woahdude

Actually the distance from your nose doesn't matter. Think of it this way. When you look at a screen or piece of paper, both of your eyes are trained to converge on the paper or screen. These images have a hidden point somewhere beyond the paper/screen where your eyes can easily focus and see the hidden 3d image.

Hold your hand 3 inches behind your phone. Move your phone away and focus I your hand. Return the phone while maintaining your focus on your hand.

That's all it takes. Different images have different focal points, so 3 inches might work for this image but not for others. Might have to experiment a bit. But if you can see your hand in focus, then you can also see these images.