07-19-2010, 07:23 PM
The camera normally idles with the aperture fully open. This lets most light in to the viewfinder, but the DoF shown may differ from the selected aperture you want to take the photo in, as the aperture is one of the contributing factors. Pressing the DoF preview button temporarily sets the aperture to the selected value so you can see what impact it has. As the aperture can only get smaller, it lets less light through and the viewfinder image gets darker.
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