Proper exposure is crucial to creating a good image, and if you’re not sure what’s going on inside the camera it can be diffi cult to grasp many of the concepts you will read about in this book. Your camera can, and will, make a lot of decisions during the shooting process, and it’s important that you understand what it is doing and why. Here I’ll cover some of the basic principles of exposure, so you have the tools to move forward and improve your photography.
Exposure is the process whereby the light refl ecting off a subject refl ects through an opening in the camera lens for a defi ned period of time onto the camera sensor. The combination of the lens opening, shutter speed, and sensor sensitivity is used to achieve a proper exposure value (EV) for the scene. The EV is the sum of the components necessary to properly expose a scene. A relationship exists between these factors that is sometimes referred to as the “Exposure Triangle.”
At each point of the triangle lies one of the factors of exposure:
- ISO: Determines the sensitivity of the camera sensor. ISO stands for the International Organization for Standardization, but the acronym is used as a term to describe the sensitivity of the camera sensor to light. The higher the sensitivity, the less light is required for a good exposure. These values are a carryover from the days of traditional color and black and white fi lms.
- Aperture: Also referred to as the f-stop, this determines how much light passes through the lens at once.
- Shutter speed: Controls the length of time that light is allowed to hit the sensor.
Here’s how it works. The camera sensor has a level of sensitivity that is determined by the ISO setting. To get a proper exposure—not too much, not too little—the lens needs to adjust the aperture diaphragm (the size of the lens opening; think of it as the pupil in your eye increasing and decreasing in size) to control the volume of light entering the camera. Then the shutter is opened for a relatively short period of time to allow the light to hit the sensor long enough for it to record on the sensor.
ISO numbers for the 7D start at 100 and then double in sensitivity as you double the number. So 200 is twice as sensitive as 100. The camera can be set to use 1/2- or 1/3-stop increments, but for ISO just remember that the base numbers double: 100, 200, 400, 800, and so on. There are also a wide variety of shutter speeds that you can use. The speeds on the 7D range from as long as 30 seconds to as short as 1/8000 of a second. When using the camera, you will not see the “1” over the number, so you will need to remember that anything shorter than a second will be a fraction. Generally
speaking, for handheld photography you should stay at 1/60 of a second for the slowest speed (or 1/30 if you have steady hands), so you will most likely be working with a shutter speed range from around 1/30 of a second to about 1/2000. These numbers will change depending on your circumstances and the effect that you are trying to achieve. The lens apertures will vary slightly depending on which lens you are using. This is because different lenses have different maximum apertures. The typical apertures that are at your disposal are f/4, f/5.6, f/8, f/11, f/16, and f/22.
When it comes to exposure, a change to any one of these factors requires changing one or more of the other two. This is referred to as reciprocal change. If you let more light in the lens by choosing a larger aperture opening, you will need to shorten the amount of time the shutter is open. If the shutter is allowed to stay open for a longer period of time, the aperture needs to be smaller to restrict the amount of light coming in.
HOW IS EXPOSURE CALCULATED?
We now know about the exposure triangle—ISO, shutter speed, and aperture—so it’s time to put all three together to see how they relate to one another and how you can change them as needed.
When you point your camera at a scene, the light refl ecting off your subject enters the lens and is allowed to pass through to the sensor for a period of time as dictated by the shutter speed. The amount and duration of the light needed for a proper exposure depends on how much light is being refl ected and how sensitive the sensor is. To fi gure this out, your camera utilizes a built-in light meter that looks through the lens and measures the amount of light. That level is then calculated against the sensitivity of the ISO setting and an exposure value is rendered. Here’s the catch: There is no single way to achieve a perfect exposure because the f-stop and shutter speed can be combined in different ways to allow the same amount of exposure. Deciding which combination to use is one of the fun, creative parts of being a photographer.
Here is a list of reciprocal settings that would all produce the same exposure result. Let’s use the “sunny 16” rule, which states that, when using f/16 on a sunny day, you can use a shutter speed that is roughly equal to your ISO setting to achieve a proper exposure. For simplifi cation purposes, we will use an ISO of 100.
If you were to use any one of these combinations, they would each have the same result in terms of the exposure (that is, how much light hits the camera’s sensor). Also take note that every time we cut the f-stop in half, we reciprocated by doubling our shutter speed. For those of you wondering why f/5.6 is half of f/8, it’s because those numbers are actually fractions based on the opening of the lens in relation to its focal length. This means that a lot of math goes into fi guring out just what the total area of a lens opening is, so you just have to take it on faith that f/5.6 is half of f/8 but twice as much as f/4. A good way to remember which opening is larger is to think of your camera lens as a pipe that controls the fl ow of water. If you had a pipe that was 1/2” in diameter (f/2) and one that was 1/8” (f/8), which would allow more
water to fl ow through? It would be the 1/2” pipe. The same idea works here with the camera f-stops; f/2 is a larger opening than f/4 or f/8 or f/16.
Now that we know this, we can start using this information to make intelligent choices in terms of shutter speed and f-stop. Let’s bring the third element into this by changing our ISO by one stop, from 100 to 200.
Notice that, since we doubled the sensitivity of the sensor, we now require half as much exposure as before. We have also reduced our maximum aperture from f/2 to f/2.8 because the camera can’t use a shutter speed that is faster than 1/8000 of a second.
So why not just use the exposure setting of f/16 at 1/250 of a second? Why bother with all of these reciprocal values when this one setting will give us a properly exposed image? The answer is that the f-stop and shutter speed also control two other important aspects of our image: motion and depth of fi eld.
You will hear the term stop thrown around all the time in photography. It relates back to the f-stop, which is a term used to describe the aperture opening of your lens. When you need to decrease the exposure, you might say that you are going to “stop down.” This doesn’t just equate to the aperture; it could also be used to describe the shutter speed or even the ISO. So when your image is too light or dark or you have too much movement in your subject, you will probably be changing things by a stop or two.