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Your D800 has an image sensor that's the same size as a piece of 35-millimeter film. This is actually one of the advantages that it has over other SLRs. Most of Nikon's other SLRs have a sensor that's the size of a piece of APS film, which is a little bit smaller than 35-millimeter film. Now, a lens projects a circular image onto the camera's focal plane, so depending on the size of your sensor, you'll end up cropping a different size rectangle out of that circle. Nikon FX lenses are engineered to project a circle that's big enough to cover the D800's 35-millimeter-size sensor.
Nikon DX lenses are engineered to project a circle that's only big enough to cover an APS-size sensor. Now, the advantage of DX lenses is that they can be made physically smaller and lighter. The disadvantage to DX size sensors is that you can't get depth of field that's as shallow when you're shooting with a smaller sensor. Also, a given focal length on a DX lens will have a narrower field of view than the same focal length on an FX lens. This is because the DX sensor is cropping a smaller rectangle out of the circle that the lens is projecting.
Now, your D800 can use either FX or DX lenses. By default, when you attach a DX lens, the camera will automatically crop a smaller area out of the center of the image. It will show you guidelines in the viewfinder that show the exact area that's being cropped. However, when you're using an FX lens, you can still opt to choose a DX crop. When you do, the field of view of your lens will narrow. To figure out how much, multiply your focal length by 1.5.
For example, if you put a 50-millimeter FX lens on your camera and then set the camera to DX mode, you'll have the field of view of a 75 millimeter lens. That's 50 x 1.5. The advantage of switching to DX mode when using an FX lens is that it lets you frame with a tighter field of view. The disadvantage is that you're cropping out many of the pixels that that big 35-millimeter-size sensor has. Personally, I would just shoot it full size and crop later in my image editor, as this would give me more options for how to use the image.
You can control all of this stuff with the Image Area command on the D800, which also offers a couple of other options. If you come into your menu system and go in the Shooting menu, down to Image area, which is, using my scrollbar, down a little ways, so this a few items down, you get a few different options here. By default, the camera sets to Auto DX crop, meaning if you put a DX lens on this camera, the camera will automatically adjust itself to work with that lens. And what it's going to do is it's going to crop out just the center part of the image sensor.
If you look through the viewfinder, you'll actually see guidelines that show you which part of your field of view will actually be captured, everything outside of the inside box is going to be lost. It's kind of amazing to see just how much bigger that FX sensor is than the DX sensor. So this is what is giving you a smaller field of view. Now, I've got an FX lens on my camera right now. But if I want, I can choose to take a DX crop. So, when I choose this, I will still see my smaller viewfinder, and any focal length that I have set on this lens will effectively be multiplied by 1.5 to get the equivalent field of view.
I can also set to a 1.2x multiplier, or I can set to this 5:4 aspect ratio, which is going to get me a slightly narrower field of view. All of these will show me the effect of field of view in my viewfinder, and they are a way of shooting a pre-cropped image. If I'm shooting JPEG files, only the cropped area will be saved. If I'm shooting raw files--I am processing with Nikon image processing software-- then the raw files will automatically be cropped when I convert them.
This doesn't get you anything that you cannot do shooting in full FX mode and using a Crop tool on your image later. It's simply a way of pre-visualizing what a smaller crop would be if for some reason you need to match the field of view of this camera to a DX camera.
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