The Canon EOS Digital Rebel XTi gives you complete control over many of the basic functions of the camera. These include exposure, sensitivity (ISO settings), color balance, focus, and image parameters like sharpness and contrast. You can choose to let the camera set any or all of these for you automatically. Or, you can opt to fine-tune how the XTi applies its automatic settings. If you want absolute creative control over any of these functions, you can set them manually, too. That's why the Digital Rebel XTi is such a versatile tool for creating images.
This chapter explains the shooting basics of exposure, either as an introduction or as a refresher course, depending on your current level of expertise. When you finish this chapter, you'll understand most of what you need to know to take photographs in a broad range of situations.
Exposure can make or break your photo. Correct exposure brings out the detail in the areas you want to picture, providing the range of tones and colors you need to create the desired image. Poor exposure can cloak important details in shadow, or wash them out in glare-filled featureless expanses of white. However, getting the perfect exposure can be tricky, because digital sensors can't capture all the tones we are able to see. If the range of tones in an image is extensive, embracing both inky black shadows and bright highlights, we often must settle for an exposure that renders most of those tones—but not all—in a way that best suits the photo we want to produce.
There are four things within our control that affect exposure, listed in "chronological" order (that is, as the light moves from the subject to the sensor):
■ Reflected, transmitted, or emitted light. We see and photograph objects by light that is reflected from our subjects, transmitted (say, from translucent objects that are lit from behind), or emitted (by a candle or television screen). When more or less light reaches the lens from the subject, we need to adjust the exposure. This part of the equation is under our control to the extent we can increase the amount of light falling on or passing through the subject (by adding extra light sources or using reflectors), or by pumping up the light that's emitted (by increasing the brightness of the glowing object).
■ Light transmitted by the lens. Not all the illumination that reaches the front of the lens makes it all the way through. Filters can remove some of the light before it enters the lens. Inside the lens barrel is a variable-sized diaphragm called an aperture that dilates and contracts to admit more or less of the light that enters the lens. You, or the XTi's autoexposure system, can control exposure by varying the size of the aperture. The relative size of the aperture is called the f/stop.
■ Light passing through the shutter. Once light passes through the lens, the amount of time the sensor is exposed to that light is determined by the XTi's shutter, which can remain open for as long as 30 seconds (or even longer if you use the Bulb setting) or as briefly as 1/4000th second.
■ Light captured by the sensor. All the light falling onto the sensor is captured. If the number of photons reaching a particular photosite doesn't pass a set threshold, no information is recorded. Similarly, if too much light illuminates a pixel in the sensor, then the excess isn't recorded or, worse, spills over to contaminate adjacent pixels. We can modify the minimum and maximum number of pixels that contribute to image detail by adjusting the ISO setting. At higher ISOs, the incoming light is amplified to boost the effective sensitivity of the sensor.
These four factors—quantity of light, light passed by the lens, the amount of time the shutter is open, and the sensitivity of the sensor—all work proportionately and reciprocally to produce an exposure. That is, if you double the amount of light, increase the aperture by one stop, make the shutter speed twice as long, or boost the ISO setting 2X, you'll get twice as much exposure. Similarly, you can increase any of these factors while decreasing one of the others by a similar amount to keep the same exposure.
F/STOPS AND SHUTTER SPEEDS
If you're really new to more advanced cameras you might need to know that the lens aperture, or f/stop, is a ratio, much like a fraction, which is why f/2 is larger than f/4, just as 1/2 is larger than 1/4. However, f/2 is actually four times as large as f/4. (If you remember your high school geometry, you'll know that to double the area of a circle, you multiply its diameter by the square root of two: 1.4.)
Lenses are usually marked with intermediate f/stops that represent a size that's twice as much/half as much as the previous aperture. So, a lens might be marked:
with each larger number representing an aperture that admits half as much light as the one before, as shown in Figure 4.1.
Shutter speeds are actual fractions (of a second), but the numerator is omitted in displays, so that 60, 125, 250, 500, 1000, and so forth represent 1/60th, 1/125th, 1/250th, 1/500th, and 1/1000th second.
