Question
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.
Answer
Actually the same 50-135 f/2.8 lens was sold by Tokina for other mounts, but it was discontinued in 2009.
I'd say the primary reason is that there's no significant savings between making a telephoto lens just for APS-C or one that's also good for full frame. In fast telephoto lenses, the majority of glass goes towards achieving the big aperture needed, the smaller image circle won't introduce very significant differences.
Another reason is that since a fast telephoto zoom is an expensive lens, many of the buyers are already considering getting a full-frame body in the future, and would dismiss an APS-C lens. This leaves a smaller pool of potential buyers, meaning less potential profit. Pentax doesn't have to worry about it, since they don't have a full frame body, nor have they announced any plan of making one.
Check more discussion of this question.
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