There's a funny caveat to this.

According to RAW, if I duck behind a rock, I get defense from cover or armor, not both.

However... if I'm concealed , which is fair interpretation of ducking behind a rock (it should qualify as "intervening terrain") I can get 1-3 setback dice that are not defense (and therefor stacks) on attack rolls towards me.

If you can't get cover, claim concealment.

Popular thing to do at my tables is shoot the lights and then use a flashlight on the baddies. Sometimes the GM gives them a boost because they can just shoot at the light, but it doesn't negative the 2 or 3 setback from the darkness.