The goal of your eyeglass prescription is to focus light entering the eye into a clear image. Traditionally, eyeglasses are made thinking about one eye at a time. If your eyeglasses can make your right eye vision clear, and your left eye vision clear, then your vision should be clear and comfortable. However, this is not always the case! Your two eyes need to work as a team.
Eyeglass lenses have side effects when manipulating light into a clear image, such as magnifying or minifying the incoming light.
If your eyeglass prescription is different between your two eyes, your eyeglass lenses may manipulate image size differently. At a certain point of difference between your two eyes, it becomes difficult and/or uncomfortable for the brain merge these images when interpreting information from your two eyes.
The greater the difference in prescription between your two eyes, the greater the difference in image size between your eyes. This image size difference, known as aniseikonia, can cause a number of symptoms.