Observe, Observe, Observe
After you talk to the child, explain you’re going to get your camera ready and back away. The child will soon lose interest in you, giving you a chance to observe the child relating to the world. Inquisitive? Intent? Distracted? Capture that in your images.
If you’re taking photos of your own child, try observing your child as if you don’t know him/her at all. You’ll probably see some things you haven’t noticed before.
