. Refer to your lensometer manufacturer's directions or contact a technician to calibrate your lensometer if the power wheel does not read "0" when the mires are in focus.

• 3

  Place a pair of eyeglasses with bifocal lenses on the lens platform so that the temples of the frame face you, and the bottom of the lenses are as close to the path of the viewfinder as possible.

• 4

  Rotate the power drum while you look in the eyepiece of the lensometer until you see three fine lines in perfect focus.

• 5

  Note the position of the power wheel. That is the amount of "ADD," or the magnifying power, of the close-reading section of your bifocal lenses.

• 6

  Check the "ADD" against your doctor's prescription. Start wearing the new glasses as much as possible to get used to them if your results with the lensometer match your doctor's prescription. Return the glasses immediately to your optician if the "ADD" does not match your doctor's specifications.