First, a short review on the purpose of shutter speed and aperture to better understand what I am trying to say in this article. Aperture controls how much light your sensor will be exposed to. On the other hand, shutter speed controls how long you want your sensor to be exposed to that amount of light.
So now, going back to flash. Flash happens . . . well, in a flash. Getting the right exposure from flash is determined by it's power settings - or amount of light that you want it to give out - and amount of that power to pass through your lens (aperture) onto the sensor. Thus, aperture settings are used to control flash exposure. It wouldn't matter then whether you use your camera's slowest shutter or fastest shutter sync speed - flash exposure will remain constant - as long as your aperture setting is constant.
Conversely, shutter speed settings are used to control ambient (or your background exposure). Ambient is a continuous light sourse, its intensity is relatively constant and the main reason why, what you need to do with it is determine how long do you want your camera sensor to be exposed to that intensity.
Let's take a look at the series of photos below. Some important points to keep in mind about this setup:
1. The camera was sitting on a tripod.
2. Flash power was constant at 1/2 power.
3. ISO and aperture were constant at 100 and f/25 respectively.
4. Shutter speed variance from shot to shot was 1-stop.
1/250 sec. f/25
At my camera's fastest sync speed of 1/250, correct exposure was achieved for my car's rear left wheel while the rest of the picture is left to dark.
1/125 sec. f/25
In this next shot, with shutter speed slower by 1-stop, flash exposure on the tire remains the same as the first picture but notice how the rest of the scene (ambient) has become brighter.
1/60 sec. f/25
A further stop slower and the tire's exposure remains the same but ambient has further brightened up.
1/30 sec. f/25
1 more stop slowed down, and details from the garage floor started to come out. Notice that the flash exposure on the tire has not changed.
1/15 sec. f/25
At a shutter speed setting of 1/15 sec., a further stop slower, this is more or less the ambient / flash ratio balance I was looking for.
1/8 sec. f/25
Taking this experiment further, shutter speeds were further varied by one stop each in this shot and the rest that follows with flash exposure on the tire remaining constant. You may say that the tires are getting slightly brighter. That's because ambient is having its effect on it as well in addition to flash.
1/4 sec. f/25
1/2 sec. f/25
1 sec. f/25
For this particular shot, flash and ambient lights have come to a ration of 1 / 1 making flash indistinguishable.
You see, you do not need to make your background dark as night and worry about blowing out your main subject with flash. Ambient can be controlled to balance flash light with available light. Learning to balance flash with ambient allows you to do more subtle lighting - making your subject, not your lights, as the star of the picture.
You should try this for yourself to fully understand the principle behind it. Reading or watching video tutorials will only give you half the story - just the idea. You have to go out there and do it for yourself.
Labels: lighting photography, technique