A stop expresses a factor-of-two change in exposure or light level. One stop more means twice as much; one stop less means half as much. Two stops correspond to a factor of four, and three stops to a factor of eight.
Stops and lens markings
The familiar full-stop aperture sequence is f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11 and f/16. Opening from f/4 to f/2.8 increases exposure by one stop; closing from f/4 to f/5.6 reduces it by one stop. The numbers are conventional rounded markings, not a sequence that increases by one each time.
Practical example
If a light measures 800 lux at the subject and a modifier reduces the reading to 400 lux, the measured loss is one stop. A reading of 200 lux is two stops below the starting point. Keep the meter position, direction and fixture settings the same when making that comparison.
Meter displays and dimmer percentages
A meter displaying f/4 plus 0.3 stop is not indicating an aperture of f/4.3; read the display convention in its manual. Likewise, moving a fixture’s dimmer from 100% to 50% is a one-stop reduction only if the measured output actually halves. Changing ISO alters the camera’s exposure setting or signal handling; it does not add light to the subject.
Related terms
Lux · Dimming Curve · Lighting Ratio · Incident Light Meter
References
Nikon: aperture and f-stop markings · Fujifilm: understanding stops