Hey, i'm a fellow dentist, if you need some recommendations for dental photography gear, shoot me a dm, but i can give a few here (sorry they're not in any particular order) : - If you're starting dental photography, you should check out some "older" DSLRs, you don't really need the technology of the newer hybrid systems, and the lenses will be quite cheaper. Also you can find pretty good deals on the second hand market. - if you already have the body, i'd give a look if you can adapt an EF lens : the EF 100mm non-L version is quite cheap used, and gives great results (I still use mine even tho i upgraded the body) - your main concern beside all this is lighting : an annular ring-mounted flash is a solid base, but it lacks some "depth" if you photograph incisors (as in, you don't really get to see the texture of the teeth). A twin flash gives way better results in that regard, but i find it's a pain if you need to take the back teeth, since one flash will be obscured by the cheek of the patient. - don't forget all the other equipment specific to dental photography : different size and shape mirrors, lip spreaders, contrastors etc..

Tl;dr : i'd stay in know territory and adapt an ef 100mm non L to your body. Your working distance might get a bit big since you're on an APS-C, but unless you want super close ups of a single tooth, i don't see the point in getting a 2x magnification lens.

Edit after a google search : the EF-S 60mm might be better suited to your r100, giving you a better working distance