There are two reasons to perform this part of the exam prior to refractive surgery. Most people understand that a dilating drop makes the pupil big. The reason this is important is that it allows us to look inside your eye with our lights to make sure the retina and internal eye are healthy prior to proceeding. The second reason is a bit more complicated but let us make sense of this for you since it is so important. To understand why it is important you must first understand the process of accommodation.
Accommodation is the process our eyes use to read.
When we read, the lens inside your eye that sits right behind your pupil gets more fat or curved. This curvature of the lens makes a person more nearsighted so they can read. During an eye exam when a person is looking at the eye chart, they can involuntarily try to focus on the eye chart and stimulate the accommodation response. This is the second reason for the dilating drops. The drop called Cyclogyl not only dilates the pupil but it relaxes the muscle that changes the shape of the lens.