Chapter Five: Capturing Light—The Exposure Parameters
ISO
With the aperture controlled by your choice of lens, and shutter speeds limited by the laws of physics and the principles of engineering, the EOS 6D has pushed the remaining exposure variable to yet another degree of sensitivity. The EOS 6D, like the EOS 5D Mark III, extends the maximum ISO to 102,400. While you will seldom need such a high ISO, in implementing that range, the range of low-noise ISOs also is extended. Of course, if you want a very long exposure, you can set the ISO to as low as 50.
Control over ISO has also expanded over the years, and today we deal with terms like ISO Speed Settings, ISO Speed, ISO Speed Range, and Auto ISO Range, all terms you will encounter in the Shoot3 menu. ISO Speed Settings is probably the simplest to explain: it’s nothing more than a convenient term used to describe the collection of menu options available for setting the various ISO variables. The first of those variables is ISO Speed, which is the numeric value used to represent light sensitivity. ISO Speed can be either AUTO or one of the values between 50 and 102,400. With ISO Speed set to AUTO, the camera will attempt to keep the shutter speed and aperture within a factory-determined set of parameters. When proper exposure cannot be achieved by using those settings, the camera will automatically change the ISO speed. The extent of that change can be limited by values you specify in the Auto ISO Range option, but if proper exposure still cannot be achieved within those limits, the camera responds as it would for any over- or under-exposed composition. The Auto ISO Range option allows you to specify both a minimum ISO value and a maximum ISO value to constrain the range of values available when the camera attempts to automatically establish the ISO setting.
Similarly, the ISO Speed Range option allows you to choose the minimum and maximum ISO values that will be available for selection. Regardless of the method you choose to use for setting the ISO value, your choices will be limited to those described by the minimum and maximum, and those between the two values.
Setting the ISO speed on the EOS 6D can be accomplished in a variety of ways:
• One option is pressing the ISO Speed Setting button and rotating the Main Dial, the Quick Control Dial, or the Multi-controller. The LCD Panel will show only a single value as you scroll through the ISO range, but if the INFO button is set to Shooting Settings, then a portion of the entire ISO array is displayed on the LCD Monitor, allowing you to see what surrounding ISO values are available for selection (see figure 5-1).
Figure 5-1. ISO speed array, from the ISO button
• By pressing the Quick Control (Q) button, using the Multi-controller to access the ISO section, and then rotating either the Main Dial or the Quick Control Dial to scroll through the array of possible ISO values, you can select the desired ISO. To see the current portion of the ISO speed array as well as a description of the probable exposure conditions appropriate for any selected ISO value, press the Quick Control button, use the Multi-controller to select the ISO Speed section, press SET, then use the Main Dial, the Quick Control Dial, or the Multi-controller to scroll through the ISO values. Press SET again to commit the selected value and return to the Quick Control screen. If you do not press SET to return to the Quick Control screen, after six seconds the exposure description will be deleted from the screen. After another four seconds, the screen will revert to the INFO screen with the new value committed (see figure 5-2).
Figure 5-2. ISO Speed displayed on the Quick Control screen
Figure 5-3. ISO Speed setting from the MENU button
• In the Shoot3 menu, navigate to ISO Speed Settings > ISO Speed, and press SET. Then either rotate the Quick Control Dial or use the Multi-controller (the Main Dial cannot be used) to scroll through the array of possible ISO values to select the desired ISO and press SET to commit it. Notice in figure 5-3 that when accessed from the MENU button, the current ISO value is shown with a blue vertical bar, while the bar associated with the current scrolled position is orange.