Most commonly, exposure settings are made using the aperture and shutter speed, followed by adjusting the ISO sensitivity if it's not possible to get the preferred exposure (that is, the one that uses the "best" f/stop or shutter speed for the depth-of-field or action stopping we want). Table 4.1 shows equivalent (same) exposure settings using various shutter speeds and f/stops.
Table 4.1 Equivalent Exposures
When the XTi is set for P mode, the metering system selects the correct exposure for you automatically, but you can change quickly to an equivalent exposure by pressing the shutter-release button ("locking" the current exposure), and then spinning the Main Dial until the desired equivalent exposure combination is displayed. This Program Shift mode does not work when you're using flash, and only applies to the next exposure you take; if you want to shift again for your next shot, you'll need to repeat the adjustment process.
In Aperture Priority (Av) and Shutter Priority (Tv) modes, you can change to an equivalent exposure, but only by adjusting either the aperture (the camera chooses the shutter speed) or shutter speed (the camera selects the aperture). I'll cover all these exposure modes later in the chapter.
Your Canon Digital Rebel XTi calculates exposure by measuring the light that passes through the lens and bounces up by the mirror to sensors located in the focusing screen, using a pattern you can select (more on that later) and based on the assumption that each area being measured reflects about the same amount of light as a neutral gray card with 18 percent reflectance. That assumption is necessary, because different subjects reflect different amounts of light. In a photo containing a white cat and a dark gray cat, the white cat might reflect five times as much light as the gray cat. An exposure based on the white cat will cause the gray cat to appear to be black, while an exposure based only on the gray cat will make the white cat washed out. Light-measuring devices handle this by assuming that the areas measured average a standard value of18 percent gray, a figure that's long been used as a rough standard (not all vendors calibrate their metering for exactly 18 percent gray) for many years.
You could, in many cases, arrive at a reasonable exposure by pointing your XTi at an evenly lit object, such as an actual gray card or the palm of your hand (but increase the exposure by one stop in the latter case, because the human palm—of any ethnic group—reflects about twice as much light as a gray card). It's more practical, though, to use your XTi's system to meter the actual scene, using the options available to you when using one of the Creative Zone modes (P, Tv, Av, M, and A-DEP). (In Basic Zone modes, the metering decisions are handled entirely by the camera's programming.) (See Figure 4.2.)
F/STOPS VERSUS STOPS
In photography parlance, f/stop always means the aperture or lens opening. However, for lack of a current commonly used word for one exposure increment, the term stop is often used. (In the past, EV served this purpose, but Exposure Value and its abbreviation has been inextricably intertwined with its use in describing Exposure Compensation.) In this book, when I say "stop" by itself (no f) I mean one whole unit of exposure, and I am not necessarily referring to an actual f/stop or lens aperture. So, adjusting the exposure by "one stop" can mean changing to the next shutter speed increment (say, from 1/125th second to 1/250th second) or the next aperture (such as f/4 to f/5.6). Similarly, 1/3 stop or 1/2 stop increments can mean either shutter speed or aperture changes, depending on the context. Be forewarned.
In most cases, your camera's light meter will do a good job of calculating the right exposure, especially if you use the exposure tips in the next section. But if you want to double-check, or feel that exposure is especially critical, take the light reading off an object of known reflectance. Photographers sometimes carry around an 18 percent gray card (available from any camera store) and, for critical exposures, actually use that card, placed in the subject area, to measure exposure. If the card is present in at least one final picture, it can be used to zero in on color balance in an image editor.
To meter properly in the Creative Zone, you'll want to choose both the metering method (how light is evaluated) and exposure method (how the appropriate shutter speeds and apertures are chosen). I'll describe both in the following sections.
The XTi has three different schemes used in Creative Zone modes for evaluating the light received by its exposure sensors. You can choose among them while the camera's exposure system is active (tap the shutter release to wake it up) by pressing the left cross key to produce the selection screen shown in Figure 4.3, and then pressing the left/right cross keys until the mode you want is selected. Press the Set button to confirm your choice.
