In a digital camera, you have one additional parameter that you can change to control exposure.

As you just learned, your image sensor measures voltages on the sensor’s surface to determine how much light struck each pixel. These voltages must be amplified before they can be measured. If you amplify the voltages more, then the sensor will effectively be more light-sensitive, because dimmer light levels will be boosted.

ISO is a standard for measuring the sensitivity of film. Digital vendors have adopted the standard for specifying the sensitivity of a digital image sensor. When you increase the ISO setting on the Rebel XS, you’re essentially making the sensor more light-sensitive. As the sensor becomes more sensitive, it will require shorter exposures to be able to “see” a scene. We’ll discuss ISO in more detail later.

As you’ve learned, the shutter sits just in front of the image sensor and opens and closes to expose the sensor to light. The shutter speed that you (or the camera) chooses determines how long the shutter will stay open.

Shutter speeds are measured in seconds, and a longer shutter speed exposes the sensor to light for a longer period of time than a shorter shutter speed. The XS has a shutter speed range of 1/4000th of a second on the fast end to 30 seconds on the slow end. The XS also provides a Bulb mode, which lets you keep the shutter open indefinitely.

In the Full Auto shooting that you’ve been doing, the camera has been calculating an appropriate shutter speed for you automatically, and you can see its decision in the camera viewfinder status display.

Figure 5-4. Here, the camera is indicating that it has chosen a shutter speed of 1/100th of a second.

On the status display shown on the back of the camera, shutter speed is the number in the upper-left corner.

Figure 5-5. Shutter speed is also shown on the rear LCD screen.

In later chapters, you’ll learn how to control shutter speed manually.

Like shutter speed, the size of the aperture (which we discussed earlier) can be controlled automatically by the camera or manually by you. Aperture size is measured in f-stops, and an f-stop number is often fractional, so you’ll see f-stops with values such as f5.6, f8, or f11.

Unfortunately, f-stops can be a little unintuitive because the way they’re measured can seem backward. A larger f-stop number, say 16, indicates a smaller opening.

Let’s try a little experiment that will make all of this a little clearer.

It’s pretty easy to see that when the aperture is smaller, it blocks (or stops) a lot of light. For this reason, when the shutter button is not pressed, the aperture opens as wide as it can go to ensure that as much light as possible makes it to the viewfinder so that you can more easily see and frame your shot. When you press the shutter button, the camera closes the aperture down to your chosen aperture setting, opens the shutter to make the exposure, closes the shutter, and then opens the aperture back up all the way so that you have a clear view out the viewfinder.

Now press and hold the Exposure Compensation button and dial the aperture to f8. Again, look at the front of the camera and press and hold the shutter button. The iris will close, but it should be obvious that it’s not closing as far as it did when the aperture was set to 16.

Figure 5-8. At f8, the aperture will be slightly larger than at f16.

Dial up some other apertures and take a look at them. When you’re finished, be sure to set your lens back to autofocus by moving the focus switch to AF.

What you should have seen is that when you choose a higher f-stop number, you’re electing to stop more light with the aperture. That is, you’re selecting a smaller opening.

Figure 5-9. As aperture size increases, less light strikes the sensor.

In this book, we’ll use a few aperture-related terms that you’ll regularly hear photographers employ. When we speak of stopping down a lens, we mean choosing a smaller aperture (larger f-number). Opening up the lens means choosing a larger aperture (smaller f-number). Shooting full wide means choosing the largest possible aperture (the one with the smallest number).

This last term can be a little tricky because, on a zoom lens, the largest possible aperture can vary depending on your current focal length. For example, if you’re using the 18–55mm lens that Canon bundles with the XS, then you have a large aperture of f3.5 when shooting with the lens at full wide but a large aperture of f5.6 when shooting with the camera at full telephoto. In other words, the aperture can’t open as wide when you zoom the lens in all the way. This is simply because of the nature of engineering a lens. (If you look at the front of the lens, you’ll see that it says “1:3.5–5.6.” This tells you the aperture range, from full wide to full telephoto.)

