Bifocals are lenses that help you see in the distance and up close. They usually are measured to sit in an eyeglass frame so that the top line of the bifocal is adjacent to the edge of the lower eye lid. An improperly measured bifocal will cause patients to have headaches as they strain their eyes trying to read. A specific method is used to measure a bifocal correctly.
Place the glasses on the person to be measured. Make sure the frames are sitting properly and comfortably. Have the person look straight ahead. It is best to be seated across from him at the same eye-level.
Start with the right eye. The right is noted as O.D. (Oculus Dextrous) on optical prescriptions, according to The Ohio State University Medical Center. Optical measurements are always done with the right eye first, then the left, to avoid confusion. Place the PD ruler so the zero is at the bottom of the frame, directly under the person's pupil, and measure to the edge of their bottom eye lid. Measure up to the edge of the lower lid and note the number on the ruler.
Repeat with the left eye, written as O.S. (Oculus Sinister) on optical prescriptions. Measure from the bottom of the frame to the edge of the lower lid with the PD ruler. Most people are not symmetrical and might have a different measurement for each eye.
Placement of a bifocal can vary depending on the style of bifocal, the prescription and the size of the frame. "All About Vision" suggests having a detailed discussion with an expert before choosing and measuring bifocals. Check with a licensed optician or eye-care practitioner to verify the exact measurement of the bifocal if you are unsure. Most opticians will measure a bifocal for free or a nominal fee.