On this Adorama video, photographer Mark Wallace takes a look at how he shot a dark background image in a brightly lit room and killed the ambient light from the image.
On this video Wallace gives one of the best explanation of the workings of both the shutter speed in a camera and the way flash sync speed works. Since there is a maximum speed between the first and second curtain of the shutter (which is a bit different for each camera but typically not much more than 1/300 second), when using a flash at any shutter speed faster than that you will get black lines in your image.
What you need to understand is that every time you use a flash you are basically combining two exposures into one. One exposure is that of the light in the room (the ambient light), the second is that of the flash. To kill of the first exposure you need to take an image without the flash and see what is the minimum exposure that will give you a dark image (a completely dark one) – to do that you can lower your ISO to the minimum, crank up your shutter speed to the fastest one that your flash will tolerate and if you still need more close the aperture down (to us much as you need to make the image dark.
Now when you turn on the flash – the only image that you are going to get is the one lit by your flash and if you want more or less light in your image – just increase or dercrease the power of your flash (just make sure you don’t aim your flash to your background and get light spill (like Wallace actually got in his image), you can do that bit lighting from the side – you can of course use more than one light (one on each side to reduce shadows if you don’t want them).
We have already looked at some of the basics of flash sync on several videos in the past including “Understanding Flash Sync Speed & High Speed Sync“, “Understanding Flash Sync Speeds” with Karl Tylor and “Flash Sync Speed and Flash Duration” with Mark Wallace and more recently high speed sync and how it works with photographer Jay P. Morgan.