Low vision has a variety of causes, including eye injury, diseases and heredity. Sometimes low vision involves a lack of acuity, meaning that objects do not come into focus. Other times, it involves the ability to distinguish colours, see contrasts or determine spatial relationships among objects. Vision can be hazy with cataracts, blurred or partially obscured in the central visual zone with macular degeneration, and distorted or blurred with diabetic retinopathy. People with glaucoma or retinal detachment can lose peripheral vision and have difficulty seeing at night.