Canon PowerShot G12 What Is Exposure?

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In order for you to get the most out of this book, I need to briefly discuss the principles of exposure. Without this basic knowledge, it will be difficult for you to move forward in improving your photography. Granted, I could write an entire book on exposure and the photographic process—and many people have—but for our purposes I’ll just cover some of the basics. This will give you the essential tools to make educated decisions in determining how best to photograph a subject.

Exposure is the process whereby the light reflecting off a subject passes through an opening in the camera lens for a defined 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 these components necessary to properly expose a scene.

The relationship between these factors is sometimes referred to as the “Exposure Triangle,” the corners of which are made up of the following:

  • 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 films.
  • 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) 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 G12 start at 80 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 is also a wide variety of shutter speeds that you can use. The speeds on the camera range from as long as 15 seconds to as short as 1/4000 of a second. Typically, you will be working with a shutter speed range from around 1/30 of a second to about 1/2000, but these numbers will change depending on your circumstances and the effect that you are trying to achieve.

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 a 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 calculat ed?

We now know about the exposure triangle—ISO, shutter speeds, 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.

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 give some additional exposure, you might say that you are going to “add a stop.” 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.

When you point your camera at a scene, the light reflecting 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 reflected and how sensitive the sensor is. To figure 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 to come up with an exposure value. Here is the tricky part: 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. See, I told you it was tricky.

Here is a list of reciprocal settings that would all produce the same exposure result. Let’s use a traditional “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 the ISO setting to achieve a proper exposure. For simplification purposes, we will use an ISO of 100. (I’ve included f-stops higher than f/8 to demonstrate the point, even though the G12 is capable of only f/8.)

Reciprocal Exposures: ISO 100

If you were to use any one of these combinations, they would each have the same result in terms of the exposure (i.e., 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. (If you’re wondering why f/8 is half of f/5.6, it’s because those numbers are actually fractions based on the opening of the lens in relation to its focal length. That’s an evasive way of saying that a lot of math goes into figuring out just what the total area of a lens opening is, so you just have to take it on faith that f/8 is half of f/5.6.) A good way to remember which opening is larger is to think of your camera lens as a pipe that controls the flow of water. If you had one pipe that was 1/2″ in diameter (f/2) and one that was 1/8″ (f/8), which would allow more water to
flow through? It would be the 1/2″ pipe. The same idea works here with the camera f-stops; f/2.8 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 in the third element by changing our ISO by one stop, from 100 to 200.

Reciprocal Exposures: ISO 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/2.8 to f/4 because the camera can’t use a shutter speed that is faster than 1/4000 of a second.

So why not just use the exposure setting of f/8 at 1/1000 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 field.


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