Built in flash techniques: 04. Use red-eye reduction
One of the problems with using on-camera flash is that the light is very close to the lens and this means that there’s a high probability of light bouncing off the back of the eye and into the camera, creating the well-known red-eye effect.
However, most cameras have a red-eye reduction mode to help tackle this problem.
This works by firing a pre-flash before the exposure so that the subject’s pupils close down to reduce the amount of light that can enter them.
In many cases this will significantly reduce the problem, however, if you have an image that still exhibits the phenomenon it can be dealt with quickly and easily in most image editing software packages.
Diffusing or bouncing the flash will also help reduce the likelihood of red-eye.
Best flash diffuser: 6 top models tested and rated
Bounce flash photography techniques in 4 easy steps
What is exposure compensation: free photography cheat sheet
10 common exposure problems every photographer faces (and how to fix them)