Most higher end DSLR and many film cameras include a button, usually located on the side of the body where the lens mounts. When you press the button, the lens “stops down” to whatever aperture you set on the camera. Without this button on your camera, you are simply guessing at the results of the various apertures.
Try this exercise: Set your mode to AV (A), set your f-stop to f5.6 and press the depth of field (DOF) button. Looking through the viewfinder, press the button a few times. Not much happens. Next, keeping your mode on AV, set your f-stop to f11 and press the button a few times. You should notice that the viewfinder image is a bit darker to you at f11. Now, keeping the camera up to your eye, set your f-stop for f16 and press the DOF button a few times. What you will notice as you press and let go of the button is the scene gets darker. But the point of the button is to show you a preview of depth of field so look more closely at the background of your image. You will see a significant difference between the background when the f-stop is set for f5.6 versus f11. When the preview in your viewfinder darkens its because you are actually seeing the lens “stop down” as it will when you take the final image. I suggest that students keep their eye looking through the viewfinder the whole time you use the DOF button so your pupil remains dilated, making it easier to see the results.
For the above exercise it helps to frame a subject that is close and include some background that is farther away too. Stay focused on the near subject for this experiment.
I photograph using the Aperture priority mode as I believe that control of the aperture is the most important tool available for landscape and nature photographers. Choosing the appropriate depth of field (combined with a shutter speed that works for the image) is one of the most creative way to make your photos “sing”.