When making flash pictures, your camera will be on manual mode—I hope so, anyway. It’s the way to go. This means you set ISO, aperture and shutter yourself.
But your flash does not need to be on manual. That is to say, flash power can still be metered and be adjusted by the camera, automatically. We call this “TTL” (Through The Lens metering). And TTL can work even when the camera itself is on manual.
When your flash is in TTL mode, the back looks something like this:
You see TTL, ETTL, TTL-BL, i.e. something containing the letters TTL. This means the flash power is adjusted automatically by the camera. If you are close to something, the flash will fire at low power; if you are far away, a high power flash will be emitted. Magic!
The alternative is that you set the flash power. Manual flash, in other words. Press the MODE button on the back of the flash and set it to Manual:
In the photo above, the flash is set to half power (1/2). It could also be set to full power (100%, or 1/1), or to quarter power (1/4), or one eight power (1/8), one sixteenth (1/16), and so on; or to some level in between.
Try this now. Set your camera to manual mode, 1/40 second, 400 ISO, f/4. Now turn on your flash and set it to manual, and set it to 1/64 power. Using the viewfinder, take a photo. Check the photo: You will probably be hard pressed to see the flash, especially if your subject is far away.
Now set it to full power (1/1, or 100%). Do the same again. Whew, probably a grossly overexposed picture!
But you probably noticed something else. You did not see the flash through the viewfinder. “Did it work?”, you may well have asked. That is because when the flash is on manual, it fires just once, at the power level you set. You do not get the metering pre-flash that it uses when set to TTL mode (a flash at 1/32 power that is used to determine the needed power level). And that preflash is the one you can see through the viewfinder. The actual flash you cannot see!
Now, an exercise.
- Find an object to photograph. With the camera set as before, and the flash on MANUAL, find the correct power level for a good picture. Aim the flash straight ahead for this exercise.
- Now move 40% farther from the object. E.g. if your original distance was one metre, make it 1.4 metres. Or if you were 4 feet away, make it 5 feet 7 inches (that is 40% farther than 4 feet).
- Now find the correct power level for this picture. How much more power did you need? And (an advanced question for mathematicians): why? (Hint: it’s actually 41.4% farther).