DoF control on Nikon D200

Ahhh yes depth of field preview....back when Canon actually was #2, they had eye control in their viewfinder, which was great btw to change focus points on the fly...and they had one corner for DOF preview.

Anyway, your viewfinder view is, by default at the maximum aperture, which also produces the least amount of depth of field, which is basically the distance away from the camera that is in focus.

At larger apertures you can have a depth of field in millimeters, while at smaller apertures it can be measured in hundreds of meters...depth of field is dependent on the film/sensor size, lens focal length and focusing distance from the sensor/film. (The crop cameras actually have the same depth of field as their full frame brothers, but you have to take away the crop factor to figure it out, so you need faster lenses with a crop camera to get smaller depth of field)

The preview button stops down the lens so you can "preview" what the final image will look like before actually taking the picture. However, this does make the viewfinder darker. But it's useful if you are trying to get several places in the scene to be in focus that you are photographing. But with digital it's not as important since you can review and zoom in.

 

if you look into the end of the lens while pushing the button you can see it working and closing the aperture blades down. try it at a few different apertures.

Like Roger said, I hardly ever use it any more since I can instantly see my results in digital. on the d300 you can change that button's function. I still have that button set to dof. I can't bring myself to change its function.