Flash Exposure

Trick Photography And Special Effects

Trick Photography and Special Effects

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When you half-press the shutter button while the flash is popped up, the T1i calculates an exposure, just as it does when the flash is down. However, with the flash up, it bases its exposure on its understanding of how much light the flash will be able to add to the scene. In Program mode, the camera will never pick a shutter speed slower than 60, to ensure handheld shooting. It will also never pick a shutter speed faster than 200, because it's not possible for the entire sensor to be exposed to the flash if the shutter speed exceeds 200.

When you take a shot after metering, the camera will turn the flash on. When it sees that enough light has been added to the scene to properly expose it at the current settings, it then turns the flash off. In other words, although it might appear that the flash is always just a quick flash of light, the T1i is actually carefully monitoring the situation and firing the flash for longer or shorter periods of time.

WARNING: Waiting for Recharge

Before the flash can fire, it must charge up, a process that is usually very speedy. As your battery weakens, though, flash recycle time can worsen. The T1i will display a busy indicator in the viewfinder while the flash is charging. When the busy indicator is displayed, you'll still be able to shoot, but the flash will not fire.

Flash with Priority and Manual modes

If you're using the flash in Shutter Priority mode, then the camera will not let you select a shutter speed faster than 1/200th. As always, it will automatically calculate an aperture that's appropriate for your speed.


In Aperture Priority mode, no matter what aperture you pick, the camera will never choose a shutter speed faster than 1/200th.

In both modes, though, you can have shutter speeds that are much slower than 1/60th. This allows you to achieve the same slow-sync shutter effect that the Night Portrait scene mode creates (you learned about this mode in Chapter 1). Slow-sync shutter means the camera combines a flash shot with a long exposure to create a scene that has good exposure in both the foreground and the background.

If you haven't read the "Understand Flash Range" section in Chapter 1, you should do so now.

The Night Portrait mode is an easy and effective way to get slow-sync effects, but if you're shooting raw or want more white balance or ISO control, then you'll need to use Aperture Priority or Shutter Priority mode.

REMINDER: Tell Your Subject Not to Move

As with Night Portrait mode, when you shoot using a slow-sync shutter, it's important to tell your subjects not to move until you say it's okay. Most people will move as soon as the flash fires, but if the camera is set for a very long exposure, this can create blur.

In Manual mode you can select any combination of shutter speed and aperture, but as you would expect, you can't choose a shutter speed faster than 1/200th of a second. As always in Manual mode, if you dial in exposure settings that would result in an over- or underexposure, the camera will warn you by flashing the Exposure Compensation display to indicate the amount of exposure error.

Flash Exposure Compensation

In addition to changing the shutter speed and aperture settings as you normally would, you can also tell the camera to add more or less flash to your scene. For example, consider the flash exposure compensation image at the top of the next page.



The flash in our left exposure is too bright, but we can dial it back using flash exposure compensation to produce the right image.

The flash has added a lot of brightness to the scene, but in the process, our subject has overblown highlights on him that make him look too harshly exposed. By using flash exposure compensation we can tell the camera to use less flash so that more of the natural light in our scene is used, and the harsh highlights are reduced. I shot the image on the right using a flash exposure compensation of -1 stop.

To activate Flash Exposure Compensation, choose Flash Control from the first Shooting menu, then choose "Built-in flash func. setting". You'll find Flash Exp. Comp in the resulting menu, just below Shutter Sync. You specify flash exposure compensation in stops, just like you do normal exposure compensation.

The image on the right has much better exposure and lacks the overblown highlights. A quick check of the histograms of the images confirms that the second exposure is better (it lacks the overexposure spike on the right side).

I didn't know ahead of time that 1/3rd of a stop would be enough, but because you can review your image and histogram as soon as you take the shot, it's easy to find the right exposure. So, if you go into a situation like a party in a dark room and plan on shooting flash, you might want to try a few tests right away to determine a proper flash exposure. Then you can shoot with those settings as long as the lighting doesn't change significantly.

TIP: Don't Forget the Histogram

Because the Tli's LCD screen isn't a perfectly accurate representation of exposure, it's important to check the histogram if you think there's a chance of overexposure. If you see a white spike on the right side of the histogram, then you'll need to adjust your flash exposure compensation and shoot again.

Improving flash range

Flash exposure compensation also lets you increase the amount of flash that gets added to a scene. However, this increase does not improve the range of your flash. Remember, all your camera can do is control how long the flash is on, but just as a desk lamp doesn't light up a bigger area if you leave it on for longer, your T1i's flash doesn't get a longer reach with an increased exposure setting.

However, you can effectively improve the range of the flash by increasing the camera's ISO. Light intensity falls off as it travels farther from its source, and when the camera's image sensor is dialed up to a higher level of light sensitivity, it will be able to pick up the fainter levels of light that have traveled farther away. Page 62 of the Rebel T1i instruction manual provides a chart that gives you an idea of how much of a range increase you can expect at different focal lengths.

In that chart, you'll see that at ISO 1600, with an 18mm lens, you can expect a range of about 50 feet. But remember, that doesn't mean objects 50 feet out will get the full flash exposure. You'll still want to review your images carefully and consider using a slow-sync flash exposure.

TIP: Consistent Flash Overexposure

If the display consistently blinks when you try to shoot a flash exposure, try lowering the ISO. If you've left the ISO at a very high setting and then go into a bright situation and try to use fill flash, the camera will probably not be able to get a good exposure. Dialing down the ISO should help.

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