Red eye

Red eye is an unwanted effect caused by using a flash close to the lens, where human eyes have red-filled pupils (and animal eyes sometimes have other colours). This comes from the flash reflecting back from the eye's retina. This is common with compact cameras, where the built-in flash (or flashcubes) are fixed close to the lens.

Prevention

Pocket Instamatic 10, with Magicube extender

Red eye can be reduced by several methods:

• Add one or more flashes shortly before the main flash and exposure; this causes the eye to close their irises, reducing the chance for reflection. Many compact cameras with built-in flash can do this, set as a "red eye reduction mode" - sometimes indicated by a crossed-out eye icon.
• Move the flash further away; with a separate gun, this can be done by holding the flash by hand or with a flash extension bar - using an extension cable from PC socket or hot shoe if necessary. With flashcubes, this could be done with a flashcube extender
• Bounce the flash off of the ceiling, if indoors. This is not usually possible with built-in flashes, but many flashguns are fitted with pivoting heads for this purpose - and others could be held by hand to point upwards. This spreads and diffuses the flash, improving the lighting generally, as well as changing the angle to reduce red eye, but can result in strange colour casts if the ceiling is not white.
• Post-process or retouch the image. Some digital cameras can do this automatically, immediately after the shot, and some photo processing software has it as an automatic function - but it can be done by simply filling in pupils with black dots "by hand". Traditional paper prints were sometimes retouched using black ink to do the same thing. The only disadvantage of this (apart from the effort of doing it) is that irises are often invisible since they were wide open when the picture was taken, so eyes lose their colour.