Shutter Speed
The length of time the shutter stays open, controlling both total exposure and how much subject or camera movement is recorded as blur.
Formula / rule
t = 1 / denominator
Measured in
seconds
In depth
The standard series doubles or halves at each full stop: 1/1000, 1/500, 1/250, 1/125 and so on. Shutter speed is the only exposure control with no optical side effect on the image other than motion rendering, which makes it the natural variable to sacrifice when depth of field and noise are already fixed by the brief.
Examples in practice
1/1000 s freezes a sprinting dog mid-stride; 1/15 s during a deliberate pan turns the same dog sharp against a streaked background.