My long-lens combo to date has been a D80 with 80-400mm zoom (the old one, of course). Recently I bought a D3200, followed by a 55-400mm zoom. Now, given the relative sensor sizes (3,872 x 2592 on the D80 vs. 6,016 x 4,000 on the D3200), I can digitally zoom the image by a factor of over 1.5 on the D3200, thus giving me an image that's equivalent to around 450mm on the D80's sensor. Is this correct?
Given that I spend a long time walking (or cycling) with my camera and lenses, the 1kg difference between the two combos makes a huge difference. The difference in image quality between the two combos is hardly noticeable. Any sense in me hanging on to the old 80-400mm? It's autofocus is slow and can't be overridden. To me, it's become a bit of a clunker.
I've done a short test, comparing the results of the 80-400mm on the D80 with the 55-300mm on the D3200. Results here: http://jeziorki.blogspot.com/2013/04/lighter-long-lens.html