Using a DSLR flash in manual mode
Why manual mode?
The built-in flash on Digital Rebel cameras is for fill only when used in Creative, Aperture Priority (AV) and Shutter Priority (TV) modes. In those modes, the exposure is set for the background and the flash fires to lighten close shadows and provide fill for close subjects.
AV mode is especially frustrating with flash, because you would be using flash in a dim environment. AV mode will result in a long exposure time that you cannot handhold!
But do not despair. Most DSLR flashes are made to be used in Manual mode, providing more control to balance the flash with ambient light. This produces more natural looking photos.
Using the built-in flash in a small room
In a small room, where the dominant light source is the flash, set the Aperture for the depth of field that you want and the shutter speed for something reasonable that can be hand-held. Then set the ISO for an underexposed picture.
When you take the shot, the flash fires to provide the light needed to bring up the exposure to the middle needle. The middle needle is what the camera thinks is a good exposure.
Let’s say you set f/8, 1/60th, at ISO 400 and the exposure meter reads one stop underexposed. When you take the picture, the flash will increase the light by one stop (to take the exposure from -1 to zero, or middle needle).
Leave all your settings the same but change the ISO to 200. Your exposure meter reads two stops underexposed. When you take the picture the flash will increase the light by two stops (to take the exposure from -2 to zero).
You could also change your aperture and shutter speed as well as ISO to fine tune your under exposure and resulting flash output.
Flash output power
Typically you want one stop underexposed for light fill flash; two stops underexposed for a more normal point-and-shoot looking flash. At three stops or more underexposed, the flash will be powerful and start to take over the exposure from ambient light.
Using the built-in flash in a large room
When taking flash pictures in a large place, the flash generally doesn’t reach the background. Now you have two exposures to set. The first one is for the background, called ambient light exposure. The second one, known as flash exposure, is for the foreground.
The key is to recognize that, by camera design or program, these two exposures are related. The relation is that however much the background is underexposed, the camera will assign that light output to the flash for lighting the foreground. The result is that the background is underexposed and fades from importance, while the foreground is properly exposed and stands out.
In Manual mode when you set the aperture, shutter and ISO combination, you are setting the exposure for the background. If you set that background exposure to be underexposed by one stop, the camera (through its design or program) will know to fire the flash at one stop of output light to bring the foreground exposure up one stop to middle needle, or what the camera meter thinks is good exposure.
Remember, this is for a large space and the flash only reaches the foreground. The flash has no effect on the background.
Alternately, if you set the background exposure to be underexposed by two stops, the camera will know to fire the flash at two stops of output light to bring the foreground exposure up two stops to middle needle. In this case, you get a background underexposed by two stops (that looks great for a sunset) and a good exposure on the foreground (great for a person standing in front of that sunset!).
Tips for using a Digital Rebel flash
- Flash exposure compensation. Remember the example where it was suggested that you set f/8, 1/60th, at ISO 400 and the exposure meter read one stop under exposed? The background would be one stop under exposed and your foreground subject would receive one stop of extra light from the flash.What if you wanted your background exposure to stay at one stop underexposed but wanted to change your foreground subject to receive two stops of light? Set the flash exposure compensation to +1. The foreground will get one stop of light from the one stop under exposed and another one stop of light from the flash exposure compensation for a total of two stops.
- When you set the camera for overexposure in manual camera mode, the flash will just fire at minimum output. It will add a little light to the foreground, but not much.
- If you set the underexposure to 16 stops underexposed, your flash will not explode. If the camera calls for more output than the flash is capable, it will just fire at maximum output. You will end up with an under exposed picture because the flash couldn’t add enough light.
A word about external flashes
A hot shoe mounted external flash is much more powerful and versatile than a built-in flash. One of the best things about almost all external flashes is that you can attach a diffuser to the flash head to soften the light. Even better, you can point the flash at the wall or ceiling to bounce the flash for a more diffused even light that makes your subject look great.
Using a DSLR flash effectively, whether built-in or external, takes some practice. But do not discount flash use. It is a powerful photography tool!