Why Flash Causes Red Eye (and How to Avoid It)

Red eye happens because flash light travels in a straight line to the eye, reflects off the blood-rich retina at the back, and bounces straight back into the lens — most likely when the flash sits close to and level with the lens.

Why it's worse in the dark

In low light, pupils dilate wider to let in more available light, which exposes more of the retina to catch and reflect the flash — this is why red eye is far more common at night or indoors in dim rooms than in bright daylight.

How to reduce it

• Turn on a room light before shooting, so pupils are already smaller.
• Have the subject look slightly away from the lens rather than straight at it.
• Use a flash mounted further from the lens axis, or bounce it, if your camera allows.
• Use your camera's red-eye reduction pre-flash setting, which briefly fires the flash to shrink pupils before the real exposure.