Diagnosis and Staging. A visual field test is performed at the initial visit or as soon as glaucoma is suspected. It evaluates vision loss due to glaucoma, damage to the visual pathways of the brain, and other optic nerve diseases.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.