Gradual painless reduction / loss of vision

Many cases where both eyes are affected will be due to the need to change glasses (refractive errors) or cataracts.

More patchy loss of vision in one or both eyes, especially in the elderly may be due to glaucoma, age-related macula degeneration or, especially in younger groups, the effects of diabetes. 

Loss in one eye only may be due to posterior capsule opacification following cataract surgery, retinal detachment, especially if preceded by flashes and/or floaters, optic neuropathy, or in children amblyopia.

Rarer causes include:

Corneal diseases such as keratoconus, band keratopathy or corneal dystrophies.

Retinal diseases

Problems with the brain

See also:
Loss of central vision, difficulty seeing detail
Loss of peripheral vision