The following slide show demonstrates the visual effect in the viewfinder when pressing the Depth-of-Field preview button (DOF button) at different aperture settings using a 100mm f/2.8 based on an object distance of about 5m.
Depth-of-field preview button
Some people have difficulty using the depth-of-field preview. They only see a darker image but can't see increased depth-of-field.
Depth-of-Field Preview A feature on advanced SLRs that closes the lens down to the actual taking aperture to enable you to see the picture's depth-of-field.
Depth-of-Field Preview Button
Canons work instantly and silently. I wish everything worked this well. On the other hand, the button is on the wrong side of the Canons so it takes a second hand to use.
Depth-of-field preview
At the front of the camera beside the lens mount (hand grip side) is the Mark III's depth-of-field preview button.
Use your depth-of-field preview button (if you have one) to stop the lens down to its taking aperture to visually check and see if everything looks sharp. Take your time and allow your eye to adjust to the darkened image in the viewfinder.
Many SLRs have a depth-of-field preview button on the front of the camera. If you've ever hit a button on your camera and then wondered why your viewfinder went dark, that's the one.
3 Make use of the depth-of-field preview button. This effectively stops the lens down to the working aperture (which makes the viewfinder dark) and lets you see the graphics at work within the composition.
Most SLR cameras have a depth-of-field preview button to compensate for this. When you press it, the aperture closes down to the set f/stop. Although the viewfinder becomes darker, you can see the actual depth of field at the selected aperture.
Many DSLRs have a depth-of-field preview button; when you press it, the lens briefly closes to the shooting aperture, showing you the real depth of field. Be aware that when you press this button, the image in the viewfinder will temporarily darken.
If your camera has a depth-of-field preview, then use it often to see what’s in focus and what isn’t.
Practice and the depth-of-field preview button that is. By stopping the lens down to your shooting aperture and then moving the filter around, it becomes much easier to see the edge of the graduation.
Fortunately, this was apparent in the viewfinder with the depth-of-field preview button pressed, so the composition was made with an eye toward eventual cropping.
To help position your ND Grad, you can set your shooting aperture and hold down your camera's depth-of-field preview button if it has one. Although the viewfinder will be dark, it will show the transition more clearly.
in an SLR camera, to switch from the aperture fully open (for viewing/focusing) to the taking aperture (for depth-of-field preview, "stop-down metering", or taking the photo).
Therefore, many older DSLRs do not provide "live preview" (allowing focusing, framing, and depth-of-field preview using the display), a facility that is always available on digicams although today most DSLRs offer live view.
Use small apertures (such as f/16, f/22, or smaller) as necessary to ensure sharp focus throughout the image frame. Also, use live view and your depth-of-field preview to help achieve critical focus and optimal aperture.
It's nice to have things like depth-of-field preview and some form of mirror lock up (or prefire). Not essential, but nice.
 See also: Camera, Depth-of-field, Focus, Lens, Exposure
  
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