Hello! I recently purchased Nikon FM2n with a Nikkor 50mm 1:1.8 lens. I've been trying to figure out the DOF preview, but I can't seem to get it. When I press the DOF down on F1.8 the image is clear, but when I press it down on F22 the image is completely black. I thought the background would be darker on F1.8 since it has a shallow depth of field (Makes the backgrounds blurry?). I also can't tell what's in the depth of field vs not, on the the other F stops. Please help and correct me if I'm wrong. Thanks.