...They probably have crop bodies because no one told them the difference between crop and FF....
.....I wouldn't look at cost too much as you can write off all this gear.
Yes, this ^^.
I'll start by stating that I know absolutely nothing about medical imaging.
I do know that I'd really prefer my medical professional not make surgical decisions based on photographs with fun house mirror type distortions.
To avoid visual compression or fish eye effects one must use a near normal or natural focal length, that natural or normal focal length being one that's as close as possible the sensor's (or film's) diagonal.
I looked up specs for a 7D that uses a typically sized Canon 1.6 crop factor sensor with dimensions of 22.3 x 14.9 mm, diagonal of that sensor is approximately 27mm.
I'm unaware of any macro lenses with a focal length near 27mm.
Diagonal of a full frame 36 x 24mm sensor is approximately 43mm.
On a B&H listing of Canon mount macro lenses
there is a Canon EF 50mm f/2.5 Compact Macro Lens
a Sigma Normal 50mm f/2.8 EX DG Macro Autofocus Lens
and a Canon MP-E 65mm f/2.8 1-5x Macro Photo Lens
All with focal lengths fairly close to the 43mm full frame diagonal, for sure a lot closer to the 43mm full frame diagonal than the 27mm crop diagonal.
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Note that the Canon EF-S 60mm f/2.8 Macro USM Lens and the Tamron G005 SP AF60mm f/2.0 Di II LD 1:1 Macro Lens are both for APS-C crop sensor bodies and cannot be used on full frame bodies.
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Regarding concerns about how close to a patient you'll need to be with these shorter focal lengths, um, you will soon be close enough as to be in the patient's mouth when actually working, is closeness of a camera really relevant?
Regarding LED ring lights, I've made and used several. As they were not real bright, a steady camera and subject are both helpful. They did indeed cast even and mostly shadowless light.
Being continuous light, you get a very good idea what your photo will look like through the finder or live view before triggering the shutter unlike with flash where you find out after.
This might be a consideration towards minimizing patient discomfort by avoiding multiple flash discharges as you dial in the exposure.
Then too, likely, once you've a small bit of experience you'll know what settings work and will need but a single flash exposure.