Imagine that some strange man wants a picture of your child. If it were the year 1750, he might commission an artist, who would then knock on your door and ask to paint a portrait. You could say no.

If it were 1850, he might hire a daguerreotypist who would ask your child to sit still while light and mercury vapor etched their image into a brilliantly polished, silvered copper plate. Again, you could say no.

And if it were 1950, the man might buy a Polaroid camera, knock on your door and ask to snap a photo of your child — and you could still refuse.