The visual field test is a subjective measure of central and peripheral vision, or âside vision,â and is used by your doctor to diagnose, determine the severity of, and monitor your glaucoma.maging of the optic nerve and surrounding tissues is an objective test that can also detect glaucoma damage and progression. This information can help your doctor make treatment decisions, and as the technology improves it will be even more useful. However, at this time it has not replaced the visual field test.