For birding FX can indeed be depressing. Even with the Tamron 150-600mm if I want to fill the frame with a small bird, I need to hire someone squeezing it against the front glass. When DX still shoots nicely filled frames, I have to start crawling through the grass in a Ghillie suit.
But with an FX I'm still shooting when it for DX no longer pays. That's the other side of the medal.
But with an FX I'm still shooting when it for DX no longer pays. That's the other side of the medal.