Understanding exposure


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Understanding exposure

  1. 1. 18/04/2009 Understanding Exposure MEMBER LOGIN FORGOT PASSWORD? Email Address: User Password: HOME FREE LESSONS ALL LESSONS NEWSLETTER CONTACT US REDEEM COUPON home :: all lessons :: free lessons :: Understanding Exposure what we offer As newer, better, faster digital how it works cameras enter the market, many of us who were hesitant at first are testimonials now taking the plunge into the sign up now world of digital imaging. Along with the promise of more resolution and better color search reproduction comes the myth that photography no longer needs to be a manual process. Today, many newcomers to photography cannot resist but to set their top-of-the- line digital SLR to the Program mode and postpone learning about any of the other modes or basic view free lessons settings available to them. enter our drawing buy a gift certificate In reality, even with the most advanced camera systems and software, photography has not changed much since the day that the very first photographic image was recorded. To this day we are still painting with light, albeit using a much more advanced arsenal of tools. redeem a gift certificate In this lesson, we will take you all the way back to the basics. We will explore the fundamental, manual process that makes photography what it is, "painting with light." By the end of this lesson, youll be able to take full control over your camera settings including shutter speed, aperture, and ISO. These basic camera settings fall under the umbrella of exposure. Understanding exposure is the first and most important step to unleashing your full photographic and artistic potential. (Most images can be clicked for an enlarged view.) Topics Covered: How Cameras Work Understanding the Digital SLR The Aperture Exploring Depth of Field The Shutter Speed The ISO Setting The Light Meter Tying It All Together Equipment Used:webphotoschool.com/…/index.html 1/18
  2. 2. 18/04/2009 Understanding Exposure Camera/Media Olympus E-510 From the very first box cameras to be mass-produced in the 19th century all the way to the most current digital cameras, the mechanics of exposure have remained relatively unchanged. The most notable difference today is that the camera is now able to do most of the "thinking" and arrive at various exposure settings automatically. Nevertheless, the camera is not right 100% of the time. More importantly, the camera does not know, nor does it care, about your personal vision for any given photograph. Therefore, in order to gain control over ones photographic vision, one must understand how a camera works and how to manually control the mechanical process of exposure. How Cameras Work First, lets explore the internal workings of a camera. Simply stated, the camera is really nothing more than a lightproof box with a hole on one side and a light sensitive material on the other side. The very first camera actually existed long before there was such a thing as film or light sensitive material. It was called the Camera Obscura (translated from Latin as "dark chamber"). The mechanics of a Camera Obscura are simple (figure 1). It is a light-tight box with a tiny pinhole on one side. Light enters through this tiny pin hole and an upside down, laterally reversed image of whatever is outside or in front of the pin hole is projected onto the interior wall of the dark box. It is really quite magical and it Figure 1 really does work. However, without any film or digital sensor, it is practically useless to photographers. The next logical step to the Camera Obscura is a basic, homemade pinhole camera. Anyone who has taken a beginning photography course in the pre-digital age was probably taught how to make one. A pinhole camera is a light-tight box with a hole on one side and a piece of photographic paper on the inside. As with the Camera Obscura, light enters through the pinhole and an image is projected onto the light-sensitive paper. By controlling the amount of light entering through the hole and reaching the paper, one is able to control the exposure of the recorded image. If this concept is still a little unclear, dont worry. For now, just remember that a camera need not have any dials or LCDs in order to be a camera. Rather, a camera is simply a dark box.webphotoschool.com/…/index.html 2/18
