You pick up your contact lens prescription and see rows of abbreviations. OD. OS. BC. DIA. SPH. CYL. AXIS. The numbers after each one determine whether your lenses will fit and whether you will see clearly.
Here is what every field on your prescription means, in the order they appear on a standard form.
OD and OS: Which Eye Is Which
OD is your right eye. OS is your left eye. These are Latin abbreviations that every optometrist uses and no patient remembers on their first visit. Your prescription will have one set of numbers for each, and they may differ slightly — one eye can be more nearsighted than the other.
SPH (Sphere): Your Vision Power
The sphere number is your prescription strength. A minus sign means nearsighted — you see clearly up close but not far away. Yilala lenses come in powers from 0.00 (plano, no correction) to -4.50. If your SPH is -4.75 or higher, our prescription range does not cover you. You would need lenses from a different source. A plus sign means farsighted, but colored contacts are rarely manufactured in plus powers.
BC (Base Curve): How the Lens Curves
Base curve is the curvature of the lens measured in millimeters. Yilala lenses use an 8.6mm BC. If your prescription specifies 8.6mm, the match is perfect. A difference of 0.1mm or 0.2mm is usually fine. A difference of 0.4mm or more will cause noticeable fit issues — the lens will either slide around or feel tight. Read the full parameters guide.
DIA (Diameter): How Wide the Lens Is
Diameter is the width of the lens measured in millimeters. Yilala lenses are 14.5mm DIA. Most prescription lenses are 14.0-14.2mm. The 0.3-0.5mm extra on Yilala lenses creates subtle eye enlargement. If your prescription specifies a DIA significantly different from 14.5mm, the fit may feel different from your current clear lenses.
CYL and AXIS: Astigmatism Correction (If Present)
If your prescription includes CYL and AXIS values, you have astigmatism — your cornea is slightly oval rather than spherical. Standard colored contacts, including Yilala, do not correct astigmatism. These lenses only correct SPH values. If you have astigmatism, talk to your eye doctor about whether toric colored contacts are available and appropriate for you. Read about colored contacts for astigmatism.
Expiration: Your Prescription Has a Time Limit
In the US, a contact lens prescription is valid for one year from the date of the exam. After that, you need a new exam. Your eyes change over time, and an old prescription can mean the wrong power or a missed health issue. Do not try to order lenses with an expired prescription.