I am looking to add a longer lens to my current zoom (a Tamron 17-50) for my Nikon D300 (but I believe this question is relevant across aps-c lens manufacturers).
There is just one currently in production — the Pentax 50-135 f/2.8 (which is Pentax mount only).
What is the reason behind this? Is there a physical reason why making a smaller DX alternative to a 70-200 is not be possible? I have seen it mentioned that "beyond 100mm the advantage in making dx lenses is gone", but not sure if this is folklore or a genuine restriction.