If you’re just getting into photography and you’ve been reading everything you can get your hands on to learn about taking great photos like I did, you’ve undoubtedly read about exposure value, usually abbreviated as EV. But exactly what is exposure value, and for that matter, exposure itself?
Simply put, exposure is the combination is ISO, aperture, and shutter speed that produces an image. A properly exposed image will neither have extremes in the highlights (overexposure), or lost detail in the shadows due to excessive darkness (under exposure).
You’ve probably also read or heard references to exposure such as “increased exposure by 2 stops”, or when dealing with overexposure, “adjust exposure compensation by -2 EV”. Most guides aren’t too forthcoming in explaining exactly what an exposure stop is, and for that matter the units of measurement. We’re going to look at that next.
What is EV?
EV is a relative unit of measure that refers to a particular combination of shutter speed and aperture. It’s known as a relative unit of measure because it has no specific units unlike, for example, units of length which might be measured in feet or meters.
The following chart indicates the shutter speed for a given f-number (aperture) and EV (source: wikipedia). You’ll notice that ISO doesn’t appear on the chart. It’s taken into account in the EV. The camera will actually meter a different EV depending on the ISO that’s been selected. That EV will then be used to determine aperture and shutter speed.
EV really doesn’t have much value though unless you are comparing it to another EV. Rather than look at absolute values of EV like shown in the chart, it’s much more useful to look at how we can increase or decrease exposure by 2 EV, for example. In this case it would be in reference to the metered exposure from the camera.
Each unit of EV is called a stop. So another way of saying, “Increase exposure by 2 EV” would be to say, “Increase exposure by 2 stops”. They both mean the same thing.
What is an Exposure Stop?
One exposure stop (i.e. 1 EV) is defined as the equivalent to doubling or halving the amount of light in a photo. And this change of light quantity can be made using any combination of adjustments to ISO, aperture, and shutter speed.
Doubling the ISO, for instance from 100 to 200, has the effect of doubling the camera’s sensitivity to light, and would correspond to an increase in exposure of 1 stop.
Doubling the aperture (lens opening) would also have the effect of increasing exposure by 1 stop. Aperture has its own measurement known as f-stops. It’s not a linear scale, however, so you’ll need to refer to a chart such as the one above to determine what constitutes a full stop.
For example, a change in aperture from f/11 to f/8 is a one full f-stop change. A change in aperture from f/1.8 to f/1.4 is only a 2/3 f-stop increase.
If ISO and shutter speed are kept constant, then a 1 f-stop change in aperture corresponds to a 1 stop change in exposure as well.
Shutter speed is measured in terms of the amount of time the shutter remains open when capturing a photo with the camera. For example, a shutter speed of 1/250 means that the shutter stays open for 1/250th of a second to record the shot.
A 1 EV change in exposure can therefore be obtained by either doubling or halving the shutter speed, assuming that ISO and aperture are kept the same. Using the above example, a shutter speed of 1/125 would mean a 1 stop increase in exposure, and a shutter speed of 1/500 would correspond to a 1 stop exposure increase.
Most modern DSLR cameras have some form of an exposure compensation setting, sometimes also known as exposure bias. This allows you to dial-in a compensation factor measured in EV units that will be applied to all photos taken.
Let’s say for example, you found that many of the photos you are taking have blown-out (overexposed) highlights. This is normally bad, because any detail that was included in the pure-white areas of the photo have been lost forever. You can attempt to mitigate this problem by setting a negative exposure compensation such as -1 EV. This will tell the camera to choose ISO, aperture, or shutter speed, to consistently underexpose every photo by 1 stop. This will most likely result in the majority of the photo being too dark, but this is easily corrected in post-production using software such as Adobe Lightroom.
Maintaining Exposure while Adjusting Settings
Having a good understanding of exposure will also help you when adjusting ISO, shutter speed, and aperture for artistic effect. The most common case of this will be when you want to adjust the depth of field to bring more or less of the photo into focus. Depth of field is controlled in large part by the aperture setting.
As an example, let’s say you want to increase aperture by 2 f-stops to get a more shallow depth of field to really make the subject of your photo stand out. To maintain the proper exposure, that means that either ISO, shutter speed, or some combination of the two need to also be adjusted to allow less light in compensation for the increased aperture.
It turns out there are 3 different ways you could do this:
- Decrease ISO by a factor of 4 (1/2 * 1/2 = 1/4)
- Increase shutter speed by a factor of 4 (2 * 2 = 4)
- Cut ISO in half and double the shutter speed
Each of the above options would have the same overall effect as far as exposure is concerned, but you may favour one of the others for practical, artistic, or quality reasons:
- You may not be able to change ISO, shutter speed, or aperture if already at the lower or upper limits of the camera’s capabilities
- Increasing ISO may not be desirable for fear of introducing more noise into the photo
- You may not want to decrease shutter speed because you are not using a tripod and don’t want to introduce camera shake
- You may not want to change shutter speed because you are trying to achieve a motion blur or freeze motion
- You may not want to adjust aperture because depth of field is already appropriate for the shot
Your camera’s program modes
You’ll really only need to worry about the above exposure adjustments if working in your camera’s manual (M) mode, in which you need to set ISO, aperture, and shutter speed explicitly.
When using your camera’s Aperture priority (A / Av) mode, shutter speed is automatically selected by the camera to achieve proper exposure. And when taking photos in Shutter priority (S / Tv) mode, aperture is chosen automatically.
It’s still very beneficial to have a good understanding of exposure, stops, and how various combinations of ISO, shutter speed, and aperture work together. I hope this guide was beneficial to you!
I’d love to hear your feedback on this guide to understanding exposure. How to you deal with exposure compensation in your photography?