Balancing Ambient Light

For the first exercise discussion, we will go over balancing flash with ambient light. Basically the concept is to pop a subject with the flash while at the same time preserving some of the pleasant ambient. So how do we do this? Well you know that a properly exposed photograph consists of the appropriate values of aperture, shutter speed and ISO. In a pure flash environment we usually set our shutter speed to the full sync speed, either 1/250th or 1/500th and adjust our aperture to allow the right amount of flash in. Why is flash only concerned with the aperture? Think of how a flash works. It is a pulse of light. No matter how fast you set your shutter speed this pulse of light will hit the camera sensor fully. So we adjust the aperture to determine how much of that light pulse gets to the sensor. As you can imagine, the larger the f-stop the more light hits the sensor.
So now we want to get some of the surrounding non flash constant light involved in our exposure. Well this is simple. Slow down the shutter speed to the point that the ambient light has time to affect the exposure. Let's take a look at a set of pictures I took of the venerable Buddha.



The first shot is without flash. Notice the room is completely dark. The next shot near full sync speed of 1/250th. Note that the subject is properly exposed and the background is dark. Only some of the flash spill can be seen on the back wall. We will learn how to control light spill in a later exercise. As I take the next few shots I slow down the shutter speed to gradually bring in the bright sunshine coming in though the windows. The exposure of the subject stays the same the whole time, no matter what the shutter speed value is. That's the key. The world changes but Buddha stays the same.
Now lets take a look at what we can do with this. This shot of Abbie(the wife, see, you remember) was taken on a very bright day with the Sun behind her.


The subject is normally exposed but I decided to underexpose the background a bit to separate her from it. Cool affect huh? For this I metered off the light behind her and increased the shutter speed to until the ambient was underexposed to my liking. This calls on the old adage that the correct exposure is up to you. Another great use of this feature is shooting a subject in front of a sunset. Sunsets are so beautiful, so of course you would want to include that in your shot.
Play with this a bit and you'll find the possibilities endless and inspiring.

Take care!
ML