■ Evaluative. The XTi slices up the frame into 35 different zones, shown in Figure 4.4. The zones used are linked to the autofocus system. The camera evaluates the measurements, giving extra emphasis to the metering zones that indicate sharp focus, to make an educated guess about what kind of picture you're taking, based on examination of thousands of different real-world photos. For example, if the top sections of a picture are much lighter than the bottom portions, the algorithm can assume that the scene is a landscape photo with lots of sky. This mode is the best all-purpose metering method for most pictures. I'll explain how to choose an autofocus/exposure zone in the section on autofocus operation later in this chapter.
■ Partial. This is a faux spot mode, using roughly nine percent of the image area to calculate exposure, which, as you can see by Figure 4.5, is a rather large spot. Use this mode if the background is much brighter or darker than the subject.
■ Center-weighted. In this mode, the exposure meter emphasizes a zone in the center of the frame to calculate exposure, as roughly shown in Figure 4.5, on the theory that, for most pictures, the main subject will be located in the center. Center-weighting works best for portraits, architectural photos, and other pictures in which the most important subject is located in the middle of the frame. As the name suggests, the light reading is weighted toward the central portion, but information is also used from the rest of the frame. If your main subject is surrounded by very bright or very dark areas, the exposure might not be exactly right. However, this scheme works well in many situations if you don't want to use one of the other modes.
Figure 4.3 Press the Metering Mode button (the left cross key) and then choose from Evaluative, Partial, or Center-weighted averaging modes.
Figure 4.6 Center-weighted metering measures the entire scene, but emphasizes the area in the center of the frame.
Figure 4.6 Center-weighted metering measures the entire scene, but emphasizes the area in the center of the frame.
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Choosing a Creative Zone Exposure Method
You'll find five methods for choosing the appropriate shutter speed and aperture within the Creative Zone settings. Your choice of which is best for a given shooting situation will depend on things like your need for lots of (or less) depth-of-field, a desire to freeze action or allow motion blur, or how much noise you find acceptable in an image. Each of the Digital Rebel XTi's exposure methods emphasizes one aspect of image capture or another. This section introduces you to all five available on the Mode Dial.
The Automatic Depth-of-Field exposure mode is a handy method to use when you want to maximize the range of sharpness in your image. The camera actually selects an f/stop that will allow as much of the subject matter you've framed as possible to be in sharp focus, and then it applies the shutter speed necessary to provide a good exposure at that aperture. The disadvantage of this mode is that you relinquish control over shutter speed, f/stop, and focus distance (which makes A-DEP a bit more like a Basic Zone scene mode than a true Creative Zone mode). You might end up with the required depth-of-field (DOF), but you might get a blurry photo because the XTi has selected a shutter speed that's too slow for handholding!
A-DEP performs its magic by examining the nine autofocus points in the viewfrnder to discover the nearest and farthest objects in the frame. Then, it chooses an aperture and focus point that supplies the required DOF (if possible) and sets the appropriate shutter speed. Obviously, you must have the lens set to autofocus for this mode to work (if not, the XTi switches to P exposure mode). The focus zones that can be rendered in sharp focus will flash red; other zones that can't be included in the focus range remain black, as shown in Figure 4.7. Press the DOF button on the front of the camera while holding the shutter release down halfway to check the range of focus. This mode won't work under all conditions, for example, with flash or if you're using manual focus. The viewfinder provides you with status information:
■ Flashing red AF points. Shows the subjects covered by the DOF range set.
■ Blinking aperture indicator in viewfinder. The desired DOF range cannot be set, because the subjects are separated too widely for sufficient depth-of-field at the smallest available aperture.
■ Blinking 30 shutter speed in viewfinder. Illumination is too dim to provide requested DOF at the current ISO setting.
■ Blinking 4000 shutter speed in viewfinder. Illumination is too bright to provide requested DOF at the current ISO setting.
