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Thanks! The max I have is 300... though not quite understand that it is 450mm equivalent.

You're welcome! It just means that because you're using an APS-C camera (cropped sensor) it 'zooms in' - only uses the central part of what the lens can produce - so a 300mm looks the same as a 450mm would on a full frame camera. For Nikon you times is by 1.5 for whatever the focal length is!

So the same lens (my Tamron in this case) on a full frame camera will have actually 300mm?

Yep! That's it!

Hmm... learned something new :)