In the factory settings for the EOS 6D, the ISO Speed is set to AUTO. The Auto ISO Range is set to a minimum of 100 and a maximum of 12,800 when the Mode Dial is set to Scene Intelligent Mode, or a maximum of 25,600 when the Mode Dial is in P, Tv, Av, or M mode. The ISO Speed Range is set to a minimum of 100 and a maximum of 25,600. The ISO values of 50, 25,600, 51,200, and 102,400 are considered expanded range values, and are represented by the characters L, H, H1, and H2, respectively. By default, within the ISO range of 100 through 25,600, ISO can be set in 1/3-stop increments. However, if you determine that you don’t need that level of precision, you can use the C.Fn I-2 menu’s ISO Speed Setting Increments option to use full-stop increments. That will certainly speed up scrolling through the list! Compare the ISO value display for third-stop increments shown in figure 5-3 with the display for full-stop increments shown in figure 5-4.
Figure 5-4. Display of ISO values, with ISO Speed increment set to full-stop
If you choose to use either the menu system or the Quick Control screen for setting the ISO, you will be able to view a range of ISO values the camera can use, but if you use the ISO Speed Setting button, the LCD Panel will display only a single value as you use the Main Dial, the Quick Control Dial, or the Multi-controller to scroll. That value will be within the specified ISO Speed Range. There’s an interesting added feature to using the ISO Speed Setting button: if you have the INFO screen open to show shooting settings, then the ISO array is displayed there. But notice that any values outside the limits set in ISO Speed Range are simply not displayed (that’s also true when using the menu system or the Quick Control screen). If allowed by the values specified in ISO Speed Range, any attempt to scroll to Auto; 50; 51,200; or 102,400 will be presented on the LCD Panel as A, L, H1, or H2 (strangely, the ISO value 25,600 is displayed as a five-digit number, not as H).
Aperture
Though the aperture is wholly a function of a lens, today’s DSLRs expect to be informed of the characteristics of the mounted lens, and will use that data—aperture being a part of it—in determining the correct exposure for each image capture. The beautiful part of this technology is that, regardless of the calculated aperture for an exposure, the lens will remain at maximum aperture until the Shutter button is pressed all the way down (or you press the Depth-of-Field Preview button). Allowing the lens to be at maximum aperture ensures that the maximum amount of light is passed into the body of the camera for use in composition and focusing. When the aperture becomes the more significant factor in an exposure, typically because of concerns over depth of field, ensure that the Mode Dial has been set to the Av (Aperture-Priority AE) mode. In Av mode, the camera will expect you to specify the aperture to be used. Then, based on the ISO, the camera will calculate the shutter speed to be used.
To set the aperture while in Av mode, simply rotate the Main Dial to the desired value. If the current aperture is replaced with the letter L when you attempt to set the aperture, the Multi-function lock switch has been set to the right; move it to the left, set your desired aperture, then (if you want to re-enable the lock) slide the lock switch to the right.
If you press the Shutter button halfway and you see the shutter speed is set to 30”, and that value is blinking, that means the settings will result in underexposure. Similarly, if you press the Shutter button halfway and notice the shutter speed is now set to 1/4000 second, and that value is blinking, that means your settings will result in an overexposure. In either case, you need to adjust the aperture setting, the ISO Speed setting, or both.
Note that, by default, when you have the Mode Dial set to M (for Manual exposure mode), the Main Dial is used for changing the shutter speed. The Quick Control Dial must be used to change the aperture.
If your preferred style of photography has some limits on the minimum shutter speed you can use in addition to your need for aperture-priority, then you may want to use the Shoot3 menu. Navigate to ISO Speed Settings > Min. Shutter Speed and either rotate the Quick Control Dial or use the Multi-controller to scroll through the possible shutter-speed values. Select the desired minimum shutter speed and press SET to commit it. The Min. Shutter Speed value is ignored in flash photography.
Shutter Speed
A half century ago, even shutter speed was often a function of the mounted lens. As the negative size got smaller and film became more sensitive to light, it became practical to implement a shutter closer to the image-capturing element, whether it was film or a digital image sensor. That has allowed shutter speeds to become quite short, with 1/4000 second being the shortest shutter speed of the EOS 6D.
