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.
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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.
For me, learning about absolute Exposure Value also made exposure click. All this abstract talk of stops suddenly took on real, concrete meaning. I could understand why certain combinations were equivalent. So don’t feel the need to memorize all the values in this article; instead just try to understand them.
The EV100 Scale
The EV100 value of 0 is the combination of an aperture of f/1.0 and a shutter speed of 1 second. Everything else is based on that. This means that your camera and lens can, without using any extra kit, use EV100s of between -1 and +21. This is one of the reasons you need special gear to take good photos of the night sky which has an EV100 of between -3 and -11, depending on what the moon, stars, and Aurora are at.
Here’s a full table of EV100 values from Wikipedia. It does a really good job of showing which combinations of aperture and shutter speed match up with which EVs.
More interesting, I think, than seeing how shutter speed and aperture match up, is to see what light levels correspond to what EVs. While your camera can go up to +21, you’re not likely to see EVs much higher than 16 in the real world.
|16||Snow on a sunny day|
|14||Hazy, some clouds|
|12||Overcast, shady areas on a sunny day, sunrise and sunset|
|9 to 11||Just before sunrise and after sunset, the blue hour.|
|8||Bright street light, bright indoor lighting|
|5 to 7||Indoor lighting. Bright window light.|
|2 to 4||Dim window light.|
|-1 to 1||Dark morning before sunrise, dark evening after sunset.|
|-2 to -3||Moonlight from a full moon.|
|-4||Moonlight from a gibbous moon.|
|-5 to -6||Moonlight from quarter moon, bright aurora.|
|-7 to -8||Stars and starlight.|
|-9 to -11||Milky Way center.|
The above table is a ballpark, but a pretty accurate one. There’ll always be some variation but if you follow it, you won’t be too far out.
Using Exposure Value
As I said earlier, understanding Exposure Value is more useful for your photography in an abstract sense than in a practical sense, but that doesn’t mean there aren’t ways you can’t use it.
If you’re shooting long exposure images with a neutral density filter, you can shoot your test shots without the filter and then, add the filter, add whatever the filter’s stop value is to your current EV, and work out your new shutter speed setting using the EV chart above. You can also use an online EV calculator; it will probably quicker and you’ll also be able to calculate EV values for ISOs other than 100.
The other way to use EVs in the real world is through the Sunny 16 Rule. This rule says that if it’s sunny out, set your aperture to f/16 and your shutter speed for a proper exposure will be 1/[Your ISO], so in our case 1/100. If you look up at the chart, you’ll see that f/16 for 1/100th of a second has around an EV100 of 15 which nicely lines up with a sunny day. The thing is, you can then use this as the base for working out correct settings for other lighting situations. A slightly cloudy day needs f/11 at the same shutter speed and ISO, a single stop more. A very cloudy day needs f/8, a heavily overcast day needs f/5.6, and the light around sunset needs f/4.
While you should always review your shots to make sure you’re not blowing out your highlights or crushing your shadows, it’s pretty neat to be able to quickly guess at camera settings and be in the right ballpark.
One of the main reasons people struggle to understand exposure is they try to learn it in the abstract. If you understand how it relates to the real world through exposure values, it’s a much simpler concept to grasp.