Before buying an external flash for your DSLR camera, take some time to look at the features available that will be of most use to you. While the most expensive flash often has the most power and most features, not everything packaged into that price may necessarily be important to you. On the other hand, if you plan to experiment with your camera and practice your technique, spending a couple of hundred dollars on a flash without the features or flexibility you need may just be a wasted investment.
Compare models that are compatible with your camera. Not all flashes are designed to work with all cameras. Most flash manufacturers explicitly state which cameras a flash is designed to work with. Companies like Vivitar include the camera manufacturer in the model number, such as "CAN" for Canon or "NIK" for Nikon.
Consult the user guide for your camera to see which flash features are compatible with your camera. For example, if you pay for a flash that supports wireless use, it will be wasted money if your camera model doesn't support wireless.
Compare the recycle times. This represents how long you have to wait between shots for the flash to recharge. If you are taking a lot of successive pictures using a full flash, having to wait 10 seconds after every shot may not be acceptable. Manufacturers may also provide an estimate of the number of flashes you can expect before having to change batteries.
Look at the flash durations or flash speeds available for each flash. These represent how long the flash light actually flashes, usually given as a range between its longest and shortest flash duration. A higher range will give you more flexibility in your choice of aperture settings when taking a photo. For night shots at a distance, a longer flash duration is important to ensure everything in the field of vision is lit.
Compare the flash guide numbers. A guide number is calculated by multiplying the distance between the subject and the flash by the f-stop required at that distance. Most guide numbers are based on an ISO setting of 100. If you are looking for a powerful flash that can crank out a lot of lumens, choose a flash with a higher guide number. Most manufacturers use a distance in meters (m) for the guide number.
Look for manual setting options if you plan to experiment with your camera. For most people, the automatic settings will almost always be enough, but if you are a serious photographer, chances are you will want to take more control of the flash as you practice your skills.
Examine the bounce angles that the flash can be used for. These range between 0 degrees, when the flash is parallel with the lens, to 90 degrees, when the flash is pointed at the ceiling. The more angles you have as options, the more choices you will have to bounce light from different directions.
Look for other advanced features that might suit your needs. A continuous-on mode with power-saving functions is important if you are taking a lot of pictures over long periods of time, like at a wedding or other event. Remote-firing, or being able to use a flash when it's not fixed to the top of the camera, is also an important feature for serious photographers. While you can get adapters that connect the flash to the camera shoe with a cable, wireless flashes are also an option today.
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