Stop

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"Stop" is the older term now generally replaced with EV or 'exposure value'. Stops relate to the stopping down of aperture, with the aperture ring having various click-points, usually at f1.4, f1.8, f2.8, f4, f5.6, f8, f11, f16, f22. To be precise, these stops should read f1/1.4, and so on, but were never marked as such on lenses. Some people still write f/1.4. Each full stop halves the amount of light entering the lens, and therefore corresponds exactly with doubling the shutter speed. By extension, photographers talk about '1 stop more of light', meaning twice as much light, and studio strobes are often selectable by 'stops' with the same numbering system as aperture stops.

G type lenses are not fitted with aperture rings, and stops are therefore purely notional. For this, and other reasons, the technical measure of doubling or halving light is now generally referred to as EV in print, although many photographers persist in saying 'stop'.