Give the camera real scale with a coin or card

Choose a coin you have in your pocket. In the next step you'll match it against a circle on the screen — then the app knows exactly how many millimeters a pixel corresponds to on your skin.

1. 1
   Choose a coin or bank card
2. 2
   Place it on your skin and match the shape
3. 3
   Done — try tattoos at 1:1 scale