Most doctors test eyesight first by utilizing an eye chart called the Snellen, which has letters in rows of decreasing size. Thereby, the lowest line that you can read clearly is where your eyesight is determined to be with 20/20 being normal, not the best that someone can have. Just keep in mind that this simple test does not mean your eyes are perfect or healthy, so the doc will perform other tests as well.
When considering what classifies good eyesight, other factors play a role in exactly how good or healthy the eyes are. For example, peripheral vision is the ability to see to the side without actually turning the eyes away from looking straight ahead. Those with tunnel vision may score great on the Snellen charts, but not have peripheral sight, which will give the doctor cause to check for other issues.
Color blindness, depth perception, focusing and coordination are determined by other tests that an eye doctor performs. Each of these can determine the extent of good eyesight and any underlying health issued that need to be addressed. If you can't decipher colors appropriately then a 20/20 eyesight measurement is not that good.
Being Able to See Near and Far
Many people can see really well at a distance of 20 or so feet, but can't seem to read the label on a prescription bottle. These people are classified as being farsighted, meaning they can see things at a distance clearly, but objects or words that are close to the eye are blurry. Just the opposite is true of those who are nearsighted, they can see just fine when things are close, but not very well at a distance.
Optometrists are the best at determining your level of need for adjustment and other eye tests will need to be performed. The 20/20 Snellen test does not give the whole picture when it comes to determining good eyesight. If you are having trouble with seeing the print of a newspaper without having to hold it at a distance, then it would be wise to get your eyes checked.
Additional Tests for Good Eyesight
Another test that is performed is called confrontation visual fields test, which determines the level of peripheral vision. It's a simple test that the doctor performs by asking you to stare at a point directly ahead as he asks you to let him know when his hand enters your field of vision. It is also a good test to check for blind spots or other eye diseases.
Extra-ocular movements test the eye muscles and coordination; you may be familiar with the 'follow-the-pen' test. Weaknesses can be determined by simply following a pen with only your eyes. The papillary reaction test can decipher how well your eye responds to light and can reveal certain neurological issues. A test called the cover test will give eye doctors insight into how well your eyes function together.
It's always a good idea to have an eye examination performed by an optometrist so that you can quickly be diagnosed and corrective measures taken. Also, having eye tests performed is the best way to reveal any underlying health issues. The sooner any concerns are addressed, the sooner they can be repaired or corrected. Good eyesight is measure by more than just your eye chart number.