Switching from the Tamron 16-300 to the Tamron 18-400 (on my d7100) gave me the additional reach I wanted for Africa safaris and other longer shots. I think it was worth it and Tamron is priced quite well. If you go much longer you have to really be able to shoot high shutter speeds or start using a tripod, neither of which will enhance your general watching experience. (But that's just me...)
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