  3. 3. 18/04/2009 Understanding Exposure Understanding the Digital SLR Camera Obscuras and pinholes aside, you might still be asking yourself how all of this relates to your brand new Canon 20D, Nikon D200, or Olympus EVOLT E-3. Well, the truth of the matter is that every one of these modern digital cameras are build around the same design concept as the basic dark box example shown above. The Digital SLR is also a dark box with a hole on one side and a sensor on the other side. SLR is an acronym for Single Lens Reflex. Single Lens Reflex means that the lens that you are looking through when you look into the viewfinder is the same lens that is taking the picture. There are other types of cameras where this is not the case, but for the purpose of this lesson, we will direct our attention to the SLR as this is the most common type of camera being used today. The diagram on the right (figure 2) illustrates the inner workings of an SLR (Single Lens Reflex) camera. In a nutshell, heres how it works. Light enters through a lens instead of a pinhole. The lens is made up of numerous glass elements, which work together to focus the light coming in. Having a lens instead of a pinhole makes it possible to let in a much larger amount of light while keeping the projected image in focus. After the light passes through the lens it encounters a mirror, which sits at a perfect 45-degree angle to the lens. The mirror reflects the light upward into a Figure 2 specially shaped prism. The prism bounces the light around its various surfaces turning the image right side up. When you look inside the viewfinder of an SLR, you are seeing the image reflected inside of the prism. Keep in mind that this entire mirror and prism mechanism is only there so that you can see the image right side up in the viewfinder prior to taking the picture. As soon as you press the shutter release, the mirror pops up and a shutter curtain opens, exposing the digital sensor or film. At this point, the light simply travels in a straight path through the lens and if youve focused the lens correctly, a sharp image is projected onto the light sensitive material (CCD or film). This is where the concept and technique of controlling exposure comes into play. One of the primary skills in photography is properly determining how much light reaches the sensor, how long the sensor is exposed to the light, and finally how sensitive the sensor is to begin with. This is what we mean when we compare photography to "painting with light." As you read on, we will revisit each of these points in detail. The 45-degree mirror described above is an essential part of every SLR, film or digital. Here are two illustrations that show the mirror inside of an Olympus EVOLT E-510 (figures 3 & 4). For demonstration purposes, the lens has been removed. When you remove a lens from an SLR body, you will see where the mirror is positioned directly behind the lens (figure 3). When the shutter release is pressed, the mirror instantly moves up, out of the way,webphotoschool.com/…/index.html 3/18
  4. 4. 18/04/2009 Understanding Exposure exposing the digital sensor (figure 4). Figure 3 Figure 4 Now that youve got a grasp on some of the basic parts that make a digital SLR or any other camera work, we can begin a more in-depth discussion of exposure and its various elements. As mentioned above, exposure is comprised of three major factors, all of which work together to control how light or dark the recorded image will be. The three factors that effect exposure are: 1. The amount of light coming in through the lens. This is controlled by the aperture setting. 2. The length of time that digital sensor is allowed to be exposed to the light coming in through the lens. This is controlled by the shutter speed setting. 3. The sensitivity level of the digital sensor or film. This is controlled by the ISO setting. In the case of traditional film, each individual roll of film has an ISO rating. The Aperture Inside every lens there is an adjustable opening called the aperture. The aperture controls how much light is allowed to enter through the lens. The larger the aperture opening, the more light can enter through to the digital sensor. The smaller the opening, the less light can enter. The relative size of the aperture is measured in f-stops. The range of f-stop settings available on the camera depends on each lens. With older 35mm film cameras, the aperture adjustment setting was usually located on the lens, and not the camera. Each lens would have an aperture ring right behind the focus ring. With most of todays digital SLRs, however, the aperture adjustment setting is located on the camera body. But regardless of how the aperture is controlled on whatever camera you are using, the range of f-stop settings have essentially remained the same.webphotoschool.com/…/index.html 4/18
