Selecting AF points

In most Creative Zone modes, you can either manually choose a single AF point , or you can let the camera automatically select the AF points. Each option has benefits and drawbacks.

Note A-DEP mode, the camera automatically selects the AF points to provide the optimal depth of field, and you cannot manually select an AF point.

The advantage of setting the AF point manually is getting the point of sharpest focus precisely where you want it to be in the image. Manual AF point selection takes a bit more time than letting the camera set the AF points, but you're assured of tack-sharp focus in the appropriate spot, provided that you don't move the camera to recompose the image after setting focus.

Tip You can simplify the method for changing the AF point by selecting among several alternatives offered by C.Fn 111-3. For example, you can choose to use only the Quick Control dial or the Multi-controller to select an AF point, which is much faster during shooting. You can learn more about Custom Functions in Chapter 4.

If you let the camera automatically select the AF point or points, the camera focuses on the subject or the part of the subject that is closest to the lens that has readable contrast. If you're shooting a portrait, automatic AF will likely set the sharpest focus on the subject's nose rather than the subject's eyes. This method is faster in terms of shooting, but very often the point of sharpest focus is not where it should be within the subject.

2.1 In this image, I used automatic AF-point selection, and the camera set the point of sharpest focus to the right side of center instead of on the center of the flower. For this reason, I use manual AF-point selection in all of my images. Exposure: ISO 100, f/2.8, 1/250 sec. using an EF 100mm f/2.8mm Macro USM lens.

2.1 In this image, I used automatic AF-point selection, and the camera set the point of sharpest focus to the right side of center instead of on the center of the flower. For this reason, I use manual AF-point selection in all of my images. Exposure: ISO 100, f/2.8, 1/250 sec. using an EF 100mm f/2.8mm Macro USM lens.

Improving Autofocus Accuracy and Performance

Autofocus speed depends on factors including the size and design of the lens, the speed of the lens-focusing motor, the speed of the autofocus sensor in the camera, the amount of light in the scene, and the level of subject contrast. Given these variables, it's helpful to know how to get the speediest and sharpest focusing. Here are some tips for improving overall autofocus performance.

♦ Light. In low-light scenes, the autofocus performance depends in part on the lens speed and design. In general, the faster the lens, the faster the autofocus performance. Provided that there is enough light for the lens to focus without an AF-assist beam, then lenses with a rear-focus optical design, such as the EF 85mm f/1.8 USM, will focus faster than lenses that move their entire optical system, such as the EF 85mm f/1.2L II USM. But regardless of the lens, the lower the light, the longer it takes for the system to focus.

Low-contrast subjects and/or subjects in low-light slow down focusing speed and can cause autofocus failure. With a passive autofocus sysem, autofocus-ing depends on the sensitivity of the AF sensor. Thus, autofocusing performance will always be faster in bright light than in low light, and this is true in both One-Shot and AI Servo AF modes. In low light, consider using the built-in flash or an accessory EX Speedlite's AF-assist beam as a focusing aid using C.Fn III-5. The AF assist beam fires twice; first, a prefire to communicate focusing distance data to the camera, then a second beam to confirm that the subject is in focus, and then the shutter fires.

♦ Focal length. The longer the lens, the longer the time to focus. This is true because the range of defocus is greater on telephoto lenses than on normal or wide-angle lenses. You can improve the focus time by manually setting the lens in the general focusing range, and then using autofocus to set the sharp focus. However, the 40D's new AF sensor is better able to detect extreme defocus situations and performs better than previous models.

♦ AF-point selection. Manually selecting a single AF point provides faster autofocus performance than using automatic AF-point selection because the camera doesn't have to determine and select the AF point(s) to use first.

♦ Subject Contrast. Focusing on low-contrast subjects is slower than on high-contrast subjects. If the camera can't focus, shift the camera position to an area of the subject that has higher contrast, such as a higher contrast edge.

♦ EF Extenders. Using an EF Extender reduces the speed of the lens-focusing drive.

♦ Wide-angle lenses and small apertures. Sharpness can be degraded by diffraction when you use small apertures with wide-angle or wide-angle zoom lenses. Diffraction happens when light waves pass around the edges of an object and enter the shadow area of the subject producing softening of fine detail. To avoid diffraction, avoid using apertures smaller than f/16 with wide-angle prime (single-focal length) and zoom lenses.

To manually select an AF point or to choose automatic AF, follow these steps:

1. Set the camera to a Creative Zone mode except A-DEP, and then press the Shutter button halfway. The currently selected AF point or points light in red in the viewfinder and are displayed on the LCD panel.

2. Press the AF-Point Selection/Enlarge button on the back top-right side of the camera. The selected AF point lights in red in the viewfinder and is displayed on the LCD panel.

3. Turn the Main or Quick Control dial or tilt the Multi-controller in the direction of the AF point you want to select and select one AF point. Or to have the camera automatically select the AF point or points, turn the Main or Quick Control dial or the Multi-controller until all the AF points are lit in red. If you're using the Multicontroller, you can also press the controller in the center to select the center AF point.

4. Press the Shutter button halfway to focus using the selected AF point or the camera's selected AF points. The camera beeps when accurate focus is achieved and the autofocus light in the viewfinder remains lit continuously. If you don't hear the beep, focus on a higher contrast area or use the built-in flash assist beam (C.Fn III-5).

5. Press the Shutter button completely to make the picture.

Tip Manual focusing is available by sliding the switch on the side of the lens from the AF to MF position. Alternately, you can use full-time manual focusing by letting the camera automatically set the focus, and then tweaking it by manually turning the focusing ring on the lens.

It's important to know that in normal shooting when you don't use the AF-ON button to focus, the AF point that achieves focus is also the point where exposure is set. In other words, whatever AF point you choose is also the point where the camera meters the scene light and determines the exposure settings to use.

But the AF-point location may or may not be the point of critical exposure. For example, if you're shooting a portrait and the model has a bright highlight on part of his or her face, the highlight is the point where you want to set the exposure to ensure that detail is retained in the bright highlight area. Obviously, that isn't where you want to set the point of sharpest focus. It's necessary then to decouple autofocus from metering by using Auto Exposure Lock, a technique discussed later in this chapter.

Tip If you're shooting in a quiet venue such as a wedding ceremony, you can silence the AF confirmation beeper. Press the Menu button. On the Shooting 1 (red) menu, select Beep, and then press the Set button. Select Off, and then press the Set button. You can verify when the camera has achieved sharp focus by watching the focus confirmation light on the far-right bottom of the viewfinder.

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