EV stands for the nondescript term “exposure value”. In plain language, an EV is a chunk of light equivalent to a full f-stop, or a full standard shutter speed, or any combination of the two adding up to a full step.
Unlike shutter speeds and f-numbers that have geometric progressions, EV steps march along in ordinary whole numbers: EV 1, 2, 3, 4….. And unlike those wrong-way f-numbers that are big when the aperture is small and vice versa, EV numbers are logical: big to represent bright light, small for dim light.
For EV readings to make sense, though, you have to state an ISO rating-the sensitivity of your film or imaging sensor. ISO 100 makes a good benchmark. At ISO 100, EV 16 is beach under blazing sun, whereas EV 0 is dark enough that people have great difficulty reading, and most DSLR camera AF systems give up.
Another nice thing about EV is that it gives you a quick, accurate guide to the range of lighting in a scene or studio. A range of EV 4 to EV 8 is 4 stops.
That’s another great thing about EV: even the fractions make sense. With EV, 0.3 is 1/3 stop, 0.5 is ½ stop, 0.7 is 2/3 stop. Period. So from EV 4.3 to EV 8.3 is 4 stops.
If EV makes so much sense, why don’t DSLR cameras use it for exposure settings? Actually, most of today’s DSLR cameras do, for exposure-compensation control. Want the picture a stop darker? Just dial in -1 EV.
Accessory flash units almost always use EV.
And handheld lightmeters often provide EV readouts. We use EV reading meters in the Pop Photo Lab to set light levels for things like AF tests. It is easy with EV.