What should I look for in a tripod for very long telephoto lenses?

For very long focal lengths, tripod stability matters a lot, but the head is just as important as the legs.

A heavier, more rigid tripod generally helps because mass resists vibration better. That said, simply adding weight to a flimsy tripod won’t fully solve the problem if the tripod/head combination is undersized for the lens.

With long telephotos, balance is critical. A gimbal head is often recommended because it supports the lens-camera combination in a balanced way. Without that, even tiny forces can cause the lens and camera to vibrate independently, which is highly visible at extreme focal lengths.

Also make sure you’re testing whether shake is really the issue. Try placing the camera on a very solid surface and triggering it with a timer. If images are still soft, the problem may be optical quality rather than tripod movement—especially with a strong teleconverter.

So: look for a rigid, substantial tripod, pair it with a head suited to long lenses (often a gimbal), and verify whether your softness is from vibration or from the lens/teleconverter setup itself.