Actually it can be harmful. Here's a quote directly from the manual that came with my SB-26 flash unit:
Quote:
DO NOT FIRE FLASH NEAR THE EYES: Firing the flash light very close to any person's eyes can injure the retina, thereby weakening eyesight or causing blindness.
|
So when taking closeups of an animal's head, use a lens that allows you a good bit of working space between the camera and the subject. Yes, you can get close up lenses that do closeups if you are right down on the subject, but that also puts the flash unit precariously close to the subject as well.
If you have any doubts about the effects of the flash on your animals, put the flash an equal distance from your own eyeball and see what it feels like to you. I think that will sufficiently answer that question.