Nikon and Canon refer to their external flash units as speed lights. Speed lights can be an effective way to get the necessary lighting to your subject, particularly when you are on the move. The advantages are that modern speed lights are smart computers almost perfectly compatible with your camera provided you are using the speed light designed for your camera. And they’re portable.
It does require a basic understanding of how your camera and speed light work together in order for this to occur. Let’s start with Sync speed. This is the optimum speed so that your sensor is completely open for the flash unit. It varies from camera to camera but is generally 1/250th of a second. Many of the more expensive cameras have high speed flash to allow you to work at faster speeds but it requires more effort. Using it may leave you without the power you need in a given situation. What happens when your camera’s shutter speed is faster than its sync speed?
Each sensor has two curtains: the front and rear. They moved vertically across the sensor. If the shutter speed is set too fast, the curtains don’t have enough time to cross the sensor, resulting in bad lighting. You also have a choice. You can set your camera on front sync or rear sync mode. Most cameras default to front sync for reasons that I do not comprehend.
The shutter speed in front or normal sync mode will default to 1/60 of a second unless you change it in custom settings to something else. The shutter speed in front sync mode is the slowest speed that the camera uses for its exposures. Think about it because if your subject is dark, the flash will be the only light you have.
When the front curtain opens, the flash fires at the beginning of the exposure, admitting very little ambient light into the image. So the place to use front curtain is where there is very little ambient light such as a studio or in places where you do not want ambient light. Almost everywhere else you should set the flash for rear sync. It helps bring out ambient light outdoors and is crucial when motion in involved. Just try using your flash on a moving object and you will see what I mean. Believe me, you want motion to show behind the moving object which both looks and is logical. The only way to obtain this is by firing the flash at the end of the exposure in rear sync. In rear sync the front curtain opens and starts to expose the sensor then just before the rear curtain begins to shut, the flash fires.
One thing about speed lights that causes them to compare unfavorably with other types of strobes is the harshness in the light. It has been compared to shining a flashlight on your subject. This is why light modifiers are so important to speed lights. Lumiques ultra soft diffuser, Gary Fong’s light dome, snoots, and grids are all examples of light modifiers. They make the light less harsh and even simulates the light obtained in a studio situation to some degree. Speed lights now come with a diffusion dome and wide angle diffusion panels are built into the flash head. If your flash is designed to work between 12 mm and 200 mm like Nikon’s new SB-900, you will need the wide angle diffusion panel if you are shooting with an extremely wide angle lens that is wider than 12 mm.
It is easy to change the power of your flash, either with the camera or with the flash itself. Because this change is exponential you should use caution when increasing or decreasing the light. A little change goes a long way. For instance:
+1 increase in exposure value gives you twice the light.
+2 gives you four times the light.
-1 gives you half the light
-2 gives you ¼ of the light
Four simple ways to influence flash power:
1. Distance from subject: Remember what I just said. Any change in the intensity of light will have a major effect on the amount of light falling on your subject.
2. Flash zoom setting should match the lens zoom but you can increase the zoom on the flash head manually. It can cause light to fall off around the edge of the image but if you are trying to get enough light on a speaker at a podium, and don’t care what else doesn’t get lit, then turn it up.
3. Camera ISO. Changing from 200 to 1600 can make a big difference in the amount of flash you send onto your subject.
4. Aperture affects light from the flash. Shutter affects ambient light. This seems to be one of the hardest things for people to understand. If you want to increase your flash, then open your aperture.
I have deliberately stayed away from using guide numbers in creating formulas for exact flash output. With guide numbers there are simply too many variables. Guide numbers not only vary from camera to camera but also from flash head zoom position. For instance, if you use an SB-900 at 35 mm, the guide number is 111. It is 89 at 24 mm and 184 at 200 mm, etc. If you need to be this exact you should get a good light meter. I use a Sekonic L-358 myself. If I tell you that your aperture should be equal to your flash’s guide number divided by the distance from you to your subject, you might as well have a drink and stop worrying about photography.