Merlyn

• Shows how much astigmatism correction you need; it may be a plus or minus number.
• If SPH or DS appears instead of a CYL value, there is no astigmatism — enter 0 or SPH/DS.

• The orientation of your astigmatism correction, measured from 1° to 180°.
• Only required when a CYL value is listed on your prescription.
• Tip: Drop leading zeros — enter 005 as 5.

• Extra power added for close-up tasks such as reading.
• Needed for reading glasses and progressive (multifocal) lenses.
• Standard single-vision distance lenses do not require an ADD value.

PD (Pupillary Distance) is the gap between your pupils, measured in millimeters. If it is not printed on your Rx, ask your eye doctor or measure it yourself with our PD ruler.

Two PD Numbers

For most adults, combined PD falls between 54–74 mm.

Distance & Near PD

When two numbers appear in this range, the higher value is for distance vision (single vision or progressive lenses) and the lower is for near vision (reading glasses).

Individual Eye PD

Also called mono PD, each eye is usually 25–37 mm.

If your Rx lists separate left and right values, choose the "2 PD numbers" option.

Prism power (in diopters) helps correct eye alignment issues such as double vision or a turned eye. Most prescriptions do not include prism. When present, each eye has two entries: prism strength and base direction.

Vertical Prism

Used for up-and-down misalignment — typically base up in one eye and base down in the other.

Horizontal Prism

Used for side-to-side misalignment — both eyes are set in the same lateral direction.

No prism on your Rx? Leave these fields empty.