On the top of the camera sits a hotshoe, a fairly standard camera interface that allows you to use a number of different accessories. It's called a hotshoe because it's an interface that provides electrical contacts through which the camera can communicate with an attached accessory (as opposed to a coldshoe, which has the same type of mount but provides no communication between the camera body and whatever is in the shoe).
You can attach an external flash to the hotshoe on the top of the camera.
You can use the hotshoe to attach special wireless or wired remotes, levels (for ensuring your camera isn't tilted), umbrellas to keep your camera dry, and other specialized accessories. But hotshoes are most commonly used for external flash units.
Even though the T1i has a built-in flash, there are a number of reasons to use an external flash. They're more powerful, so they can cover a longer range and offer more latitude for exposure adjustment. Flash units all provide a specification called a guide number that indicates how powerful the flash unit is. For example, on page 214 of the Rebel Tli's instruction manual, you'll find a listing for Guide Number: 13/43 (ISO 100, in meters/feet). This number represents a constant that you can use to determine the range of the flash at a given aperture. For example, a guide number of 43 indicates that the flash can fully illuminate an object roughly 10 feet away when shooting at f4. (Ten feet multiplied by 4 is 40.)
These numbers used to be important for calculating manual exposures and for figuring out how to expose when using multiple flashes. If that last paragraph didn't make sense to you, don't worry, because you really don't need to keep track of this stuff anymore: the camera and flash will do it all for you.
Canon sells three different flash units that are compatible with the T1i, as well as two special flash attachments for macro shooting. You can see these on page
196 of the instruction manual. The Speedlight 430EX II has a guide number of 141 feet/43 meters, whereas the Speedlight 580EX II, Canon's largest external flash unit, has a guide number of 191 feet/58 meters. So, if you're willing to spend the money (and carry an extra component), you can pack a lot of illumination power into your camera bag.
Having the extra power doesn't just mean you can light up a subject farther away. With a larger flash, you'll be able to fill a larger area with more light.
External flashes also offer the advantage of being able to slave multiple units together. With multiple flashes, you can create complex lighting solutions. You've seen how you can use a fill flash to improve images shot under direct sunlight. With multiple flashes you're in control of several light sources, allowing for a tremendous amount of lighting creativity. The use of multiple flashes is far beyond the scope of this book and is something that most shooters don't need.
However, external flashes also give you the option to tilt and bounce the flash, rather than aiming it directly at your subject. This is probably the greatest advantage of an external flash, and when combined with the stronger power of these units, the tilt/swivel capability allows you to create very natural-looking lighting.
One of the biggest problems with flash pictures is that the flash is always located in the same location as your camera. In other words, the main source of illumination for your subject is located just a little above your head. This is very rarely how real-world light sources work, so flash pictures always look like...well, flash pictures.
In the real world, we're used to light coming from overhead, and we're also used to those light sources being larger than a single bulb the size of the T1i's flash head. Lighting that shines onto the front of a person, from a very small source, simply doesn't look natural to us.
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