Right, they don't know, and that is why you should have to know how to flash right for full control of the automatic flash!
I came up with this when I started to use the slave flash that I built. You can find the original instructable here:
In the meantime I even built a second version of it that is much smaller and much cheaper and easier to built:
So now, let's get started!
Step 1: Aperture - Time (and Flash ?)
You can vary either the exposure time, the aperture of your lens or the sensitivity of your film / sensor. While you normally only change the first two, because it is enough freedom to get good pictures.
The pictures of this step show you what happens when you chose shorter exposure times for a given situation and aperture. In the lower right corner of each image you can read the exposure time, the aperture and the iso-sensitivity.
The first picture has a correct exposure, the other ones are more or less under-exposed.
But when a flash is involved the degrees of freedom are reduced because you have an additional light source that can only be adjusted in a very small range (see next step).
Step 2: Guide Numbers of Flash-lights
But furthermore the guide number tells you something about the amount of light that will come out of your flash light.
And it does so in a way that you can directly use to calculate the correct aperture for your scene.
For an aperture A and a distance D to the object you want to light up (in meter), the guide number L is defined as
L = A * D
For a given situation you can then calculate the aperture by A = L / D.
Example: The guide number is 40 and the distance is 5m, then you have to use aperture 8.
Step 3: Determine the Correct Flash-parameters
You want to take a picture in a rather dim situation, that is why you use the flash light.
You measure the exposure time to be 1/20s for f2.8. That is enough to get a correct exposed picture (picture 1). Now you want to use the flash to light up the scene. Say you have a flash with L = 32 (which is a good value) and the object is 4m (D) away from you. The correct aperture would be:
A = L / D = f8
Now take a picture with flash at f8. As you can see in the second picture the background is very dark.
Why is this? You changed the aperture from f2.8 to f8, that means you closed it down three stops but didn't change the exposure time.
So the background is underexposed by three stops.
Step 4: Using Flash and Ambient Light
What do we have to do?
Well we could adjust the exposure time to be 1/4s, which would give us a well exposed picture without flash. If we then would use the flash in addition it would give us an overexposed picture by one stop. To make this right there are some possibilities:
- close down the aperture one stop further (if possible) and so reducing both flash and ambient-light by one stop. -> f11
- or we could reduce the sensitiviy from ISO 400 to ISO 200
- or we could choose 1/8s and reduce the flashlights energy to half if this is possible
You got the trick?
Step 5: Sequence to Get to Good Flash Pictures
- Measure or determine the amount of ambient light to take a good picture without flash.
- Adjust the correct aperture for your flash: Aperture = guide number / distance
- Choose if you want to have ambient light an then change the aperture accordingly
- Adjust the exposure time to give you a correct exposure.
- Take the picture!