  5. 5. 18/04/2009 Understanding Exposure Most lenses have the following standard range of f-stop settings available: [f/2.8, f/4, f/5.6, f/8, f/11, and f/16] For the purpose of this tutorial, we will not delve into the complex equation from which these f-stop numbers are derived. Much more vital to ones understanding of exposure are the following aperture related facts: The smaller the f-stop number, the larger the aperture opening inside the lens, and the more light is allowed to enter. The larger the f-stop number, the smaller the opening, the less light can enter. The list of apertures given above is the standard list expressed in full stop increments. A "full stop" increment can be expressed as either twice or half the amount of light as the f-stop before it. In other words, an aperture setting of f/5.6 will allow twice as much light in through the lens as f/8 or half as much light in as f/4. This will make more sense later, when we discuss how the shutter speed interacts with the aperture. Many current Digital SLRs will allow the aperture to be set in 1/3-stop increments. This allows for f-stops like f/9 and f/10, which allow more precise exposure control, but this can be confusing for the absolute novice. Some lenses will allow for additional stops at the widest and smallest aperture settings. For example, some lenses will allow apertures as wide as f/2, f/1.8, or even f/1.4. These lenses are considered fast lenses because they let more light in, which makes them useful for low light situations. We will explore this concept later in the lesson. The following five images (figure 5) are illustrations of 5 full-stop increments in aperture, ranging from f/11 at the smallest lens opening to f/2.8 at the largest lens opening. The window towards the bottom of each image shows a picture of an actual aperture inside of a lens, shot through the back of a 35mm film camera. For each shot, the shutter speed and the ISO setting were kept constant in order to demonstrate the effect of the aperture on the exposure of this shot. figure_override figure_override figure_override figure_override figure_override Figure 5webphotoschool.com/…/index.html 5/18
  6. 6. 18/04/2009 Understanding Exposure As you can see, a larger lens opening (represented by a smaller f-stop number) lets in more light, which causes the image to be lighter. A smaller aperture lets in less light, which causes the image to be darker. In this example, the third aperture setting (f/8) resulted in the most accurate overall exposure for this particular lighting situation, shutter speed, and ISO setting. At this point, it is important to remember that the shutter speed and ISO settings were kept unaltered throughout. As we will soon demonstrate, each of these other exposure settings play an equal role in determining exactly how much light is recorded. Exploring Depth of Field Every exposure setting (aperture, shutter speed, and ISO) has its own unique side effect, which can be used by the photographer to achieve specific creative goals. The main side effect of aperture is depth of field. Depth of field is a photographic term that relates to how many things remain in focus along an imaginary line drawn from the lens into infinity. The wider the aperture or the bigger the lens opening, the less will be in focus. The smaller the lens opening, the more will remain in focus across a longer distance range. For example, imagine that you are photographing a portrait and the model is in front of a distant mountain landscape. With an aperture setting of f/2.8, you either have the model or the mountain in focus, but not both at the same time. However, with an aperture setting of f/16 (or the smallest lens opening), both the model and the background will be in focus. Heres another example (figure 6). This image of a branch set against a distant background of foliage is a composite of two shots taken at different aperture extremes. The left portion of this comparison shows the result of using f/2.8 as the aperture setting. This wide aperture setting caused the foreground to sharp, while the background was rendered completely out of focus. This is known as a "shallow" or "limited" depth of field. On the right, an aperture setting of f/22 was used. There the background is almost as Figure 6 sharp as the foreground. This is referred to as "long" or "great" depth of field. Another important thing to notice in the example above is the shutter speed setting, which is dramatically different for the f/2.8 and the f/16 portions of the comparison. This will make more sense when we discuss the shutter speed later on in this lesson. For now, just keep in mind that by closing down the aperture from f/2.8 to f/16, you significantly reduce the amount of light coming in through the lens. In order to keep the overall exposure the same for both images, you have to compensate by using a longer shutter speed. In other words, you have to let the light in for a longer duration in order to have the same amount of light recorded on the digital sensor.webphotoschool.com/…/index.html 6/18
