Power
If you work in this field, you could very well be all-powerful. As long as you stay within the constraints of the prescription and the walls of your own store, of course. Also, you've got to keep in mind that the customer is always right. But, hey, you do have unlimited power to be helpful and courteous.
However, there really isn't wiggle room to stray from the specifications of the doctor's prescription and the customer's aesthetic desires. Opticians have the ability to express their creativity when helping the customer make choices, but the final say-so ultimately belongs to the customer.
Moreover, the choices that you make when creating the work order for the lab techs determine what the dimensions and strength the lenses will be. If you do your job correctly, the customer will receive a pair of glasses that helps them easily get through their daily lives. If not, at the very least they'll be dissatisfied, and at worst may have serious difficulties getting around or completing day-to-day tasks.