Optometry Prescription

What's the purpose of 2 year expiration on optometry prescription?
What's the danger of using optometry prescription over two years? If an optometrist violates this law, do they lose their license to practice? Is this an actual California government law or is it just an accepted practice among optometrists to be consistent?

Question about optometry prescriptions etc?
So I lost my glasses in June. I put off doing anything about it for over three months. I finally ordered new glasses online, using my old prescription which was already almost two years old when I lost them. When I put them on, the right eye seems waaay different than it was before. Which do you think is more likely...a All the eye strain from going w o glasses for over three months made my right eye prescription change drastically.b The eyeglass manufacturer screwed up.