  7. 7. 18/04/2009 Understanding Exposure The image on the right (figure 7) is a classic example of great depth of field being used to keep the foreground and background completely sharp. In this case, an aperture setting of f/16 was used to maintain sharp focus from the closest plants to the farthest pine trees. Figure 7 This next image (figure 8) is an example of shallow depth of field. Here the photographer used an aperture of f/2.8, while carefully focusing on the single flower in the foreground. This resulted in a completely blurry background, which helps to isolate the subject. If a smaller aperture had been used, the flower might have been lost amidst a very busy background. Using shallow depth of field is a great way to keep the background elements simple. Figure 8 Before we move on, let’s review the key points that you need to understand about aperture. The aperture is an adjustable opening inside of the lens, which controls or limits the amount of light entering through the lens and reaching the film of digital sensor. The aperture setting on the camera consists of a series of f-stops. The larger the f-stop number, the smaller the opening in the lens. The smaller the opening in the lens, the less light can come in and the darker the image will be. Depth of field is a side effect of aperture. The smaller the lens opening is, the greater the depth of field. The Shutter Speed While the aperture setting is responsible for limiting the amount of light entering through the lens, the shutter speed is responsible for limiting the length of time that the film or digital sensor will be exposed to that light. Shutter speed is the second factor that helps to control the total quantity of recordable light. In essence, the shutter speed is a direct counter- balance to the aperture. Just like apertures, shutter speeds are measured in full stop increments. Here is a list ofwebphotoschool.com/…/index.html 7/18
  8. 8. 18/04/2009 Understanding Exposure standard shutter speeds (measured in seconds or fractions of a second). [BULB, 1 sec, ½, ¼, 1/8, 1/15, 1/30, 1/60, 1/125, 1/250, 1/500, and 1/1000] It is worth noting that most modern cameras will allow you to set the shutter speed in third stop increments. For the sake of keeping things simple, we will only concentrate on full stops for this lesson. Many cameras will also allow shutter speeds as fast as 1/4000th of a second and/or as slow as 30 seconds. Most if not all SLR cameras offer an alternative shutter speed setting called BULB. When the camera is set to BULB, the shutter will stay open as long as the shutter release is pressed. This makes it possible for much longer, manually-timed shutter speeds that can be in excess of several minutes or even hours in length. As you can see from the list of standard, full-stop shutter speeds listed above, each consecutive shutter speed (from left to right) is half the time of the preceding speed. 1/250th of a second is half the time or half the amount of light reaching the sensor as 1/125th of a second. We will explore this concept in more detail later. In its basic form, the mechanism that makes the shutter speed work consists of two opaque curtains which move in front of the sensor exposing it to light. When the shutter release is pressed, the mirror pops up and out of the way. Then, the first curtain moves to the right, revealing the sensor and exposing it. To end the exposure, the second curtain follows the first and covers the sensor. Then the mirror can come back down and into position. When shutter speeds are faster than 1/15th of a second or so, the second curtain will actually begin moving before the first curtain has reached the end. This causes a small window or slit to move laterally across the film. The shorter the exposure time, the narrower this window will be. Keep in mind that this description is based on a much older style of shutter found in older 35mm film cameras. Modern cameras feature more advanced versions of essentially the same process. The series of images shown below (figure 9) illustrate how the shutter speed affects the exposure. For each of these examples, the aperture and the ISO settings have been kept constant. The window below each image illustrates how the curtain shutter mechanism moves across at different speeds. figure_override figure_override figure_override figure_override figure_override Figure 9webphotoschool.com/…/index.html 8/18
  9. 9. 18/04/2009 Understanding Exposure From this series of illustrations, you should be able to understand that as the shutter speed gets shorter, less light is allowed to reach the sensor. Less light results in a darker image. In addition to controlling the length of the exposure and the total amount of light reaching the sensor, the shutter speed is also responsible for either freezing movement or blurring movement. Just as depth of field is a side effect of aperture, motion blur is a side effect of shutter speed. This is where your creative decisions come into play. When the shutter speed is slow (i.e. 1 full second), the shutter is open for a relatively long time. During this long exposure, anything that is moving (subject and/or camera) will cause motion blur in the image. If the exposure is really short (i.e. 1/1000th of a second) whatever is moving will inevitably become frozen. It