I'm new to DSLR photography, but have been snapping pictures with a pocket camera for years. I have a 50mm f/1.8 prime lens on my D200, and it takes photos with beautiful bokeh. But I feel like I'm overlooking something by not using the depth of field preview button on the camera. I have read about this button online, but can't quite wrap my head around what exactly it is supposed to do. Or perhaps it's working just fine, and I don't actually understand it.
Anyway, I apologize if this sounds like a dumb question but I'm hoping someone could explain it in simple terms :)
The whole system is designed to keep the viewfinder bright by keeping the lens open to its widest aperture until the shutter button is pressed. The preview button is there simply to allow you (the photographer) to judge just how limited or extensive the depth of field will be when you take the image. If your lens is set to 1.8, nothing will happen when you press the DoF Preview button. If the lens is set to minimum aperture (16, 22 on this one?), the viewfinder will get dark but you should also notice that the DoF will deepen substantially. Intermediate results at intermediate aperture settings.
While the DOF preview button was important when shooting film in some instances, it's all but obsolete in digital. I typically assign the button to some other useful function. For a DOF preview, I shoot a picture and chimp on the LCD to decide on DOF and aperture settings. It's far easier to see on the LCD than in a very dim viewfinder at small apertures.