Essilor SV -

Most people only require a Standard lens (1.5 index). If your SPH is ±3.00 or higher you'd benefit from thinning. Or if your Cyl ±2.00 or higher. 

If you select a slimmer lens and you don’t need it, we’ll downgrade and refund the difference. If we think you’d benefit from slimmer lenses, we’ll confirm before upgrading.