Chapter Five: Capturing Light—The Exposure Parameters
ISO
With aperture controlled by your choice of lens and shutter speeds limited by the laws of physics and principles of engineering, the EOS 5D Mk III has pushed the remaining variable to yet another degree of sensitivity. Whereas the EOS 5D Mk II offered an ISO speed range up to 25,600, the Mark III extends its speed range to 102,400. While you will seldom need such a high ISO, in implementing that range, Canon also extended the range of low-noise ISOs. As a general rule, whatever ISO you chose on the Mark II, you can multiply that value by four and expect approximately the same amount of noise. Of course, if you want a long exposure, you can set the ISO to as low as 50, as you could with the Mark II.
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 of which you will encounter on the SHOOT2 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 ISO variables. The first of those variables is ISO Speed, which is the numeric value used to represent the effective light sensitivity of the image sensor. 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, but the extent of that change can be limited by values you specify in the Auto ISO Range option (if proper exposure still cannot be achieved within those limits, the camera responds as it would for any over- or underexposed 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 to be available for when the camera attempts to automatically establish the ISO setting to be used.
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 for setting the ISO value, your choices will be limited to those described by the minimum and maximum, and the values in between them.
Setting ISO on the EOS 5D Mk III can be accomplished in a variety of ways:
• By pressing the ISO Speed Setting / Flash Exposure Compensation button and rotating the Main Dial.
• By pressing the Quick Control (Q) button, using the Multi-controller’s joystick to access the ISO section, then rotating either the Main Dial or the Quick Control Dial to scroll through the array of possible ISO values and select the desired ISO, then press SET to commit it and return to the Quick Control screen.
• On the SHOOT2 menu, navigate to ISO Speed Settings > ISO Speed>, then either rotate the Quick Control Dial or use the Multi-controller’s joystick to scroll through the array of possible ISO values to select the desired ISO, and press SET.
As the EOS 5D Mk III comes from the factory, 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; and 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 set the C.Fn1 menu’s ISO Speed Setting Increments option to use full-stop increments. That will certainly speed up scrolling through the list!
If you choose to use either the menu system or the Quick Control screen for setting ISO, you will be able to view each and every ISO value the camera can use. However, if you use the ISO Speed Setting / Flash Exposure Compensation button approach, the LCD panel will display only a single value as you use the Main Dial to scroll, and that value will be within the range specified for ISO Speed Range. There’s an interesting feature added when you use the ISO Speed Setting / Flash Exposure Compensation button approach: If you have the INFO. screen open to show shooting settings, then the entire ISO array is displayed there. But notice that any values outside the limits set in ISO Speed Range are grayed out and are not accessible (that’s also true when using the menu system or the Quick Control screen). If allowed by the values specified in ISO Speed Range, then 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).
Figure 5-1. ISO Speed displayed on the Quick Control screen
Aperture
Though 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 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) 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 to the right.
If you press the shutter button halfway and see the shutter speed is set to 30” and that value is blinking, it means the settings will result in underexposure. Similarly, if you press the shutter button halfway and notice the shutter speed is now set to 8000, and that value is blinking, it means your settings will result in an overexposure. In either case, you need to adjust the aperture setting, change 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 main style of photography places limits on your minimum shutter speed but also requires aperture-priority, then you may want to adjust the minimum. Go to the SHOOT2 menu, navigate to ISO Speed Settings > Min. Shutter Speed>, then either rotate the Quick Control Dial or use the Multi-controller’s joystick to scroll through the array of possible shutter-speed values to select the desired minimum shutter speed. Press SET to commit it. The Min. Shutter Speed value is ignored in flash photography.
Shutter Speed
In the past, even shutter speed was a function of the mounted lens. But as the frame size got smaller and film became more sensitive to light, it became practical to implement a shutter closer to the image-capturing element, which was first film and later a digital image sensor. That has allowed shutter speed to become quite short, with 1/8000 second being common in today’s better DSLRs.
Varying shutter speed not only is a factor in establishing the correct exposure, but is also a primary means of displaying or limiting the appearance of motion on the part of the subject. It’s also a factor in image sharpness. For handheld photos, too long a shutter speed allows human-body motion—even motion caused by a heartbeat—to significantly impact 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 is shown in three different styles, depending on its absolute value:
1. For fractions of a second in the range of ¼ second to 1/8000 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).
2. 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.
3. Similarly, 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 see the aperture is set to the maximum of the lens (e.g., f/2.8), and that value is blinking, it 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, it means your settings will result in an overexposure. In either case, you need to adjust the shutter speed setting, change the ISO speed setting, 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
Having determined that the elements of exposure are effective image-sensor sensitivity (ISO), the amount of light allowed to fall on the sensor per unit time (aperture), and the amount of time that light will be allowed to fall on the sensor, we then need to determine just what part of our composition should be evaluated for brightness. The EOS 5D Mk III offers four means to accomplish that:
1. Evaluative metering 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 juxtaposed with very dark areas. Unlike partial and spot metering, which tend to optimize an exposure for a small portion of a composition at the expense of the balance of the photo, 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 many values against 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, any 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 that it would seem to change color. Similarly, if the camera were focused on the sled, then the sled would have fairly good detail, and the snow might look reasonable, but the ski outfit would be so dark as to become unrecognizable except as a silhouette. Using a larger area for determining exposure, and giving consideration to numerous points in that area, evaluative metering does a fairly good job of determining a reasonable exposure without introducing either large washed-out areas or large overly dark areas.
2. Partial metering measures approximately 6.2% of the center of the viewfinder area. This is a form of metering that is especially useful for capturing a subject against strong backlighting. Recognize, though, that to determine exposure settings for that type of photo, partial metering essentially ignores the area outside the central circle and the focus point has no bearing on the determination of the exposure.
3. Spot metering is a special form of partial metering that meters only a central area of approximately 1.5% of the viewfinder area. Therefore, it ignores an even larger portion of the image. It, too, ignores the focus point in its calculations.
4. 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’re faced with a subject that is quite different in brightness than its surroundings, this mode can help a great deal to ensure that the subject is well exposed and the surroundings are not lost.