One of the things that is obvious as I examine my photos, is that much of my time is spent at either the ultra-wide or at the long limit of my available lenses. And at the long telephoto end, I've been stuck at 300mm (35mm equivalent) since I sold my Micro 4/3s equipment and moved to Sony mirrorless.
Well, I'm all excited because I'm awaiting delivery of a Sigma 400mm f5.6 long telephoto prime lens. It's an older lens, from the 1980's, but it has a stellar reputation and despite being built like a tank, it is also relatively light and small. I found it online at KEH rated as "excellent +" for less than $200. This is with a seller that has a reputation for undervaluing their goods. All of the gear I've bought from them over the years has been better than they have described. Hot damn!
It's a Pentax K mount so I had to source a PK-to-Sony E mount adapter, and now I will have a 600mm f5.6 (35EQ) lens, complete with built-in tripod mount. I LOVE long telephoto lenses for the great separation they give to pop something out against an out-of-focus background. And simply being able to pull a subject in so close is really the reason for such a lens.
Can't wait to get this puppy and get out and shoot with it!