How do you take a picture of mew on Pokemon Snap?

When you first see mew, you'll have to throw either Apples or pester balls at it in order to break its force field. After you've thrown enough, mew will appear outside of its force field, that's when you take the photo before it gets away. Some advice when mew is outside its field is to hit it with an Apple or pester ball, this will prevent it from running away for a few seconds and you will be able to float closer to it and get a better picture.