When it comes to flash units for your DSLR camera, there is a lot more to consider than price alone. Each manufacturer offers flashes with features and capabilities that affect what you're able to do with it. Whether you want a powerful flash with manual settings or a lighter unit that works automatically with your camera, check your camera's user manual to review the features and options it has that can be used with different flash units.
Comparing Guide Numbers
Comparing the guide numbers for camera flash units is one of the most useful ways of determining which flash has the most power. A higher guide number indicates a more powerful flash. The guide number is determined by multiplying the distance between the flash and the object being photographed by the f-stop number needed for the proper exposure. Guide numbers are almost always calculated using an ISO setting of 100. The distance can be measured in either feet or meters, but most manufacturers give a guide based on both measurements.
Automatic and Manual Settings
For casual photographers and those who want to take a quick snapshot without adjusting their own settings, a flash that can be automatically controlled by the camera itself is a good choice. If you like to take control over the lighting of your photographs, a camera that also includes a manual setting is a better choice. A manual setting lets you fire the flash independently of the camera's settings using a continuous burn -- continuous, that is, for as long as the shutter is open.
The power of a flash unit is determined by the duration of the flash, not by how bright it can shine. Because the camera's shutter is only open for a fraction of a second, a short flash duration illuminates the object you are focused on but can leave the background dark. A long duration gives the light enough time to strike close and distant objects and reflect into the camera before the shutter closes. Almost all flash units you might attach to a DSLR are powered independently of the camera. A benefit of weaker flashes is that they are lighter, using only two batteries at most. More powerful units have four batteries.
When looking for a new flash, there's no reason to limit yourself to flash units made by the same manufacturer of your camera. Flash units made by other manufacturers can be completely compatible with your camera. Companies like Vivitar and Sunpak, for example, make flash units designed for different camera brands. If your camera is designed for a specific feature, such as a wireless flash, check the product description of the flash to see if it can utilize that feature, regardless of who makes the flash unit.
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