Even if you've only had your camera for a short period of time, you've almost certainly heard people describe exposure in terms of a "stop." It is a pretty typical expression used to describe a change in camera settings, but what exactly does it mean? Read on to find out.
Specifically, one stop is a doubling of exposure, or a doubling of the amount of light that reaches your camera's sensor. So you might say that an image that is overexposed by one stop, which means that you let in twice as much light as you needed to in order to get a correct exposure. If an image is underexposed by one stop, then that means that you let in half as much light as you needed to get the correct exposure.
The word "stop" can be applied to any of the three corners of the exposure triangle — aperture, shutter speed or ISO. So, for example, if you increase your ISO from 100 to 200, you are changing your exposure by one stop. If you increase the ISO from 100 to 200 and also reduce the shutter speed from 1/250 to 1/125, you are increasing your exposure by two stops. Any time you double the exposure or the amount of light that gets into your camera, that's a stop, regardless of which setting you manipulated to make that happen..
If I just lost you, let's break it down in terms of each individual point in the exposure triangle, starting with the simplest.
When you increase your ISO, you increase your camera's sensitivity to light. The numbers make it simple to calculate stops—when you double the ISO, you double the exposure, and that equals a one stop increase. So, ISO 200 is one stop above ISO 100, and ISO 400 is one stop above ISO 200, and two stops above ISO 100. That easy enough.
Shutter speed is a measurement of time—an exposure is made when your camera's shutter opens to let light in, and the amount of light that gets in is determined by how long the shutter remains open. The longer it's left open, the more light gets in. Figuring out stops based on shutter speed is slightly less straightforward than it is for ISO, it just involves some doubling of fractions as well as whole numbers, and it's done in the opposite direction—a shutter speed of two seconds is a stop above one second, and four seconds is a stop above two. To keep this straight, you just need to visualize what happens as the shutter speed gets slower—at two seconds, you're letting twice as much light into your camera as you are at one second, so you're doubling the exposure or increasing it by one stop. When you get into fractions, you double the bottom number for a faster shutter speed—something I'm sure you remember from primary school math even if you don't use it a lot today. So 1/250 is twice the speed of 1/125, but because at that speed only half as much light reaches your sensor, the smaller number of 1/125 represents a one stop increase in the exposure.
Now you'll notice that for some shutter speeds, the number isn't precisely doubled. For example, 1/60 is a one stop increase over 1/125, but twice 1/60 is actually 1/120. So yeah, it's an approximation, and I can't tell you much more than that other than to reassure you that despite those small inconsistencies the system works the way it is supposed to.
Of course to complicate matters, most modern cameras also let you work in thirds of a stop, so you'll have two additional settings between the whole stop settings. Here's what that looks like:
1/15, 1/20, 1/25, 1/30, 1/40, 1/50, 1/60, 1/80, 1/100, 1/125, 1/160, 1/200, 1/250
The bold numbers represent whole stops, and the numbers in between represent 1/3rd stop increments.
Now here's where you need to take things a little more on faith. Aperture is calculated in a more complicated way, and you don't really need to be aware of those calculations so much as you just need to be aware of each f/stop number along that scale of doubling. That f-number represents the size of the diaphragm inside your camera, the one that controls the amount of light that reaches the sensor during that however-many second exposure. When you double the exposure or increase aperture by one stop, you double the size of that diaphragm opening. For each doubling in size, there is a corresponding f-number:
f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45
So f/1.4 is a stop above f/2, and f/2 is a stop above f/2.8.
As you can see, you can't do anything so simple as double the f-number to find the next stop up, you really just need to remember all these numbers.
Like shutter speed, modern cameras also have some f-stops that don't correspond to the numbers on this scale. These are 1/3rd stops, for example, between f/2.8 and f/4 you'll have two additional settings representing a third stop each: f/3.2 and f/3.5. Not all cameras offer 1/3rd stops, so don't be surprised if you look at yours and find whole stops only. Just like with those 1/3rd stop settings between shutter speed stops, the 1/3rd stop settings for aperture give you a little more flexibility to fine tune your settings, but you can still get very high quality photos using only full stops.
Putting it all together
Now the reason why you have three different settings you can use to add or subtract stops from your exposure is because each setting not only alters the amount of light that reaches your camera's sensor, it also alters the way the image appears visually. A slower shutter speed, for example, will not only add stops to the exposure, it will also create motion blur if there's anything in the scene that happens to be moving. Conversely, a faster shutter speed will freeze fast-moving action, which will allow you to photograph sporting events and other fast-action scenes without motion blur.
Aperture changes your image in different ways—a narrow aperture (represented by larger f-numbers) will allow you to capture broad depth of field, or sharp detail from foreground to background. A wide aperture will give you shallow depth of field, which means you'll get a sharp subject and soft blur on background details.
Finally, ISO can affect the quality of your image—a photo shot with a low ISO tends to be very good quality, with a nice range of tones and good separation between colors. At higher ISOs, you might see the quality degrade a little, and at very high ISOs you might start to get digital noise, which is that sandy or grainy quality some images have, especially when shot in low light.
So when deciding which settings to manipulate in order to add or subtract stops from your exposure, you need to consider your goals for the image. For example, let's say you are photographing a landscape and your settings are f/11, 1/125 and ISO 100. When you review the shot on your camera's screen, you notice that the highlights are blown out, and you determine that you're probably about a stop overexposed. Because you typically want to acheive broad depth of field in a landscape, you'll probably want to make adjustments to your aperture—referring back to the f/stop scale from above, you can see that f/16 is a stop below f/11, so all you need to do is make that adjustment and you'll get the correct exposure.
Now, let's say you're shooting a sporting event and the light is a little low, and your settings are 1/500, ISO 400 and f/5.6. When you review your images you determine that they look like they're about two stops underexposed. You can't really adjust your shutter speed any slower than 1/500, because you know you'll start to get motion blur in your fast moving subjects. You need to find that extra two stops somewhere, but you don't want to double your ISO and then double it again, because you'll end up with a lot of noise in your shot. And you don't want your aperture to be too wide, because it will start to become difficult to keep your subjects on the plane of focus while they're moving back and forth across the field. In this case, you can comprimise by adding a stop from each setting—you double the ISO to 800 and you widen the aperture to f/4. That lets in two stops more light, and you get the correct exposure.
Now what if you got a good exposure, but you want your image to have a different appearance, such as more blur in the background? In that case, you borrow light from one setting and compensate for it by changing another setting. For example, if you're shooting a portrait at f/11, 1/125 and ISO 100 and you want a blurrier background, you can borrow two stops of light by widening your aperture to f/5.6. That will give you the blurry background you're going for, but unless you compensate using another setting, you're going to end up with an image that's two stops overexposed. So you make your shutter speed faster—1/500 instead of 1/125, which lets in two stops less light than your orignial setting. The result is the same exposure, but a blurrier background.
I hope this is all starting to make a little bit of sense to you. It does start to become second nature after you've had some experience playing with your settings, but to really understand it I think you need to spend some time in manual mode, watching what happens to your images as you add or subtract stops of light. Think of it as an experiment and don't let your fear of manual mode deter you—the images you capture during this experiment don't have to be winners, they just have to teach you something. A few hours of experimenting should be plenty, and then you'll almost certainly find yourself talking in stops naturally.
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