What in the world is Flash High Speed Sync?? Well, we first must answer; what is Flash Sync? Flash Sync has to do with your flash shutter synchronization speed. Typically, most cameras have a shutter max flash sync speed of 200th or 250th of a second. This is the shortest amount of time for the shutter to remain open and the image be affected by the flash. If you set your camera above that speed then the flash will catch the shutter partially or fully closed. This will cause part of the photo to be underexposed (not a good thing). There is a minor caveat to this but (isn’t there always) that will be another blog post.
The next question to answer is; why would you need a faster shutter speed than 250th of a second when you know the burst of light from the flash will likely freeze any movement from your portrait subject? If you guessed ambient light then you are correct. For portrait work you typically want a wide open aperture to ensure that you have separation from your subject and the back ground. That means you are letting in a great deal of light. On a bright sunny day with an aperture of f2.8 you might have a shutter speed of 1/2000th of a second for proper exposure. If you want to use a flash to light your subject or fill in the shadows on the face then you will either have to stop down the aperture, which brings the background into focus (not desirable for some portraits) or use High Speed Sync.
High Speed Sync allows you to raise your shutter speed beyond the typical 250th of a second. It does this by emitting several pulses of light to ensure the sensor is fully exposed to the flash. Sounds great! So, why would you not always have your camera/flash set to High Speed Sync? In photography there is always a trade off… By emitting several pulses of light the power of the flash is reduced. This means that the flash it self will have to be much closer to the subject and will not be able to light as much of the area (which is great for outdoor portraits). Check with your camera manufacturer to see if yours supports this mode. I know for Nikon users it is called Auto FP. Thanks and check back with us soon!