Flash + synchronization cord
The flash is the tool which generates the photons captured by the lens and ultimately the camera sensor. It is the second most important tool behind the lens and should reflect you budget allocation. The flash must be powerful, detachable from the camera, and attached by a synchronization cord or triggered remotely by the camera. Taking the flash “off camera” is imperative as this allows positioning along the barrel of the lens (axial lighting), creating shadow to demonstrate depth by angling the light at a 45-degrees to the subject (texture lighting), control the location of the reflection on the cornea, and minimizing shadow or uneven illumination on extreme close up photographs. A powerful flash is needed to overcome the small aperture required to good depth of field in extreme close up photographs.
Ring flashes are excellent for illuminating cavities, such as oral cavity, gynecological, and surgical fields, however they produce flat-appearing images because their diffuse lighting, do not provide any shadow. This flat lighting is useful when photographing skin eruptions where color accuracy is important. Ring flashes produce even illumination of structures that have multiple contours such as the male and female genitalia. A ring flash is not a necessity for medical photography but does expand the capability of your camera system.