In photography, we talk a lot about “stops”: it’s the standard measure of exposure where an increase of one represents a doubling of the amount of light hitting the sensor or film. One thing that a lot of photographers don’t realize is that exposure actually has an absolute scale. Let me explain.
RELATED: What Is a “Stop” in Photography?
Exposure Values and Stops
When you learn the basics of the exposure triangle—shutter speed, aperture, and ISO—it’s important to know that there are multiple combinations of aperture and shutter speed that give the same exposure, even if the photo might look different because of your chosen aperture or shutter speed. For example, if you were shooting a portrait outdoors and wanted a shallow depth of field, you might go with f/2.0 for 1/2000th of a second; a few moments later if you instead decided to shoot a landscape, you could use f/16 for 1/30th of a second. In both instances, the exact same amount of light hits the sensor, so the brightness and exposure of everything will be identical, but the photos will look totally different because of the different aperture and shutter speed.
But how do you know which combinations to use? Sure, you can go with trial and error, but there is actually a definitive scale that’s seldom taught. Both f/2.0 for 1/2000th of a second and f/16 for 1/30th of a second have an Exposure Value at ISO 100 (EV100) of 13. There are lots of other combinations that also have an EV100 of 13 like f/8 for 1/125th of a second or f/4 for 1/500th of a second.
And here’s where things get even neater: an EV100 of 13 actually corresponds to some real-world lighting conditions. A cloudy day or the sky just before sunrise generally has an EV100 of 13, so any combination of aperture and shutter speed that also has an EV100 of 13 will work perfectly.
Why Exposure Value is Worth Understanding
Before going further, I want to step back and explain why EV is worth understanding; it’s unlikely you’ll ever need to break out EV tables to calculate what shutter speed to use while you’re on a shoot.
Instead, what an understanding of EV gives you is a deeper understanding of what your camera is doing and why. I’m a big believer that every photographer can benefit from knowing what’s going on with their camera when they press the shutter button. It’s this kind of knowledge that lets you pick the right light meter mode or autofocus settings without just guessing.
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from How-To Geek http://bit.ly/2GPVEdx
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