A) First, a little depth-of-field review. When you focus your lens, you’re focusing on a specific distance. All the objects at that distance will be in focus. Depth of field is the distance range in front of and in back of the specific focus distance where the image is still acceptably sharp. To control depth of field, you change your aperture. The smaller the aperture, the bigger the depth of field-the greater the range of “things” that are in focus. Depth-of-field preview gives you a chance to see what that range is.
Normally when you look through your viewfinder, the lens aperture is wide open, letting in a maximum amount of light. This makes it easier for you to set focus and compose your shot. The depth-of-field preview button engages the aperture according to your exposure setting. Unfortunately, when you adjust the aperture, you also adjust the amount of light that passes through the lens. If you stop down the lens, there’s less light coming through, so the image in your viewfinder gets darker. It’s not broken; it’s just a matter of optics.
The trick to using depth-of-field preview is to concentrate on little details that you can make out in the darkness. It takes a little practice, but it’s a useful tool.