Robert Bailey Loc: Canada

DXOmark.com gives the Nikon D610 a score of 94;
the D700 gets a score of 80.
I would definitely go with the 610.
(I own the D750 myself.)

billnikon Loc: Pennsylvania/Ohio/Florida/Maui/Oregon/Vermont

I use a crop sensor for wildlife and aircraft. My FX glass on a FF Nikon Body just does not get me close enough. I would suggest a Mint D7200 or D7500. Or, better yet, a refurbished D500.

mas24 Loc: Southern CA

Actually all of the Nikon FF cameras are good. I would stay away from buying a D600, it had problems which resulted in the D610. For a used full frame, a D800E would be a very good choice. I know someone who owns that camera. And he loves it. The first full frame camera I saw on display, ten years ago was the D700. It is built like a tank, 12.1 megapixels. Before buying one, I would check with Nikon USA, to see if they still are repairing it? Lack of repair parts can sometimes become an issue. Good luck.

Paladin48 Loc: Orlando

amfoto1 Loc: San Jose, Calif. USA

Personally I wouldn't buy or use an FX camera for either wildlife photography or most aircraft photography.

I'd rather use a DX camera, which allows smaller, lighter, more hand-holdable and less expensive lenses to be used (both DX and FX lenses, doeesn't matter because focal length is focal length).

For example, I use crop sensor cameras a lot to shoot sports, wildlife... often with an easily hand-held 2.5 lb. 300mm f/4 lens (little larger or heavier than most 70-200mm f/2.8s). I also use a full frame camera, though it's for other things. In order to use the FF in the same way and for the same purposes as the APS-C and 300mm f/4, I'd have to fit the FF camera with an 8 lb., $9000 500mm f/4 lens that's anything but hand-holdable. With the 500mm, I'd also need to use a sturdy tripod that's pretty close to the weight of the lens (and at least another $500 cost... though upwards of $1000 is more likely). Now Nikon does offer that neat FX Nikkor AF-S 200-500mm f/5.6 VR... Great lens at a nice price around $1500... but a stop slower and still pretty hefty... yer gonna want at least a monopod.

A DX camera also may well have other advantages... faster frame rate, more advanced AF system.... and at a lower cost.

For example, a 24MP D7200 DX matches the D610 FX for resolution and they both have double that of the nearly ten year old, 12MP D700. The D7200 and D700 use a very similar 51 point autofocus array (15 of which are cross type), but while that array covers a large part of the image area in the DX camera, it's concentrated in the center of the FX's viewfinder. The D610's autofocus system is a step down from the other two... 39 point with 9 cross type. The D7200 also is able to autofocus in lower light conditions.... it's rated to -3EV, which is approx. moonlight. That's two stops lower light levels than the two FX cameras.Using only it's center AF point, the D7200 also is able to focus an f/8 lens & teleconverter combo, such as a 1.4X on an f/5.6 lens or a 2X on an f/4 lens. Both the FX cameras are "f/5.6 limited", meaning they would not be able to autofocus at all with those combinations. They need at least f/4 lens with a 1.4X or f/2.8 with a 2X.

Low light focusing capabilities go hand in hand with high ISO/low light imaging capabilities. There you might see an advantage with some FX cameras... except that's not the case with either of these. In fact, they can't even be set to the high ISOs that the D7200 offers. D700 ISO range: 200-6400 (expandable to 100 and to 25600). D610 ISO range: 100-6400 (expandable to 50 and to 25600). D7200 ISO range: 100 to 25600 (expandable to 102400). Compared to those FX cameras, the D7200 offers two stops higher ISO both in it's native range and in the expansion range. Are those high ISOs actually usable? Well, everyone has different requirements, so only you would be able to say for sure. But many users feel the D7200 is excellent to 12800 and usable beyond that. Some extra noise reduction work might be needed, but depending upon the intended use, images might be acceptable. You have to push both those FX cameras into their expansion ISOs to match what the D7200 can do without expansion. In most cases, the expansion ISOs can be "iffy" at the very least. Essentially they are under-exposed images that the camera's software has "pushed" by a stop or two. That amplification of the image always greatly increases the appearance of noise, too.

Between these three, the fastest frame rate really isn't all that different. The D700 is the slowest with a top rate of 5 frames per second. The other two can shoot at 6 fps. None of them are "speed demons". Also, none of them have particularly large buffers... all can handle about 16 to 18 RAW (NEF), so can only shoot relatively short bursts before the camera needs to pause to clear the buffer. Nikon 21MP D7500 is higher performing in this respect... with 8 fps and able to buffer 50 RAW images. Even higher performing is the D500 which can shoot at 10 fps and up to 200 images. Beware using high frame rates, though.... lots and lots of images means a whole lot more time sitting at a computer editing them!

D7200 and D700 have 1/8000 top shutter speed and 1/250 flash sync. D610 is 1/4000 tops and 1/200 sync. All three are durability rated for 150,000 actuations ("clicks"), though as they say "your mileage may vary".

Image quality... well at a pixel peeping level the D610 would probably win, thanks to larger pixel sites on a less crowded sensor. But here's the dirty little secret: Unless you make huge prints... bigger than 16x24"... YOU would be the only person who ever actually sees all that "full frame goodness" in your images. For most peoples' actual purposes... "normal" size prints, online display and sharing... the only time that FX images are truly superior is when the photographer is sitting in front of their computer monitor looking at the original image ridiculously large on their monitor, before sizing it down and turning it into something actually usable. A 24MP image displayed "at 100%" on modern flat screen monitor at the native resolution most use is roughly equivalent to making a FIVE FOOT WIDE print and viewing it from 18 or 20 INCHES away. That's something most people will never do in "real life". Even if they make such a large print, in order to see it they'll be much farther away. In reality, most people never print much larger than 13x19" and even that's relatively rare. But, hey, maybe you're different!

So, what about cropping? Wouldn't the FX camera give you more to work with? Definitely not in the case of the older, 12MP D700. And with the D610 you'd be losing most of the benefit of FX by heavily cropping your images. By the time a 24MP FX image is cropped down to DX size, there's less than 10MP remaining. Compared that 24MP resolution for the same area with a DX format D7200. With distant objects, the "free 1.5X teleconverter effect" of the DX camera is a very distinct advantage... putting far more "pixels on target" than the FX cameras can. Despite being a full frame camera and fine in 2008, image quality of the older D700 will be less than any of these more modern models.... DX or FX.

In the end, for telephoto work such as much wildlife and aircraft photography require, a recent DX model like the D7200 is much better choice than an older and/or lower resolution FX camera.

D7200 brand new is about $1000 right now and, since it's not that old a model, relatively lightly used copies of it can be found for around $800. Refurbished are avail., too for about the same price. (Note: Nikon refurbished have same warranty as some retailers offer on used).
D610 is available new, too, for $1500... or refurbished for around $1100... or used for around $900.
D700 can only be found used and seems to sell in the $600 to $700 range. With a nearly 10 year old model, check the shutter count because it may have "high mileage".
Since I mentioned them and for comparison...
D7500 is selling new for $1250, is fairly new model so there aren't a lot of used, and when there are they selling for around $1100.
D500 sells new for $1900 and used or refurbished for around $1500.

For your stated purposes, I really think an FX camera would be a mistake. Many people think FX is always "better" and will instantly improve their photos somehow. It won't. Depending upon what you've got now, an upgrade to a different DX camera would almost certainly be a better choice. Unless you already have a D7200 or one of the other recent DX models, of course.

sirlensalot Loc: Arizona

Personally, I would opt for the 750.