In the days before point and shoot cameras - back when SLR cameras didn't have any automatic features at all, everyone who wanted to take photographs had to understand the basic principles of exposure. Film was expensive and you just couldn't afford to waste a shot on a guess, so you had to really understand how your camera functioned and what settings you needed to use to ensure a correct exposure.
Today we have the great luxury of the "auto" setting, and because of that a lot of us have gotten lazy. So lazy, in fact, that there are some people who love photography, own DSLRs and consider themselves to be photographers on at least some level that still don't understand those very basic principles of exposure. And for those of us who do have a basic understanding, a quick review is never a bad thing. Ready?
In literal terms, aperture is the size of the opening in the lens that allows light to pass through. The bigger the opening, the more light is allowed to pass. Aperture is measured in terms of "f-stop," which basically just refers to the "fraction" (or "f") of light that is "stopped" before it reaches the camera's sensor.
Aperture can be a little confusing because the larger the f-number, the smaller the opening in the lens and the less the amount of light that reaches the sensor. (Hint: if you think of the f-number in terms of a fraction instead of a whole number, it starts to make more sense.) In practical terms, this means that the less light there is in a scene, the wider you need that opening to be and the larger the aperture: f/2.8 lets in a lot more light than f/11 or f/22.
But aperture does more than just regulate the amount of light that passes through your lens. It also has an impact on depth of field, which simply means that the amount of your scene that remains in focus will vary greatly depending on which f-stop you choose. If you want both your subject and background to be in sharp focus, you will need to choose a smaller aperture, such as f/22. If you want only your subject to be in sharp focus, with the background blurred out, choose a larger aperture, such as f/2.8. Of course depending on a few other things such as the amount of ambient light there is and whether or not you brought along a tripod, you may not always be able to select the f-stop you want (which is where ISO comes in, explained later in this article).
The second basic principle of exposure is shutter speed, which means exactly what it sounds like it means: the amount of time it takes the shutter to open and close. The shutter is that little device inside your camera that allows light in and then shuts it out - just like the shutter on a window. The shutter is what captures the moment that becomes your photograph. Think of it as the equivalent of covering your eyes with both hands: take your hands off your eyes for just a split second, and you will see a brief, frozen moment; but take your hands off for a few seconds and you will be able to follow some of the motion in a scene. Your camera's shutter does the same thing.
Shutter speed is measured in terms of seconds or fractions of a second. A speed of 1/1000, for example, is literally 1/1000th of a second. A shutter speed of 1/60 is just 1/60th of a second, which is still pretty fast in terms of the way humans see the world, but for a camera is a lot slower than 1/1000.
The world moves pretty fast, so to freeze action (such as a sports game or a running horse) you will need to choose a fast shutter speed. Conversely, if you want to capture motion blur you will need to choose a slower shutter speed. Like aperture, the shutter speeds that will work in any given situation depend greatly on how much ambient light there is in a scene. Darker scenes require that you allow more light in, which means you will need to choose a slower shutter speed. So you may not be able to freeze the action during a soccer game at dusk, or if that horse starts kicking up its heels 30 minutes before sunrise. And on a very bright day it may be difficult to capture motion blur in a waterfall or on a tourist-packed footbridge.
Related: Shutter Speeds You'll Use Everyday
ISO is the third basic exposure principle, and it can help you in situations where you want to select a certain aperture or shutter speed but may not be able to because of the amount of light that is present (or is not present) in your scene.
ISO stands for "International Standards Organization," which makes no sense at all unless you understand the history of ISO, which originally referred to film speed. In the early days of film, there were multiple standards for measuring film speed, and it wasn't until the 1970s that an international standard was finally developed (hence the acronym "ISO"). In those olden-days, ISO referred to a film's sensitivity to light, and could only be changed when the photographer put a new roll of film in his camera. Today ISO refers to the camera's sensitivity to light, and it can be changed on the fly, frame by frame if the photographer chooses.
The ISO number doubles as the light sensitivity increases. So an ISO of 1600 is twice as sensitive to light as an ISO of 800, which is twice as sensitive as 400 and so on.
So how does this help you? Well, let's say you are shooting that horse kicking up its heels in the pre-dawn light. If you are using an ISO of 100 (which is a setting that makes your camera less sensitive to light), you will not be able to freeze the action and you will most likely end up with a lot of blurry horse. That's because the camera won't have enough light to capture the shot, so it opens the shutter for longer to get enough light. That's what causes the blurry photos. If you change your ISO to 1600 or 3200, however, you'll be able to use a much faster shutter speed and increase your chances of getting perfect frozen-in-time shots of bucking and cavorting. At high ISOs, you'll also be able to ditch that flash and still get sharp photos in low light.
Of course, it is generally preferable to use a lower ISO, because in most cameras images shot with a low ISO are better in quality, with greater detail and less noise. As advances in DSLR technology continue, however, this becomes less of a problem. Many modern DSLRs are capable of producing images at high ISOs that have very little noise and impressive detail compared to their predecessors of only a few years ago.
Related: ISO Explained
So now that you've mastered these three basics, it's time to abandon that "auto" setting for good and start thinking about each scene in terms of how you want it to look in the final image. Do you want the background to be blurry or in focus? Pay attention to your camera's f-stop. Do you want to freeze action or capture motion blur? Adjust your shutter speed. And are you shooting in low light or bright light? Change your ISO accordingly. If you can get to the point where these three principles come naturally, you may never need to use that "auto" setting again, and good riddance.
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