Optometrists examine the internal and external structures of the eyes to diagnose eye diseases like glaucoma, cataracts and retinal disorders. In addition, they can evaluate the eyes for systemic diseases like hypertension and diabetes and vision conditions like nearsightedness, farsightedness, astigmatism and presbyopia. Optometrists also do testing to determine the patient's ability to focus and coordinate the eyes and to judge depth and see colors accurately. They prescribe eyeglasses and contact lenses, low vision aids, vision therapy and medications to treat eye diseases.