When a lens is focused on a subject, there is a certain range behind and in front of the subject that remains sharp. This range is called "depth of field", and one way to control it is to adjust the aperture. The photos show how depth of field varies with the aperture selected: A. Large apertures (e.g., f/1.7) yield a shallow field of sharp focus, rendering the background and foregraund unsharp; B. Small apertures (e.g., f/22) yield greater depth of field with more of the scene in focus. Refer to depth-of-field scale on the lens to determine approximate depth of field. To check more closely, use preview switch on camera body.
At a given aperture and focal length, depth of field also varies with subject distance: When the lens is focused on a close subject, depth of field is less; when focused on a distant subject, depth of field is greater.

Preview switch
The preview switch can be used in all exposure modes to close down and lock the lensī aperture. This enables you to determine whether depth of field is sufficient. The switch is hinged and folds up against the handgrip when not in use. To use:
  1. Focus on main subject.
  2. In A or M mode, set desired aperture. In P or S mode, meter scene in the usual way.
  3. With film advanced, press preview switch partway down and release it. Lens aperture will now be locked at the setting displayed in the data panel. Also, "F" blinks in data panel while preview switch is used.
  4. Look through the viewfinder to determine depth of field. When lens is closed down, autofocusing does not operate and it is not possible to adjust aperture setting. Also, image in viewfinder may appear dark, but this has no effect on exposure.
  5. To cancel preview operation: Press preview switch all the way down and release it. Aperture setting can now be adjusted by using the up/down controls. Preview operation is cancelled automatically if shutter is released with lens stopped down.