Figure 4.7 A-DEP mode can provide automatic depth-of-field for many types of subjects.
Because of the limitations of A-DEP mode, I don't favor it. However, it's fun to play with and may come in handy in certain situations, especially when you're shooting quickly and don't have time to manipulate depth-of-field manually.
In Av mode, you specify the lens opening used, and the XTi selects the shutter speed. Aperture priority is especially good when you want to use a particular lens opening to achieve a desired effect. Perhaps you'd like to use the smallest f/stop possible to maximize depth-of-field in a close-up picture. Or, you might want to use a large f/stop to throw everything except your main subject out of focus. Maybe you'd just like to "lock in" a particular f/stop because it's the sharpest available aperture with that lens. Or, you might prefer to use, say, f/2.8 on a lens with a maximum aperture of f/1.4, because you want the best compromise between speed and sharpness.
Aperture priority can even be used to specify a range of shutter speeds you want to use under varying lighting conditions, which seems almost contradictory. But think about it. You're shooting a soccer game outdoors with a telephoto lens and want a relatively high shutter speed, but you don't care if the speed changes a little should the sun duck behind a cloud. Set your XTi to Av, and adjust the aperture until a shutter speed of, say, 1/1000th second is selected at your current ISO setting. (In bright sunlight at ISO 400, that aperture is likely to be around f/11.) Then, go ahead and shoot, knowing that your XTi will maintain that f/11 aperture (for sufficient DOF as the soccer players move about the field), but will drop down to 1/750th or 1/500th second if necessary should the lighting change a little.
A blinking 30 or 4000 shutter speed in the viewfinder and status LCD indicates that the XTi is unable to select an appropriate shutter speed at the selected aperture and that over- and underexposure will occur at the current ISO setting. That's the major pitfall of using Av: you might select an f/stop that is too small or too large to allow an optimal exposure with the available shutter speeds. For example, if you choose f/2.8 as your aperture and the illumination is quite bright (say, at the beach or in snow), even your camera's fastest shutter speed might not be able to cut down the amount of light reaching the sensor to provide the right exposure. Or, if you select f/8 in a dimly lit room, you might find yourself shooting with a very slow shutter speed that can cause blurring from subject movement or camera shake. Aperture priority is best used by those with a bit of experience in choosing settings. Many seasoned photographers leave their XTi set on Av all the time.
Shutter priority (Tv) is the inverse of aperture priority: you choose the shutter speed you'd like to use, and the camera's metering system selects the appropriate f/stop. Perhaps you're shooting action photos and you want to use the absolute fastest shutter speed available with your camera, or you might want to use a slow shutter speed to add some blur to an otherwise static photograph.
Shutter priority mode gives you some control over how much action-freezing capability your digital camera brings to bear in a particular situation. For the action photo shown in Figure 4.8, I set the shutter speed to 1/1000th second to freeze the pole vaulter as he passed over the bar, and let the Rebel XTi choose the right aperture.
However, you'll need to keep the amount of existing light in mind when choosing your preferred shutter speed. Otherwise, you'll also encounter the same problem as with aperture priority when you select a shutter speed that's too long or too short for correct exposure. I've shot outdoor soccer games on sunny Fall evenings and used shutter priority mode to lock in a 1/1000th second shutter speed, only to find my XTi refused to shoot when the sun dipped behind some trees and there was no longer enough light to shoot at that speed, even with the lens wide open.
Like Av mode, it's possible to choose an inappropriate shutter speed. If that's the case, the maximum aperture of your lens (to indicate underexposure) or the minimum aperture (to indicate overexposure) will blink.
Program mode (P) uses the XTi's built-in smarts to select the correct f/stop and shutter speed using a database of picture information that tells it which combination of shutter speed and aperture will work best for a particular photo. If the correct exposure cannot be achieved at the current ISO setting, the shutter speed indicator in the viewfinder will blink 30 or 4000, indicating under- or overexposure (respectively). You can then boost or reduce the ISO to increase or decrease sensitivity.
