Originally posted by kjg48359 As a side note, on the modern camera bodies, the "on" switch has a "diaphragm" icon when you toggle it over (past the "on" setting),the camera will stop down the lens for you (to aid in previewing the depth-of-field).
Some of the Pentax bodies DOF preview has to be mapped to one of the button. Also note there are two DOF preview modes: Optical and Digital. Optical will stop down the lens (provided it has a lever) and you can see the results in the viewfinder. Digital will take an exposure into the image buffer (not saved to the card) and you can view it in the LCD screen.
If you are using manual focus lenses this should be set to "Optical". Provided the body supports this function (flagship models) operating the optical DOF preview in manual mode will stop down the lens and turn on the meter and exposure bars. You can then set the exposure with the eDials and/or the aperture ring (if any) on the lens.