Full-field ERG tests the entire retina’s electrical response to light. You get electrodes on the cornea or skin, dark-adapt for ~20 mins, then flashes of light stimulate rods and cones. It records a-waves (photoreceptor activity) and b-waves (inner retinal response). Used to diagnose widespread retinal diseases like retinitis pigmentosa or Leber congenital amaurosis. Results show overall retinal function, not localized areas.