Very quick history lesson: Old cameras used to stop down as you changed the aperture, fine for taking, not fine for focus and composition checks, as the viewfinder would become darker, the more you stopped down.
In the mid sixties cameras started to appear with full aperture metering through the lens. Through couplings on the cameras aperture ring you could stop the lens down, and the meter would compensate, but the aperture would stay open, so that the viewfinder was nice and bright to assist focus / composition. The aperture diaphragm would close with the exposure.
If you are metering during depth of field preview and using this as the exposure guide, you are reducing exposure drastically.
Trust the camera. Use dof_preview for checking depth of field, nothing else.
On some FD type cameras the stop down button has two positions.
One for stop down preview, and one for stop down lock.
This was required for the FD series because some cameras and lenses pre-dated full aperture TTL.