is almost like taking a thin slice out of time. In some situations you might prefer to freeze the moving subject. Other times, you might want to use a slow shutter speed to add motion blur to your shot as an aesthetic element. Perhaps the most important point to remember is that the shutter speed must be fast enough to freeze any camera shake that can occur by the movement of your hands and body while holding the camera. If a fast enough shutter speed is not possible, then the camera must be kept still using a tripod, monopod, or other camera support. Rule of Thumb: To prevent motion blur due to camera shake when hand holding the camera, make sure that the shutter speed is greater than the focal length of the lens. For example, if you are using a 60mm lens, make sure that the shutter speed is 1/60th of a second or faster to prevent camera shake. If your lens is a 25mm wide-angle lens, then you can use a shutter speed as slow as 1/30th, or if you have really steady hands, maybe even 1/15th of a second. Here is an example of intentional motion blur (figure 10). The amount of available light in this situation was extremely low, which required a very slow shutter speed of about 15 seconds. The camera was mounted to a tripod in order to prevent any motion blur due to camera shake. During the 15-second exposure, a large truck with bright headlights drove past the camera. The truck was rendered as several long streaks of light, while the other parts of the image -- the road and landscape in the background -- remained Figure 10 absolutely sharp. This next shot of a dog frozen in mid-stride is an example of how a fast shutter speed can be used to freeze action. Here, the photographer used a shutter speed of 1/2000th of a second, which resulted in every element, including the tiny droplets of water splashing around the dog, to be recorded as absolutely still andwebphotoschool.com/…/index.html 9/18
  10. 10. 18/04/2009 Understanding Exposure tack sharp. Figure 11 Keep in mind that in order to use a super fast shutter speed like 1/2000th of a second, there must be enough available light to record a proper exposure in such a short amount of time. In this example, the bright midday sun offered plenty of light to warrant the use of a fast shutter speed. Whereas in the previous example taken at night, there was not nearly enough light to use such a fast shutter speed. The ISO Setting The third factor that directly affects exposure is the ISO setting. ISO is an acronym for the International Standards Organization. In relation to the photographic process, ISO is a measurement of film or digital sensor sensitivity. In other words, the ISO rating tells us how sensitive a film or a digital sensor is to light. Therefore, the ISO rating determines how much light is needed for an accurate exposure. In the days of film, every roll of film came with its own ISO (or ASA) rating, expressed in full stop increments as follows. [25, 50, 100, 200, 400, 800, 1600, and 3200] The higher the ISO, the more sensitive the film is to light. An ISO 200 film is twice as sensitive (or 1 stop more sensitive) to light as an ISO 100 film and requires 1 stop less light to achieve the same exposure. The same principle applies to digital photography, with one major difference. The sensitivity of a digital sensor is adjustable. This means that the same range of ISO ratings can be dialed in as a setting right in the camera. No longer is it necessary to match the appropriate film to any given lighting situation. Now we have the entire range of film speeds built in to the camera and accessible on demand. The side effect of ISO is grain or digital noise. The higher the ISO setting, the more noise is present in the image. Digital noise lowers the overall clarity of the image. This can be a really tough compromise, especially in situations where both a fast shutter speed and high image quality are important. The illustration below (figure 12) compares two extremely different ISO settings. Here, we can see the effect of these settings on this composite shot of several, fast-moving billiard balls.webphotoschool.com/…/index.html 10/18
  11. 11. 18/04/2009 Understanding Exposure In this scene, the light level was relatively low, as it would be in a pool hall type of setting. ISO 100 is a relatively low ISO setting, which makes the sensor less sensitive to light. This means that more light or a slower shutter speed is needed for an accurate exposure. The result in this case, however, is motion blur. When the camera is set to ISO 1600, less light is needed for the same exposure, which allows the use of a faster shutter speed. This helps to freeze the billiard balls that are in Figure 12 motion. However, notice that the higher ISO setting here also caused more noise in the image, which significantly obscures the fine details in the subject and lowers the clarity of the image. (Click on the image for an