It is easy to reduce this reflection:
- use bounced flash – bounce the flash off of the ceiling or use a diffuser card (if your flash is equipped with a swivel head)
- take the flash off camera and hold it to one side or higher
- use a diffuser lens over the flash head (you can make a simple diffuser using a small piece of white cloth or you can buy one)
- tell the subject to move the angle of their head
- for studio lighting, used diffusers or umbrellas compatible with your flash heads
- position the lighting further back to decrease the power hitting the subject
- use theatre make up or ask the subject to change to less troublesome forms of make up
- move the model to a different location (this can help to reduce stray light reflecting from windows etc
These simple measures should be enough to ensure that stray reflected light is kept to a bare minimum or eliminated completely.