Varying the shutter speed not only is a factor in establishing the correct exposure, but also a primary means of displaying or limiting the appearance of motion by the subject. It’s also a factor in image sharpness; with handheld shooting, too long a shutter speed allows human-body motion—even motion caused by a heart-beat—to have a significant impact on your final picture.
For shutter-priority shooting, set the Mode Dial to Tv, then rotate the Main Dial to scroll through the range of available shutter speeds.
The shutter speed display uses three different styles, depending on the absolute value:
• For fractions of a second in the range of 1/4 second to 1/4000 second, you will see only the value representing the denominator (e.g., 8 would represent 1/8 second, and 400 would represent 1/400 second).
• For fractions from 0.3 second through 0.8 second, the value is expressed in the form 0”n, where n is the fractional tenth of a second.
• Full-second shutter speeds are displayed in the form n”, in which n represents the integer number of seconds. Therefore, a display of 3”2 would indicate a shutter speed of 3.2 seconds.
In a manner much like that for setting the aperture, if you press the Shutter button halfway and you see the aperture is set to the maximum of the lens (e.g., f/4.0) and that value is blinking, that means the settings will result in underexposure. Similarly, if you press the Shutter button halfway and notice the aperture is set to the minimum of the lens (e.g., f/22), and that value is blinking, that means your settings will result in an overexposure. In either case, you need to adjust the shutter speed, the ISO speed, or both.
Note that, by default, when you have the Mode Dial set to M (for Manual), the Quick Control Dial is used to change the aperture. The Main Dial must be used for changing the shutter speed.
Metering
We’ve determined that the elements of exposure include the image-sensor sensitivity (ISO), the amount of light allowed to fall on the sensor per unit of time (aperture), and the amount of time that light will be allowed to fall on the sensor. Now, you need to determine just what part of the composition should be evaluated for brightness. The EOS 6D offers four means of accomplishing this:
• Evaluative Metering. This is considered the “work horse” metering option. Using all 63 exposure metering zones, this system is able to recognize complex exposure problems caused by very bright areas being juxtaposed with very dark areas. Partial Metering and Spot Metering tend to optimize an exposure for a small portion of a composition at the expense of the photo’s balance, but Evaluative Metering considers brightness from a 7x9 grid of the entire area of the composition. It factors in the brightness of the selected point of focus (so, be sure the camera is focused on the correct area of the scene), and then maps those values to a database of thousands of images to determine the optimum exposure.
Imagine a dark blue ski outfit and white sled in the snow on a bright day. If Partial Metering or Spot Metering were used and focused on the ski outfit, the snow would be so blown out that it would essentially disappear in the final photo. Most detail in the sled would be lost, and the ski outfit would be muted to the point of losing color. Similarly, if the camera were focused on the sled, then the sled would have pretty good detail, and the snow might look reasonable, but the ski outfit would become dark enough to be unrecognizable except as a silhouette. Using a larger area for determining exposure, and giving consideration to numerous points in that area, Evaluative Metering determines a reasonable exposure without introducing large, washed-out areas or large, dark areas.
• Partial Metering meters approximately 8.0 % of the center of the viewfinder area. This is a form of weighted metering that is especially useful for capturing a subject against strong backlighting. Recognize, though, that in determining exposure settings for this photo, Partial Metering will essentially ignore the area outside the central circle, and that the focus point will have no bearing on the determination of the exposure.
• Spot Metering is a special form of Partial Metering that meters only a central area of approximately 3.5 % of the viewfinder area (the area enclosed by the circle shown on the viewfinder). It, too, ignores the focus point in its calculations.
• Center-weighted Average Metering allows the entire scene to contribute to the final result, with the center of the composition contributing more to the weighted average in the calculation. Here, again, the current focus point is not considered. This metering mode is essentially a blend of the Evaluative Metering mode and the Partial Metering mode: all 63 zones provide input, but as you approach the center of the image, more and more emphasis is placed in the calculations. When you are faced with a subject that is quite different in brightness from its surroundings, this mode can help a great deal to ensure that the subject is well exposed, and the surroundings are not lost.