enlarged view to see this effect more clearly.) Before we move on to see how all three exposure factors, aperture, shutter speed, and ISO, are used in relation to one another, let’s recap some of the most important information we’ve covered so far. Here is a list of points that you should be clear on before proceeding to the next portion of this tutorial. Exposure is a process for controlling the amount of light that is allowed to record on the digital sensor. The main objective of making an exposure is to record just the right amount of light so that the image is neither too dark (underexposed), nor too light (overexposed). The aperture setting controls the amount of light coming in through the lens. The shutter speed setting controls how long the sensor is exposed to the light coming in through the lens. The ISO setting controls how sensitive the digital sensor is to light. With all of this in mind, one might ask: "How is it possible to know which shutter speed and aperture settings to use for any given situation? Surely, you can’t just pick random exposure settings out of a hat! There must be some method for arriving at an accurate exposure without having to do a ton of trial and error!" The Light Meter Fortunately there is such a method. In the next and final portion of this lesson we will tie all of this information together by exploring the most essential tool - aside from the camera - that every photographer must know how to use: the light meter. A light meter is simply that - a device that measures the amount of light in a scene. Light meters come in several varieties and usually have several modes of operation. In general, there are two main categories of light meters: incident and reflective. Incident light meters measure light that is falling onto the subject, while reflective meters measure light that is reflecting from the surface of a subject. Within these two categories, there are many different types of light meters. Some are separate devices and some are built into the camera. Some measure ambient or available light and some measure flash or studio strobe lightswebphotoschool.com/…/index.html 11/18
  12. 12. 18/04/2009 Understanding Exposure measure ambient or available light and some measure flash or studio strobe lights. In this lesson, we will concentrate on the type of light meter that is built into the camera. Every modern SLR has a built in light meter, which is an essential tool for determining proper exposure. How does a light meter work? What exactly does it do? Essentially, a light meter measures the strength of light and determines an appropriate shutter speed and aperture combination based on the ISO setting selected. It is then up to you, the photographer, to decide whether to use that specific aperture and shutter speed combination suggested by the meter, or to adjust these settings to tailor to your own creative vision. All light meters are calibrated to read the light as middle gray. Whether the subject is all white, all black, or a mix of different tones, the light meter will average everything together and offer up an exposure that will render the metered area(s) as a middle gray tone. This might sound confusing at first, but we will clarify this point with some examples below. Most modern digital SLR cameras offer several metering modes. The most common metering modes are described and illustrated in the diagram below (figure 13). Figure 13 The best way to explain how the light meter reading translates into an actual exposure is with the following experiment. After reading through the next section, we recommend setting up a i il i tf lf t if hi th ltwebphotoschool.com/…/index.html 12/18
  13. 13. 18/04/2009 Understanding Exposure similar experiment for yourself to see if you can achieve the same results. As previously mentioned, the light meter is calibrated to read everything it sees as a middle gray tone. To demonstrate this, we positioned our model, Mary, in an ordinary scene with flat, overcast lighting. We asked Mary to hold three 8x10 cards: a white card, a black card, and a Kodak 18% gray card. Note: The Kodak 18% gray card is an inexpensive light metering tool, which can be purchased at virtually any camera shop. It is called the 18% gray card because it reflects 18% of the light falling on it. In terms of light reflectance this is considered a perfect, middle gray. Therefore, this gray card can be used as a metering guide in conjunction with a light meter to arrive at accurate exposures in any lighting condition. We strongly recommend getting your hands on one, as it will definitely come in handy in more situations than you might realize. To use the built-in meter in your camera, press the shutter release down halfway. This will activate the meter and you will see the current shutter speed and aperture settings displayed inside the viewfinder. Next to the shutter speed and aperture is an exposure