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This chapter is from the book

What Is Exposure?

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. For our purposes, I will just cover some of the basics of exposure. This will give you the essential tools to make educated decisions in determining how best to photograph a subject.

So what is exposure? It is the process whereby the light reflecting off a subject reflects 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.”

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 films.
  • Aperture: Also referred to as 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.

So 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, in the case of a mirrorless camera like the NEX-6, the sensor is activated for a relatively short period of time to allow the light to record on it before the shutter covers it and ends the exposure.

ISO numbers for the NEX-6 start at 100 and then double in sensitivity as you double the number. So 200 is twice as sensitive as 100. If you select your ISO manually, you are able to select in full-stop increments (ISO 100, 200, 400, and so on up to ISO 25,600). If you place the camera in Auto ISO, it will determine the proper ISO from 100 to 3200 in 1/3-stop increments. There are also a wide variety of shutter speeds that you can use to achieve a proper exposure. The speeds on the NEX-6 range from as long as 30 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. 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 can require 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 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 reflecting off your subject enters the lens and is allowed to pass through to the sensor for a period of time 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, and an exposure value is rendered. Here is the tricky part: There is more than one 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, known as exposure equivalencies, 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 the ISO setting to achieve a proper exposure. For simplification purposes, we will use an ISO of 100.

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. For those of you 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. This means 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 but twice as much as f/11. 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 have a pipe that is 1/2” in diameter (f/2) and one that is 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 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.

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/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 field.

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