This next bit should be pretty intuitive. As you choose a faster shutter speed, you will have more ability to freeze motion in a scene. That is, when the shutter is open for a very short time, a moving subject will be frozen. When the shutter is open for a longer time, a moving subject will be blurry and smeared.

Figure 5-10. With a slower shutter speed, you can blur your subject to create a more dynamic image.

You may think that blurry and smeared is inherently bad and that you would always want your images sharp and clear. But consider the following two images:

If we shoot this scene with a fast shutter speed so that it is completely frozen, then we no longer see this as a moving Ferris wheel. In fact, it looks stopped. Blurred motion is often a way to introduce a dynamic feel to your images. When shooting a dynamic scene, you’ll want to think about what best conveys the sense of dynamism that you feel there. Is it a perfectly frozen, fast shutter speed choice? Or a blurred, slow shutter speed choice?

There’s another concern with shutter speed, which is the fact that your hands and body aren’t always steady. You don’t want to choose a shutter speed that’s so slow that the natural shakiness of your hands will cause the image to be soft or blurry. In Chapter 6, we’ll discuss how to calculate the slowest possible shutter speed for handholding in any given situation.

As you saw when you looked at the front of your lens with the shutter button held down, when you choose a higher f-number, the aperture in your lens is closed off more and blocks more light. Obviously, this will make your final exposure darker. But changing aperture has another effect on your image.

As you go to a smaller aperture, the depth of field in your image gets deeper. Depth of field is simply a measure of how much of your image is in focus.

Figure 5-11. The image on the left has a deep depth of field, while the image on the right has a very shallow depth of field.

Here, the figure on the right has a very shallow depth of field. Notice that the background is out of focus, and even some parts on the front and back of the dandelion are slightly out of focus. The image on the left has very deep depth of field, with everything in the image in focus.

Depth of field is measured around your point of focus. So, if you’ve chosen an aperture that gives you, say, 10 feet of depth of field, then an area 10-feet deep will be in focus centered around the distance you focused at. What’s more, about a third of that depth will be in front of the point of focus, and the other two-thirds will be behind.

Figure 5-12. About a third of the depth of field in your scene will be in front of your subject, with the remaining two-thirds falling behind your subject.

Isn’t it usually best to have everything in your image in focus? Not necessarily. Sometimes, having a sharply focused background can distract the viewer from your foreground subject. Portraits are the most common example of a shallow depth of field application. If you choose a larger aperture for a portrait, you’ll create a softer background that will bring more attention to the subject of your portrait.

Just as you’ll make shutter speed choices to control motion stopping, sometimes you’ll need to consider how depth of field can be used to better express the subject or scene you’re shooting.

Also, be aware that every lens has an aperture “sweet spot.” On most lenses, if you stop down too far, you might suffer a sharpness penalty. Depending on how big you intend to print your final image, this penalty may be irrelevant.

So, you can control how sharp or blurry a subject is by choosing a slower or faster shutter speed, and you can control how deep the focus is by choosing a larger or smaller aperture.

However, each of these choices will also render your scene brighter or darker. For example, if you decide you want to use a very fast shutter speed to stop a fast- moving object, then your scene might end up underexposed. The faster speed will allow less light to hit the image sensor, and your final image might end up dark.

You can compensate for a faster shutter speed, though, by using a wider aperture. A wider aperture will allow more light to pass through the lens and will restore a good level of brightness to your scene. Of course, a wider aperture might mean less depth of field. So, if you want to stop motion and have a deep depth of field and a scene that is properly bright, you might have a problem. Balancing all of these factors is one of the tricky things about photography.

To better understand this balance, you need to learn a new term.

Let’s go back to the subject of stops for a moment. As discussed earlier, a stop is a measure of light. When you double the amount of light that hits the sensor, we say that you have increased the amount of light exposure by one stop. Conversely, if you halve the light, you decrease the exposure by one stop.

