da* 16-50 f2.8 artificially restricted to f2.8?

Well, that certainly does seem like a lot of stopping down (possibly two stops)! On the 16-45, it shifts very little by comparison. I think you need to hear from someone with the 16-50 to indicate how another one behaves. Depending on where the aperture blades (diaphragm) is located, it might be not be as dramatic as it looks, and depending on size of the rear element. The stop-down shift might appear very small on the rear side.

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JNR
www.jamesrobins.com