indicator, which usually has markings ranging from -3 to 0 to +3. Adjust the aperture or shutter speed until the indicator reads 0. This is the exposure that the light meter deems accurate for that particular amount of light. When metering, remember that the ISO setting will also affect what the light meter will deem accurate because the ISO setting determines how much light the sensor actually needs. Remember to avoid shutter speeds that are slower than 1/30th of a second (depending on the lens, as was discussed earlier) in order to prevent motion blur. For the following examples, we used the Center-Weighed metering mode in order to limit the metering area to just the center of the viewfinder. This mode excludes any light values around the edges of the frame. We set the camera to ISO 200 and kept the aperture at its widest setting of f/2.8. First, we asked Mary to hold an 8x10" inch white card. To take the meter reading, the photographer stepped in close, almost filling the frame with the white card. Then he proceeded to meter the card (figure 14). The light meter suggested 1/500th of a second as the correct shutter speed. Remember that the light meter is designed to read everything as middle gray. The result, then, is underexposure. 1/500th of a second at f/2.8 with ISO 200 rendered the white card as a nice middle gray tone or about two stops darker than it actually was in real life. And of course, this caused all of the other tones in the image to be two stops darker as well (figure 15).webphotoschool.com/…/index.html 13/18
  14. 14. 18/04/2009 Understanding Exposure Figure 14 Figure 15 Next, we repeated the same process using a black card (figure 16). This time, the meter suggested an exposure of 1/30th of a second (same aperture, same ISO setting). The result, as you might guess, is overexposure. The meter performed its function flawlessly, turning the tone of the black card into a middle gray tone, which in turn recorded all of the other tones in the image two stops lighter than they should be (figure 17). Figure 16 Figure 17 Then, we conducted the experiment once more, this time using a middle gray card. The light meter suggested 1/125th of a second as the shutter speed (figure 18). This resulted in an accurately exposed image, with all tones rendered as close as possible to how they are perceived in real life (figure 19).webphotoschool.com/…/index.html 14/18
  15. 15. 18/04/2009 Understanding Exposure Figure 18 Figure 19 With this concept of metering in mind, the next step is learning how to visually perceive middle gray in whatever subject matter you are photographing, and then take your light meter readings from that. For example, green grass is generally a good middle gray. Let’s say you are photographing a big white house surrounded by a lush green lawn. If you take the meter reading from the house, you will render the house as a middle gray and cause the image to be underexposed. If you meter off the green grass, then you will be more likely to achieve an accurate exposure. This is primarily why an Auto exposure mode cannot always guarantee optimal exposures. Of course, modern digital cameras feature highly advanced metering algorithms, which can average the house and the grass together and arrive at a fairly accurate exposure. Nevertheless, if the scene is predominantly dark or light, the camera will not intuit your intention is to keep it that way and will try to compensate by making the image unnecessarily lighter or darker. Tying it All Together Now that we’ve covered the basic exposure mechanisms of a camera, which work together to record light onto a digital sensor or a piece of film, let’s consider how we can apply these techniques to achieve a desired creative outcome in our images. As you already know now, the shutter speed, aperture, and ISO settings work together to control the exposure. And the light meter can measure a given lighting condition and offer up a combination of shutter speed and aperture settings to use with a given ISO setting. There is no such thing as a single "correct" exposure in any given situation. In fact, there are always many different possible combinations of settings, which can produce the same exposure. However, as you are already aware, each of the settings (aperture, shutter speed and ISO) has their unique set of limitations and side effects. The combination of settings you end up selecting is ideally driven by the look you want to achieve for your photograph. For example, let’s say you metered a scene made up of predominantly green foliage. With the ISO set to 200, the light meter suggested an exposure of 1/500th of a second at f/5.6, which turned out to be an accurate exposure. Based on this reading, we automatically know that the following list of aperture and shutter speed combinations will produce an identical exposure with an ISO setting of 200: 1/30 @ f/22webphotoschool.com/…/index.html 15/18