Consider these shutter speeds:

Each one is double the previous shutter speed. In other words, there is a one-stop difference in the amount of light exposure generated by each of these speeds.

Now consider these apertures:

This one is not so obvious because most of us aren’t too familiar with calculations of circular area. But, trust me when I say that each one of these apertures represents an opening that’s twice as big as the previous aperture. In other words, there’s a difference of one stop of light exposure between each of these apertures.

When it comes time to balance motion stopping power with depth of field with overall illumination, you can take advantage of the fact that both shutter speed and aperture can be adjusted by the same amount in opposite directions. In other words, the two values have a reciprocal relationship.

For example, let’s say your light meter recommends an exposure of 1/500th of a second at f8. It doesn’t know how much motion stopping you might want, or how much depth of field, so it’s simply trying to recommend a shutter speed and aperture that will give you a good level of illumination.

But if you decide that you want more motion stopping power and so increase shutter speed from 1/500 to 1/1000 (one stop), you’ll run the risk of darkening your image. But, you can open your aperture from f8 to f5.6 (one stop) to compensate for that one stop of darkening that you introduced with the shutter speed change.

Because of this reciprocal relationship, many different shutter speed/aperture combinations yield the same overall exposure. That is, many combinations produce an image of equal brightness. However, some of those combinations might produce an image with more depth of field than others, or some might produce an image that has blurrier motion than others.

Figure 5-13. These two images have reciprocal exposures. That is, they have the same overall illumination, but the left one was shot with a combination that had a slower shutter speed than the right image.

For the moment, don’t worry about how you manage all of this on your camera. In this chapter, the goal is to understand the concepts. We’ll get to the application and the actual controls in the next chapter.

In the days of manual cameras, shutter speed and aperture controls used the progression of settings that we’ve looked at here, with one stop of exposure difference between each setting (however, they provided a wider range than what I’ve shown).

However, it’s possible to adjust shutter speed and aperture by intervals that are smaller than a whole stop, and by default the XS will use values that are fractions of stops. So, as you adjust the shutter speed control on the XS, you might see a progression that goes like this:

I’ve put the traditional one-stop increments in bold. The other values are increases of a third of a stop. All the same reciprocal rules apply when dealing with fractional stops. These fractional stops simply give you a more granular, finer-level of aperture control.

FOOD FOR THOUGHT: IMAGE ANATOMY

You need to learn an additional bit of theory if you want to get the most of out your image-editing software. Just as good film shooters needed to have a certain understanding of film and paper chemistry, as a digital shooter you need to know something about how a digital image is constructed.

Pixel is a pretty common term these days, and I’ve been using it off and on already, but just in case there’s any confusion about it, here’s a definition: Pixel stands for picture element and is used to refer to any individual unit of information in a raster image. A raster image is simply an image that’s made up of colored dots. So, the image on your computer screen is a raster image made up of pixels.

Similarly, your digital camera produces a raster image of different-colored pixels.

A pixel’s color is simply a numerical value, with a different number representing each color that the pixel can be. The range of color that any individual pixel can be is limited only by the amount of memory that your imaging device devotes to that pixel. For example, your computer monitor probably uses 24 bits of data for each pixel. With 24 bits you can count from 0 to roughly 16 million, which means that any pixel on your screen can be one of roughly 16 million colors. Your Rebel XS uses 14-bit pixels, which means it can record a huge variation of color.

When you were a kid, you probably learned about the color wheel and learned you can mix the primary colors of ink together to create other colors. You probably also learned that as you mix colors together, they get darker until they eventually turn black.

Light works mostly the same way. The primary colors of light—red, green, and blue—can be mixed together to form every other color of light. But, as you mix the primaries of light, they get brighter until they eventually turn white.

A digital image is actually composed of three separate channels. One contains all of the red information in the image, the second contains all of the blue, and the third contains all of the green.

We’ll be referring to this three-channel color process at various places in this book.

Figure 5-14. Unlike ink, when the primary colors of light (red, green, and blue) are mixed together, they get lighter and lighter until they become white.