The idea is to look at the scene and observe which crosses are rotating. You will notice that in your peripheral, the crosses appear not to rotate (although the

The idea is to look at the scene and observe which crosses are rotating. You will notice that in your peripheral, the crosses appear not to rotate (although they are, and you can check that by focusing on them). This gives you an idea of how large your fovea is. On a retina Mac I had to double the scale value to get reasonable results.