F-stop

The aperture's setting, written as f-numbers: each stop halves or doubles the light. The dial where exposure and depth trade.

The f-stop scale is logarithmic bargaining: f/2 to f/2.8 halves the light, and every stop bought or spent moves two prices at once, exposure and depth of field. Wide open, the image goes shallow and luminous; stopped down, deep and demanding of light.

Cinematographers speak in stops the way musicians speak in intervals, a stop over, half a stop under, because the unit is perceptual: one stop feels like one step everywhere on the scale. The craft is choosing which price to pay, light is addable, depth is not, and holding the choice consistent across a scene's coverage.