Fog

Fog takes the background away. Whatever was behind the subject is gone, so the frame holds one person and a pale wall of air, and the audience has no idea what is out there.

That absence is the point. A scene in fog is a scene where something can arrive without warning, and the shot never has to show a threat to imply one.

Thick fog cuts visibility to a few metres. Anything past that distance loses its contrast and then disappears, so the far planes come back as flat nothing.

It also changes the light. Every source scatters, so a lamp becomes a glow with no clear edge and a hard key softens into something with no shadow left to give. Backlight is what makes fog read: put a source behind it and the beam gains a body.

On a set this is a fog machine and a fan. In a prompt it is a distance, so say how far you can see.

Use it when the world should shrink to what is within reach. It suits a search, or a scene where somebody is lost.

Avoid it when the location matters. Fog deletes the location, so a shot that was meant to say where you are says nothing at all.

Mist sits low and leaves the air above it clear, so you can still see the far side of a field. Fog fills the whole frame from the ground up. If the sky is visible, it is mist.

Haze is the thin version and it does the opposite job, because it separates the planes rather than deleting them. Fog removes depth. Haze builds it.

Akira Kurosawa shoots the forest sequences of Throne of Blood (1957) in heavy fog, so paths and buildings appear and vanish and the geography stops being reliable. John Carpenter makes the same condition the antagonist in The Fog (1980), where the bank rolling in from the sea is the thing the town is afraid of.

The frame comes back empty. Fog with nothing near the camera is a grey rectangle, so give the shot a near object such as a lamp or a railing and let the eye hold on to it.

The other result is mist. Models soften the request into a pretty low drift unless you state the visibility, so say the distance is cut to a few metres and the background is gone.