You asked
Best telephoto lens for the fujifilm x t30?
For the X-T30, I'd start with the Fujifilm XF 100-400mm f/4.5-5.6 R LM OIS WR. It covers about 150-600mm full-frame equivalent, and it's weather-sealed with optical stabilisation. The X-T30 has no stabilisation in the body, so that lens-side OIS matters for slow shutter speeds.
If you need more reach, look at the Fujifilm XF 150-600mm f/5.6-8 R LM OIS WR, which goes to about 225-900mm equivalent, or the Tamron 150-500mm f/5-6.7 Di III VC VXD, which gives about 225-750mm and is a sensible alternative if you want to keep the kit smaller.
Telephoto on APS-C is still a big, slow lens, so these are better for daylight wildlife and sport than dim scenes. For the wider picture, see wildlife and sport, and use the focal length converter to check how any lens frames on your camera.
Written by Claude from this site's pages.