02-07-2015, 08:57 PM
Heard of constructive/destructive interference? The defocus characteristic with a possible bright centre, and bright outline means that along lines of contrast, you can get brighter and darker areas next to it. In the Siemens star, the lines are not parallel so the spacing distance is changing. At times, the spacing will be just right that it boosts contrast, which gives you the what looks like in focus areas. Other times, they cancel each other out and you get grey, like the region between the two contrast rings. I'd consider it a side effect of the defocus characteristic.
<a class="bbc_url" href="http://snowporing.deviantart.com/">dA</a> Canon 7D2, 7D, 5D2, 600D, 450D, 300D IR modified, 1D, EF-S 10-18, 15-85, EF 35/2, 85/1.8, 135/2, 70-300L, 100-400L, MP-E65, Zeiss 2/50, Sigma 150 macro, 120-300/2.8, Samyang 8mm fisheye, Olympus E-P1, Panasonic 20/1.7, Sony HX9V, Fuji X100.