> By all accounts the 12-24mm is pretty damn good from 18mm - that's the best part of the lens anyhow
I don't agree. The 18-24 part is the best part of the lens when on DX. The corners of FX are not so great. That's not surprising in the least, because this lens is designed to be a DX lens, not an FX. Optimizing the FX corners wasn't part of the design programme.
> I often go to ISO 3200, and by then the D300 is pretty grainy, certainly compared to my Fuji X100, and i think the D600 will be a stop better than that.
That's a good reason to be thinking FX. If you're really at ISO 6400 a lot, FX is definitely better.
> Oh and the FX viewfinder is something I have missed since going digital.
Yep, definitely agree on that.
> But you're right, it's an expensive step
There is an undercurrent of "FX = better" out in the Internet, and that isn't necessarily the case. For some folks - for example anyone who shoots low light sports - there is clearly NO substitute other than bringing your own massive lights. For folks doing landscapes, it's really pretty hard to justify the expense.
The issue is that the format change usually brings a lens realignment. And that's not inexpensive. You have the 12-24 and 17-55. Replacing them with the 16-35 and 24-70 is a $3500 outlay, probably offset less than half by the sale of the older lenses. For those with the 70-200 already, it may also require the addition of (say) a 300/f4, which is another $1400+. And a lot of weight and bulk. The optics are terrific, but $2000 for the body plus at least $2000 for lens upgrades and maybe as much as $4000? One really needs to require the FX advantages to the tune of $4000-$6000.
And it doesn't even stop there. You probably need to change L-brackets, remotes, accessory eyepieces, extra batteries, MDB grip etc. (Ask me how I know this...)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!