exposure stops

In photography, a stop is a relative unit of measurement denoting overall exposure of a photograph. Each stop is always double the exposure of the previous stop, and half of the next stop. For example, to raise the shutter speed by “two stops” means to make it two times as bright by doubling the time the shutter is open.

Measurement name(s) Type Rising example
Shutter speed Linear scale \({1\over200}\ \mathrm{s}\)\({1\over100}\ \mathrm{s}\)
Film speed
Sensor speed
Linear scale ISO 100 → ISO 200
Aperture size
\(\sum(\sqrt{2})^{n}\), round to nearest tenth \(f/4\)\(f/2.8\), where \(4=(\sqrt{2})^{4}\) and \(2.8=(\sqrt{2})^{3}\)
Stops in other units
For the latter of the pairs of example stops above, the exposure should be exactly two times greater, relative to the former stop.

The term stop comes from aperture stop, the physical medium used to block incoming light in an optical system. One-stop units are also known as EVs (Exposure Values). Additionally, shutters, film, sensors, and apertures that take in more light significantly faster (and as such produce fully-exposed images faster) are also known to be “faster”. For example, a 50mm prime lens with a large aperture of f/1.4 would be considered a “fast” lens, and ISO 3200 film is “fast” film. Conversely, the same lens at f/16 would be “slow”, and ISO 100 “slow” film.

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