Exposure language
EV is an abbreviation for exposure value. It is a way to express exposure in one number, i.e. the combination of shutter speed, aperture and ISO. To be quite honest, I doubt that many use EV’s today, as most rely on the exposure meter in their cameras, possibly in combination with the histograms.
But you can study EV as a way to understand exposure better. For example, if you shoot at f/4 and 1/125th, you are at EV 11, but if you open up the aperture to f/5.6 and maintain the shutter speed, you are now at EV 12. This is referred to as going one stop up. So walking up and down the EV scale is referred to as stops of light. The light is doubled or halved for each stop. This gives you a “settings independent” way of talking about exposure, and you can say go one stop up in aperture and one down in shutter speed and arrive at the same EV. So shooting at f/4 and 1/125th gives the same EV as shooting at f/5.6 and 1/60th. I half the light that comes though the lens (f/4 -> f/5.6) but double the exposure time (1/120th -> 1/60th). Both are EV 11.
For simplicity I have kept the ISO out of the description above, but it is the same logic: doubling of halving the ISO is the same as going one stop up or down in exposure. So the ISO scale is: 100 – 200 – 400 – 800 – 1600 etc. Many cameras allow you to select ISO values more granularly than full stops, and that can be handy, but remember to double/half to make a full stop difference.
Another area where EV is useful is when it comes to the spec list for a camera. The cameras dynamic range is expressed as an EV. For example the Nikon D750 over at DxOMark is listed as having a dynamic range of 14.5 EV’s, whereas the Nikon D810/D850 has 14.8 EV’s. Higher is better, as it enables you in one shot to span across more difference in light in the scene, without blowing out the highlights or loosing details in the shadows. So if dynamic range is important for your kind of shooting, then the EV for a camera you consider buying could prove an important part of your decision making.