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An external flash is essential for properly exposed and pleasing flash shots. Built in flashes can do an ok job of giving you the additional light you need, as well as providing some fill flash, but they have limited range. In addition, when shooting portraits with a flash it is often desirable to attach a diffuser to the flash and/or tilt the flash so it can bounce off of a wall to give more pleasing lighting.
The three pictures above were all taken with the 550 EX. It does great as a fill flash and it can provide a good amount of light in very dark conditions as you can see in the above pictures. The photo on the right was taken outdoors at night in a very dark area.
The flash has incredible range and does a great job of outputting the correct amount of light to give nicely exposed photos almost every time. It also has a infra red beam and sensor that allows the camera to focus perfectly in incredibly low light, or even no light situations. This sensor also has remarkable range. This feature alone makes the flash a must have. When the light levels get too low, auto focus no longer functions, and if the light is low enough it is also not possible to see well enough to manually focus. With the 550 EX attached you will get very rapid auto focus at any level of light. Of course if the light levels get too low and the distance to your subject is too great even this auto focus assist feature on the 550 EX will fail.
In conclusion the Speedlight 550 EX is an essential accessory that will give you great exposure, pleasing fill flash, and has a wonderful auto focus assist feature.