A circular argument is logically flawed - it commences from the premise that what is to be proved is true. There's no such problem with the circle in art, however. It has long been considered a powerful symbol of simplicity, perfection, continuity and eternity.
A simple, but perfect, geometric shape, the circle can be found everywhere. From the humble donut to stunning modern architectural creations, circles lend themselves to interesting images.
These images were taken over a couple of months, so the equipment varies. 1 and 3 were Olympus E-30 and Zuiko Digital 14-54mm f2.8-3.5 during one of our lunchtime photo walks, appropriately themed "circles". No. 2 was Olympus E-30 and Zuiko Digital 50-200mm f2.8-3.5. No 4 was Olympus E-5 and Zuiko Digital 7-14mm f4.
The 7-14mm is an amazing lens. For such a wide field of view (14-28mm full frame equivalent), it provides an extremely sharp and well corrected rectilinear image. Arguably the finest ultra-wide angle lens available on any camera system, and one of my favorite photographic tools.