The XTi's recommended exposure can be overridden if you want. Use the EV setting feature (described later, because it also applies to Tv and Av modes) to add or subtract exposure from the metered value. And, as I mentioned earlier in this chapter, you can change from the recommended setting to an equivalent setting (as shown in Table 4.1) that produces the same exposure, but using a different combination of f/stop and shutter speed. To accomplish this:
1. Press the shutter release down halfway to lock in the current base exposure, or press the AE Lock button on the back of the camera. (The latter method actually makes the next step easier for me.)
2. Spin the Main Dial to change the shutter speed (the XTi will adjust the f/stop to match).
Your adjustment remains in force for a single exposure; if you want to change from the recommended settings for the next exposure, you'll need to repeat those steps.
Sometimes you'll want more or less exposure than indicated by the Digital Rebel XTi's metering system. Perhaps you want to underexpose to create a silhouette effect, or overexpose to produce a high-key look. It's easy to use the XTi's Exposure Compensation system to override the exposure recommendations. Press the shutter release halfway, then press and hold the Aperture/Exposure Compensation button (to the upper right of the LCD panel), and rotate the Main Dial to add or subtract exposure. The exposure scale in the viewfinder and on the status LCD indicates the EV change you've made. The EV change you've made remains for the exposures that follow, until you manually zero out the EV setting with the Main Dial. EV changes are ignored when using M (manual exposure) or any of the Basic Zone modes.
Part of being an experienced photographer comes from knowing when to rely on your Digital Rebel XTi's automation (including P mode and Basic Zone settings), when to go semiautomatic (with Tv or Av), and when to set exposure manually (using M). Some photographers actually prefer to set their exposure manually, as the XTi will be happy to provide a viewfinder indication of when its metering system agrees that the settings provide the proper exposure.
Manual exposure can come in handy in some situations. You might be taking a silhouette photo and find that none of the exposure modes or EV correction features give you exactly the effect you want. Set the exposure manually to use the exact shutter speed and f/stop you need. Or, you might be working in a studio environment using multiple flash units. The additional flashes are triggered by slave devices (gadgets that set off the flash when they sense the light from another flash, or, perhaps from a radio or infrared remote control). Your camera's exposure meter doesn't compensate for the extra illumination, so you need to set the aperture manually.
Even though, depending on your proclivities, you might not need to set exposure manually very often, you should still make sure you understand how it works. Fortunately, the Digital Rebel XTi makes setting exposure manually very easy. Just set the Mode Dial to M, turn the Main Dial to set the shutter speed, and hold down the Aperture/Exposure Compensation button while spinning the Main Dial to adjust the aperture. Press the shutter-release button halfway or press the AE Lock button, and the exposure scale in the viewfinder shows you how far your chosen setting diverges from the metered exposure.
Selecting an Autofocus/Exposure Zone Manually
If you're using P, Av, Tv, or M shooting modes and Evaluative metering, you can select the autofocus zone manually (and thus the zone linked to the metering system). As I mentioned in Chapter 1, the Digital Rebel XTi uses seven different focus points to calculate correct focus. In A-DEP, or any of the Basic Zone shooting modes, the focus point is selected automatically by the camera. In the other Creative Zone modes, you can allow the camera to select the focus point automatically, or you can specify which focus point should be used.
To set the focus point manually, look through the viewfinder and then press the AF point selection button on the extreme upper-right corner of the back of the camera, and use either the cross keys (which move highlighting directly to a point) or the Main Dial (which rotates through all the points in turn) to select the zone you want to use. As you cycle among the focus points, each will be illuminated in turn, and will be highlighted on the status LCD. Press the Set button to switch back and forth between the center autofocus point and automatic selection. In Evaluative metering mode, the focus point you select will be emphasized in calculating exposure, as shown in Figure 4.9.
