In some low-light scenes, you may want to shoot using ambient light without using the flash. However, the camera may have trouble establishing good focus due to the low light. The autofocus-assist (AF-assist) beam from the flash is invaluable in these situations to help the camera establish focus. In these kinds of scenes, you can disable the flash firing and still use the flash's AF-assist beam to help the camera to establish focus.

To enable or disable flash firing but still allow the camera to use the flash's AF-assist beam for focusing, follow these steps:

Also, with Custom Function, C.Fn-7, you can choose whether the AF-assist beam is fired by the camera's built-in flash or by an accessory Speedlite. If this function is set to 1: Disable, the AF-assist beam is not used. If you've set the Custom Function on the Speedlite for AF-assist beam firing to Disable, the AF-assist beam is not used regardless of what option you choose on the camera for C.Fn-7. You can also select option 2: Only external flash emits so that only the Speedlite's AF-assist beam is fired and the built-in flash AF-assist beam does not fire.

A disadvantage of flash exposure in portraits of people and pets is unattractive red in the subject's eyes. There is no sure fix that prevents red eye, but a few steps help reduce it. First, be sure to turn on Red-Eye Reduction on the XSi/450D. This option is set to Off by default. Then, before making the picture, have the subject look at the Red-Eye Reduction lamp on the front of the camera when it fires at the beginning of a flash exposure. Also have the room well lit. Before you begin, ensure that the Flash Firing is set to Enable on the Set-up 2 (yellow) menu. To turn on Red-Eye Reduction, follow these steps:

One way to modify flash output is by using Flash Exposure Lock (FE Lock). Much like Auto Exposure Lock (AE Lock), FE Lock allows you to meter and set the flash output on any part of the scene.

With FE Lock, you lock the flash exposure on a midtone area in the scene. The camera calculates a suitable flash output, and locks or remembers the exposure, and then you can recompose, focus, and make the picture.

FE Lock is also effective when there are reflective surfaces such as a mirror, chrome, or glass in the scene. Without using FE Lock, the camera takes the reflective surface into account and reduces the flash, causing underexposure. Instead, set FE Lock for a midtone area in the scene that does not include the reflective surface, and then make the exposure.

To set FE Lock, follow these steps:

FE Lock is a practical technique to use when shooting individual images. But if you're shooting a series of images under unchanging light, then FEC is more efficient and practical.

Flash Exposure Compensation (FEC) is much like Auto Exposure Compensation in that you can increase or decrease flash exposure up to +/-2 stops in 1/3-stop increments. A positive compensation increases the flash output and a negative compensation decreases the flash output. The compensation is applied to all flash exposures until you reset the compensation back to 0. As with non-flash exposure, the camera is calibrated for an "average" scene of 18 percent gray, and flash exposures can be thrown off by very light and very dark subjects or scenes. As a result, very light subjects may need increased flash exposure ranging from +0.5 to +1.5 stops. Dark-toned subjects may need a flash reduction of approximately −1 to −2 stops. Some experimentation is required because subject and scene tonality varies.

If you use flash compensation to create a more natural-looking portrait in daylight, try setting negative compensation it between −1 and −2 stops. Also be aware that in bright light, the camera assumes that you're using fill flash to reduce dark shadows and automatically provides flash reduction. Experiment with the Rebel XSi/450D in a variety of lighting to know what to expect.

It's important to note that if you use FEC, you may not see much if any difference with negative compensation because the XSi/ 450D has Auto Lighting Optimizer turned on by default for all JPEG images. This feature automatically corrects images that are underexposed or have low contrast. So if you set FEC to a negative setting to reduce flash output, the camera may detect the image as being underexposed (too dark) and automatically brighten the picture.

Figure 7.2. This image was taken with the built-in flash with no flash output modifications. The main light is from a window to camera left. A large silver reflector is positioned to camera right. The dark shadow behind the dog is a disadvantage of using unmodified flash. Exposure: ISO 100, f/4, 1/15 second, Av mode, custom white balance.

If you're using FEC, it's best to turn off Auto Lighting Optimizer. Just go to the Set-up 3 (yellow) menu, and then press the down cross key to select Custom Functions (C.Fn). Press the Set button. Press the right or left cross key until the C.Fn II: Image, Auto Lighting Optimizer screen appears and the number 6 is displayed in the box at the top right of the screen. Press the Set button to activate the option controls. Press the down cross key to select option 1: Disable, and then press the Set button. The Auto Lighting Optimizer remains disabled in Creative Zone Modes until you enable it again.

Figure 7.3. For this image, flash exposure compensation was set to −1 Exposure Value (EV). The lowered flash exposure provides a more natural appearance than straight-on flash and lightens the shadow behind the dog. Exposure: ISO 100, f/4, 1/15 second, Av mode.

If you use an accessory Speedlite, you can set FEC either on the camera or on the Speedlite. However, the compensation that you set on the Speedlite overrides any compensation that you set on the XSi/450D's Set-up 2 (yellow) camera menu. If you set compensation on both the Speedlite and the camera, the Speedlite setting overrides what you set on the camera. So set compensation either on the Speedlite or on the camera, but not on both.

FEC can be combined with Exposure Compensation. If you're shooting a scene where one part of the scene is brightly lit and another part of the scene is much darker — for example, an interior room with a view to the outdoors — then you can set Exposure Compensation to −1 and set the FEC to −1 to make the transition between the two differently lit areas more natural.

To set FEC for the built-in flash, follow these steps:

To remove FEC, repeat these steps, but in Step 2, press the left or right cross key to move the tick mark on the Exposure Level meter back to the center point.

Figure 7.4. For this image, I used the Canon 580 EX Speedlite and bounced it off a white card held above the flash head. The exposure on the schnauzer is good, but the background shadow is still objectionable. Using another Speedlite to light the background would eliminate the shadow. Exposure: ISO 100, f/4, 1/6 second, Av mode.