Non-dedicated electronic flash units can't use the automated E-TTL II features of your Rebel T2i; you'll need to calculate exposure manually, through test shots evaluated on your camera's LCD, or by using an electronic flash meter. Moreover, you don't have to connect them to the accessory shoe on top of the camera. Instead, you can use an adapter in the hot shoe to provide a PC/X connector, and use a shutter speed of 1/60th second or slower.
You should be aware that older electronic flash units sometimes use a triggering voltage that is too much for your T2i to handle. You can actually damage the camera's electronics if the voltage is too high. You won't need to worry about this if you purchase brand new units from Alien Bees, Adorama, or other vendors. But if you must connect an external flash with an unknown triggering voltage, I recommend using a Wein Safe Sync (see Figure 7.19), which isolates the flash's voltage from the camera triggering circuit, and provides a PC/X adapter for plugging in non-dedicated flash units.
Another safe way to connect external cameras is through a radio-control device, such as the transmitter/receiver set shown in Figure 7.20. It piggybacks onto the PC/X adapter you've fastened to your hot shoe, and plugs into your adaptor's PC/X connector. It transmits a signal to a matching receiver that's connected to your flash unit. The receiver has both a PC connector of its own as well as a "monoplug" connector (it looks like a headphone plug) that links to a matching port on compatible flash units.
Finally, some flash units have an optical slave trigger built in, or can be fitted with one, so that they fire automatically when another flash, including your camera's built-in unit, fires.
Figure 7.19 A voltage isolator can prevent frying your T2i's flash circuits if you use an older electronic flash, and it provides a PC/X connector, which the Rebel T2i lacks.
Figure 7.20 A radio-control device frees you from a sync cord tether between your flash and camera.
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