This is the most common type of macro flash. The flash forms a ring around the end of the lens.
An adapter is screwed on to the lens filter thread. The doughnut-shaped flash slides into the adapter, and can spin freely. It's best if the front of the lens doesn't rotate when focusing, but it's not a deal-breaker as the flash can turn. Adapter rings are provided to fit lenses with different filter sizes. A step-down ring can be used (if a suitable one can be found) for larger filter sizes, but vignetting (dark corners) might occur in the photo.
Most ring flashes have two flash tubes inside the ring. The flash tubes don't run the full length of the ring, so the light isn't perfectly circular. Translucent plastic over the flash tubes acts as a diffuser.
