You can try Tokina 100mm , it may be a good option for you, even if you gonna upgrade your camera in future.

You may refer to 135 MM from Nikon as you mentioned you are looking for dental photography. A Longer focal length would be a good idea.

In my humble opinion, you should take into consideration the size of the lens (bigger size = bigger image = longer distance to take the photo). If you are looking for a high precision and detailed photography of all the small aspects of your cases then go for 100mm or 135mm. On the other hand, if you go for a smaller lens you are going to be able to take photos closer to your patient intra and extraoral, but for small details you may not get closer to your objective. Normally, for daily practice smaller lens are easier for ortho and bigger lens for prostho.

When choosing the brand, if you can pay the price for the Nikon ones I would go for a Nikon. Once they are compatible for your body camera and after long use the wear usually is lesser than "white" brands.

Regarding the photo equipments, I believe the lens is the most important, following the light (flashes from the camera [ring/anelar] and soft boxes) and at last the camera body.

I hope to have answered your question!

Dear Dr. Csaba Dudás,

this is a serious issue that I have also faced myself, and unfortunately, there is very little information on what the optimum lens size is for crop sensors. The Nikon d3400 has an APS-C sensor, which will "zoom in" compared to a Full-Frame sensor. Using a "big" macro lens, such as the Nikon 105mm with an APS-C sensor, will require you to have a very long distance from your patient to fit everything in your picture. This will make focusing difficult and make you stand on your tip-toes unless you are very very tall. Therefore, for a d3400 and all APS-C sensor DSLRs (such as d3000, d5000, and d7000 series), I think that the 85mm macro lens by Nikon is the most suitable.

I just hope this was on time!