Using the Nikon D700 in DX Crop Mode?

I use a 50mm 1.8G lens on a D5000, if I put it on a full-frame camera like the D700 does it get wider? I read that if you put a DX lens (not sure if it's a DX lens) on a D700 the camera automatically changes to DX sensor format (24x16).So if I look through the viewfinder will the focal length look the same as on a DX D5000? I'm confused about DX and FX full-frame.

Added (1). Ok so 50mm on full frame "looks" 15mm wider than on a cropped body. On a D700 if you want it to look like 75mm is there a setting where the camera can be cropped? If there's a setting for that, that would be cool because you can you use two different angle of views.

The 50 you're talking about is not a DX lens. Your D5000, along with all other DX cameras (also called crop-bodies) have sensors that are 1.5 times smaller than a full frame sensor/35mm film. The 50 will be a 50 on the D700 (on any FX or 35mm SLR) and 75mm on your D5000 (or any other crop body).

If you want a D700 to have a field of view of a crop sensor, in post processing, you crop the image. It's that simple.

What you are really interested in is the horizontal field of view.
A 50mm lens on a crop sensor has a HFOV of 18°.
A 50mm lens on a full frame sensor has a HFOV of 27°.

A 75mm lens on a full frame sensor has a HFOV of 18°.

As the previous poster mentioned, the 50mm lenses Nikon makes are FX lenses, thus will work on FX and DX cameras.

People normally buy full frame cameras to get the best possible photos. Why then would you want to use a 50mm lens in cropped mode and use less of the sensor?

If you want to crop it, just do it in post processing.

Here is the DX thing;

DX cropped cameras have a smaller sensor. Therefore, when using a lens on a DX camera, the lens "appears" to have a focal length shift of 1.5x. However, in reality, the photo is being cropped inside the camera rather than in post processing. Afterall, that is why they are called cropped cameras.

So a full frame lens on a FX camera will look like it's native focal length.

A full frame lens on a DX camera will have the appearance of a 1.5x longer lens as it is cropped.

A DX lens on a DX camera will still have the appearance of a 1.5x longer lens. In that regard, a 50mm DX lens will appear the same as a 50mm full frame lens on a DX camera.

The reason the lens is DX is that the manufacturer basically removed the outer diameter of the lens as you are not using that part of the lens anyway on a DX camera. That saves money. You see this mostly with wide angle lenses as it is significantly more expensive to make a full frame wide angle lens than a DX wide angle lens.

And a DX lens on a FX camera doesn't work too well, since the DX lens has a smaller diameter. While the DX lens will look like it's native focal length, the lens diameter will be too small, and the result is vignetting (loss of the image around the corners).

The only advantage of using a DX lens on a DX camera is cost. A FX lens will work as well (and perhaps better) on a DX camera as the camera is only using the lens center "sweet spot", which tends to be sharper on most lenses. But a DX lens will be smaller and lighter - so I suppose that might also be an advantage.

And there's one point to consider. Although when used on a cropped camera, a DX lens appears to have a longer focal length, the other characteristics of the lens does not change.

An example is a wide angle lens. All wide angle lenses to some degree have perspective distortion. This distortion is worse as the lens gets wider. But even at 35mm, there will still be some perspective distortion.

Now if you put a 35mm lens on a DX camera, it would appear like a 52mm lens, but it would retain the distortion of the 35mm lens, as it is still a 35mm lens.

And when compared to a 50mm lens on a full frame camera, while the photos would have the same field-of-view, the DX photo would show more perspective distortion.

While this is not terrible in itself, just realize what is going on.

Here is a webpage that might help explain it.

http://www.althephoto.com/concepts/crop.php