During the 4 decades I attended hundreds of lectures illustrated with thousands of intra and extra oral photographs. Three of them were good. The rest were masterpieces of illustrative skill but all you could see was teeth. It was like looking at the front grilles of Buicks, but at least the Buicks flossed...Thus my general advice to anyone wanting to take dental pictures is - don't.
If you insist on it, however, the best way I know is to use a digital SLR or mirrorless camera. I used to advise people to use ring-flash units for the illumination but that has stopped with the advent of the white LED light. Nowadays you can get a decent ring-shaped LED set that runs on AAA batteries or a mains adapter and mount it on the front of nearly any decent camera. If you then set the ISO of the camera to 400-800 ( or higher if it is a modern camera ) you can put the mode to "A" and set an aperture of f:8 to f:16. If you put your lens on manual and at the shortest focusing distance you can generally make a rather decent extra-oral shot. An excess of decency.
If you need a closer shot or one that goes far back in the arch, the true macro lens may be necessary. Costly, but close-focussing, and it lets you stand back a bit from the customer as you work... Some of the customers have breath that makes this a relief.
If you need to have more light - really - you can use the ring flash systems. Nikon, Sigma, Canon, and Metz make them. They are in various sizes and degrees of sophistication, and some of them are automatic enough to work most of the time. But there is an almighty pop as they go off that might spook the patient. Plus they generally are more complex so you end up with a system that your staff might not find as easy to use. If you consistently get overexposure or out of focus results it is advisable to beat the nurse with a stick.
What you do with the results is different than the old days. The Carousel slide tray full of VMK preps is long gone. Slide shows on Powerpoint and Show Off can be integrated with text, sound, and music. There is no way to describe the experience of a professional presentation on gingival recession when it is accompanied by John Cage.
Please note that the above notes also apply to dermatology illustration, but with knobs on. Pulsating multi-coloured knobs...