  16. 16. 18/04/2009 Understanding Exposure 1/30 @ f/22 1/60 @ f/16 1/125 @ f/11 1/250 @ f/8 1/500 @ f/5.6 1/1000 @ f/4 1/2000 @ f/2.8 Remember that each change in shutter speed or aperture represents either half or twice the amount of light. This is called a full stop increment. So if we change the shutter speed from 1/500 to 1/2000, then we are letting in two stops less light. To compensate for this, we would need to open the aperture two stops from f/5.6 to f/2.8. If we were to change the ISO setting from 200 to 100, then we would be making the digital sensor one stop less sensitive. This would mean that we would need one stop more light to produce the same exposure. Therefore 1/500 @ f/5.6 with ISO 200 is equal to 1/250 @ f/5.6 with ISO 100. At this point, the preferred combination becomes creative decision. If your priority is to freeze action, then 1/2000 @ f/2.8 is your best bet. If you want to maximize depth of field, then go with 1/30th @ f/22. Here are a couple more examples of how exposure decisions are made with purely creative goals in mind. The image below (figure 20) was taken about 1 hour after sunset. The light was rapidly escaping. In fact, when this image was taken, there was barely any light left in the sky at all. Believe it or not, this is often the best time to photograph. The last trace of light after sunset can be extremely unique and surreal. Because of the dim lighting conditions, an extra long shutter speed had to be used in order to record the tiny bit of light that was illuminating this landscape. The camera was mounted to a tripod to prevent motion blur. The ISO setting was set to 100 to ensure maximum image quality. The aperture was set to f/16 to ensure that the cliff wall in the foreground and the cliffs in the distant background would both be in focus. After taking a few light meter readings from different surfaces in the composition, the photographer was able to calculate a shutter Figure 20 speed of 4 minutes, which required the use of the BULB setting and a stopwatch (figure 20). In this case, the photographer could have altered the ISO setting and the aperture setting to gain as many as 10 stops of extra light. However, this would have compromised his creative vision for this image. The severe motion blur, caused by the long exposure, turned the crashing waves into a soft mist, while the tripod ensured that everything else in the shot thatwebphotoschool.com/…/index.html 16/18
  17. 17. 18/04/2009 Understanding Exposure was motionless would be rendered sharp. Here is a different example, which shows how to handle low light situations without using a tripod. This dog portrait was also shot after sunset with a limited amount of available light. To pull this off, the photographer used a high ISO setting of 1600. At ISO 1600, the light meter suggested an exposure of 1/60th @ f/2.8, just fast enough to hand hold the camera and prevent any motion blur. Of course, the side effect of using such a high ISO setting is digital noise and a reduction in image clarity. However, for this image, the compromise was worth it and the extra noise, in our opinion, Figure 21 adds a kind of impressionistic feeling that seems to work well for this photograph. These examples make it clear that photography is really a world of compromises. The camera does not see things the same way that the human eye perceives. Sometimes, the camera can see things that we can’t perceive. Most of the time, it is the photographer’s job to manipulate the camera into seeing things a specific way. This is essentially the foundation of creative image-making. Hopefully, you can see why the autopilot settings on your camera are really not as smart as they may appear. When you set your camera to the Program mode, for example, the computer in the camera will use the built-in light meter to determine optimal shutter speed and aperture settings based on averages and “common scenarios” that have been preprogrammed by the manufacturer. In reality, the camera does not know whether you want shallow depth of field, great depth of field, frozen action, or motion blur. The only “computer” that can make these decisions is your brain. In conclusion, we suggest reviewing the topics in this lesson a few times. When it comes to manual exposure, practice is the only way to improve your proficiency. So, if you truly want to take photography into your own hands, and develop your own personal vision, then you must, at all costs, avoid the Program mode whenever possible. With enough practice, you will be able to manually set the aperture and shutter speed with more accuracy and more vision than any digital SLR can do on its own. And remember to practice and have fun! That’s what photography is all about!webphotoschool.com/…/index.html 17/18
  18. 18. 18/04/2009 Understanding Exposure Copyright © 1995-2009 Photoflex, Inc.webphotoschool.com/…/index.html 18/18