Adjusting Exposure with ISO Settings
Another way of adjusting exposures is by changing the ISO sensitivity setting when you're using one of the Creative Zone modes. Sometimes photographers forget about this option, because the common practice is to set the ISO once for a particular shooting session (say, at ISO 100 or 200 for bright sunlight outdoors, or ISO 800 when shooting indoors) and then forget about ISO. The reason for that is that ISOs higher than 100 or 200 are seen as "bad" or "necessary evils." However, changing the ISO is a valid way of adjusting exposure settings, particularly with the Canon Digital Rebel XTi, which produces good results at ISO settings that create grainy, unusable pictures with some other camera models.
Indeed, I find myself using ISO adjustment as a convenient alternate way of adding or subtracting EV when shooting in manual mode, and as a quick way of choosing equivalent exposures when in automatic or semiautomatic modes. For example, I've selected a manual exposure with both f/stop and shutter speed suitable for my image using, say, ISO 200. I can change the exposure in 1/3 stop increments by tapping the ISO key (the up cross key). The menu on the LCD, shown in Figure 4.10, will display a list of ISO settings, which you can then choose using the up/down cross keys or by rotating the Main Dial and pressing the Set button to lock in your choice. Only one-stop increments are available: ISO 100, 200, 400, 800, and 1600. I can keep my preferred f/stop and shutter speed, but still adjust the exposure in manual mode by varying the ISO setting.
Figure 4.10 Choose from ISO 100 through ISO 1600 in the ISO Speed menu.
Or, perhaps I am using Tv mode and the metered exposure at ISO 200 is 1/500th second at f/11. If I decide on the spur of the moment I'd rather use 1/500th second at f/8, I can tap the ISO button and switch quickly to ISO 100. Of course, it's a good idea to monitor your ISO changes, so you don't end up at ISO 1600 accidentally. ISO settings can, of course, also be used to boost or reduce sensitivity in particular shooting situations.
Bracketing is a method for shooting several consecutive exposures using different settings, as a way of improving the odds that one will be exactly right. Before digital cameras took over the universe, it was common to bracket exposures, shooting, say, a series of three photos at 1/125th second, but varying the f/stop from f/8 to f/11 to f/16. In practice, smaller than whole-stop increments were used for greater precision. Plus, it was just as common to keep the same aperture and vary the shutter speed, although in the days before electronic shutters, film cameras often had only whole increment shutter speeds available.
Today, cameras like the XTi can bracket exposures much more precisely using auto exposure bracketing (AEB). When this feature is activated, the XTi takes three consecutive photos: one at the metered "correct" exposure, one with less exposure, and one with more exposure, using an increment of your choice up to +2/-2 stops. Choose between increments by setting Custom Function 06 to 0 (1/3 stop) or 1 (1/2 stop). In Av mode, the shutter speed will change, while in Tv mode, the aperture will change.
Bracketing can also be used with image editor features like Photoshop's Merge to HDR capability, which allows combining multiple images taken at different exposures to incorporate the shadow, midtone, and highlight detail from each into one photo with a longer tonal range. Your camera needs to be placed on a tripod, of course, to ensure that each of the three images will differ only in exposure.
Using AEB is trickier than it needs to be, but you can follow these steps:
1. Press Menu button and press the Jump button to skip to the Shooting 2 menu. Scroll to the AEB position and press the Set button. A green dot appears on the plus/minus scale.
2. Use the left/right cross keys or Main Dial to spread the three dots to the range you want for your bracketed set. For example, with the dots clustered tightly together, the three bracketed exposures will be spread out over a single stop. Separating the cluster produces a wider range and larger exposure change between the three shots in the bracket set, as shown in Figure 4.11.
3. Press the Set button to enter the settings and the Menu button to exit the menu system. The bracketing icon appears on the LCD display.
4. Take your three photos. You can use single shooting mode to take the trio of pictures yourself, use the self-timer (which will expose all three pictures after the delay), or switch to continuous shooting mode to take the three pictures in a burst.
Figure 4.11 Rotate the Main Dial or use the left/right cross keys to set the increment between bracketed shots.
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