Everyone knows what "red eye" looks like in pictures of people--that weird red glow in the pupil that is caused by flash reflecting off of the retinal surface in the eye. Flash photos of pets can cause the same thing but I call it "green eye" because with most cats and dogs the result is either a green or blue-green pupil. It looks positively demonic!
You can use your red-eye reduction flash mode to get rid of the effect in both people and pets, but I hate that mode. To eliminate the red eye the flash fires a series of pre-flashes that cause the pupil to contract and thereby avoid reflections from the retina. The trouble is that people and pets can see the pre-flashing and it makes them aware that you're taking their picture--and usually ruins the moment.
With people it's easy enough to just keep the flash in its normal mode and then change your position slightly so that the flash is not firing directly into their eyes. If you shoot from slightly to the side, or above or below your subject, that slight shift in position pretty much solves red eye. With pets, if you start to move, what happens? They think it's time to play or get a cookie and they follow you. I've found with my cat that the better thing to do is to have the camera ready (I shot this photo in my office while sitting at my computer) and then just wait for her to look away naturally. Once her head is turned slightly there's no chance of red eye. If I had put on the red-eye mode or tried to move my position, she would not have that nice natural pose she has here.
So, with pets, just be sure they're not looking directly at you (especially at your eye level) and wait for a natural moment. As long as their head is turned you won't get "green eye."
11 hours ago