Tilt-Shift

Tilting the lens moves the plane of focus off its usual square angle, so a narrow band of the picture is sharp and everything above and below it falls away.

Seen from above, that band reads as a miniature. Human eyes associate a shallow focus band with looking at something small and close, so a real city photographed this way reads as a model of one.

Tilting the lens relative to the sensor swings the focus plane with it, which is described by the Scheimpflug principle. The focus plane is no longer parallel to the sensor, so it can cut through a scene at an angle.

The height of the camera decides whether the result reads as a miniature or as a fault. Looking down from a roof or a hill, the effect works. Level with the subject, a sharp band across the middle of frame just looks like a broken lens.

Shift is the other half of the movement, and it slides the lens across the frame without tilting it. That corrects converging verticals in architecture, which is a different job from the miniature look.

Use it on cities, crowds, traffic and rail yards seen from above, and for a sequence that should feel staged instead of lived in.

Avoid it at eye level and avoid it on faces. The effect depends on looking down, and it reads as a mistake from anywhere else.

Shallow focus comes from a wide aperture. The sharp zone is a plane parallel to the sensor, so a whole slice of the scene at one distance stays sharp.

A tilt-shift cuts the sharp zone across the frame at an angle, so things at the same distance can be sharp and soft at once. That is the check: same distance, different sharpness means tilt.

David Fincher shot the Henley regatta sequence in The Social Network (2010) with a tilt-shift, and the rowers read as toys on a model river. The effect separates the sequence from the rest of the film without a single cut away from it.

The model returns an ordinary shallow depth of field, which is a wide aperture and not a tilt. Say a narrow band of sharp focus runs across the frame with blur above and below it.

The other failure is a shot from ground level. Ask for a high angle looking down, or the miniature effect never arrives and the frame just looks wrong.