The text says that circles are less easy to find and they are dependent on real objects. They are a powerful boundary and carry greater weight of importance compared to other design features.
Ring: ISO 800 100mm f5.6 1/500
In the photograph above, despite there being an implicit triangle in the design, whose boundaries are the tree branches, the Gunnera plant on the left and the lake edge, it is the lifebelt which leaps out of the photograph. I saw this image in precisely that way - unfortunately in my haste I did not consider the camera setting which I had used - an ISO of 800 was unnecessary here.
Red Cabbage Tree: ISO 200 105mm f8 1/200
The cabbage on this vegetable stallattracted my attention - even though we have the strong, white core of the cabbage pointing inwards - diagonals - the main design feature to me are the concentric rings provided by the cabbage's own structure. The shallow depth of field, created by the close up and use of a telephoto setting adds to ensuring that the focus of the viewer's attention is the heart of the cabbage.
Door decorations: ISO 200 28mm f9 1/80
I consider this to be an example of an implicit circle which has been created by the "ring" of metal "flowers" arranged on this door. The eye is not distracted by the shape to the right, it is always led to the centre by virtue of strength of this circular arrangement. I used a square framing to this image as I felt this was the most appropriate way to present it. Somehow, circles work well in a square, perhaps because they have, in common, a regularity and tightness of shape.