Did you know your glasses prescription doesn't work for contacts?

Most people assume they can just hand their glasses prescription to a contact lens retailer and order online. Turns out there's a physics problem with that.

The distance between a corrective lens and your eye changes the effective power of the prescription. When glasses sit ~12mm from your eye and contacts sit directly on it, the math shifts — sometimes significantly for higher prescriptions above ±4.00D.

Opticians adjust for this automatically but rarely explain it. Patients walk away with a contact lens prescription that looks different from their glasses Rx and have no idea why. I ran into this enough times that I built an eyeglass to contact lens prescription calculator to handle the conversion properly.

The other thing nobody talks about upfront is the money. Depending on your prescription type and brand, the annual difference can be substantial — worth checking how much do contacts cost before committing to a lens type.

When you switched from glasses to contacts, what tripped you up most?