Originally posted by Snipsnap This requires a long zoom lens and ideally <F/4 lens to enable faster shutter speeds.
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I have done some research, and it seems like the Nikon D7100 is the way to go if I do decide to switch
There are reasons to consider a switch. But IMHO it would make most sense if you are ready to invest ~ $2000 to do it (cost of switch assuming you sell and buy used). At the amount of money you invested so far, Pentax offers good value.
Esp., the combo D7100 + *new* 80-400/4.5-5.6G zoom offers you another league of resolution and AF tracking. But at a price. But that lens is a unique offer in the Nikon system currently only paralleled in the Sony system. Some people switch systems for this lens alone
Others consider its prize to be crazy. If you are among them, I'd better not switch. Another option would be the Canon 400/5.6 though.
If you are not ready to invest this much, I don't know. You'll loose 24 MP cropping performance and a 400mm lens sharp enough to support it. And lenses <F4 at this reach start at $5000, so I don't even mention them (full frame plus 500-800mm prime lenses costs you 10,000$ but is yet another league). So, you may not see improved results when switching away from Pentax. Or look at Canon 7D + 400/5.6. But it is no zoom and has less cropping headroom.
And only few Nikon or Canon shooters buy Sigma tele lenses. Which means they can be even harder to buy used than for Pentax. Be prepared to buy Nikkor glass.
So, my advice in summary: How serious is it? Is it for the occasional snapshot? Then stay with Pentax. Is it becoming a serious hobby where you need produce "best of its kind" photography? Then invest another $2000+ and switch.
(disclaimer: I am shooting Pentax, Nikon, Nokia, Apple).
Personal note for you: Considering you just started using a K-30 and hasn't moved to a K-5II with DA*300/4 yet, I count you into the occasional snapshot group until you have gathered more experience (the DA*300/4 is better value than the same lens from Nikon because the latter isn't stabilized. And the Pentax DA* 60-250/4 is one of the best possible options available in
any system, a zoom as sharp as a prime). The source of most early frustration is (lack of) experience, not equipment. Done that, been there. You'll know by yourself when it is time to invest another $2000. Until then, don't switch and become more experienced. One element often underestimated by beginners is patience.
Originally posted by Adam The new 120-400mm lens combined with the D7100 ...
Adam, you probably mean the 80-400 I was mentioning. I say it because Nikon has a 200-400mm which costs a fortune but may be the best tele zoom money can buy. Short of Pentax 250-600 no money can buy anymore. But even the 80-400 is the price of five or six K-30...