Originally posted by Iowa Tom Where do I find the custom function one?
Not "custom function one ...", but rather "custom function. One ...". That is, it isn't the first custom function, but further down the list.
You also need to know that the camera cannot read the aperture ring, so you have to shoot in M mode. Use the green button to get the camera to stop down momentarily, meter, and set an appropriate shutter speed. Or hold the DOF preview knob as long as you like, watch the meter display, and adjust the shutter speed and//or aperture.