It is easy, you needn't worry that. Even if you have poor vision, you can wear prescription eyeglasses or contact lenses during the test. But your license will state that restriction on the back. Since DMVs may slightly different from state to state, it is very important to verify vision requirements with your local DMV office. And the DMV will administer the test either by having you read a chart on a wall in the distance or with machines called Optec 1000's. You just follow their steps. Good luck.
Even if you don't have perfect vision, there is no need to worry about it. Because if you have vision problems and need to wear glasses or contact lenses, you are allowed to wear them during the eye exam. But your license will state that restriction on the back.
If you have a lower prescription, you may want to remove your glasses before the test to avoid getting a restricted license. However, make sure you are able to see cleary enough to operate a motor vehicle safely. The best way is to check with the vision requirements with your local DMV office.