Scheimpflug Principle
The rule that when a lens is tilted, the subject plane, lens plane and image plane all intersect along a single common line.
Formula / rule
Subject, lens and image planes share one line
Measured in
degrees
In depth
The principle explains what a tilt movement actually does: rather than deepening focus, it swings the plane of sharpness away from parallel so it can lie along a receding surface. A small tilt produces a large swing, and the depth of field becomes a wedge widening with distance rather than a slab.
Examples in practice
Two degrees of forward tilt lays the plane of focus flat along a table top, rendering it sharp from front edge to back.