Fill Flash Basics for Outdoor Photography
Flash often confuses people. It all happens in the blink of an eye and your eyes simply can't tell what your camera will end up seeing. Many photographers rely on trial and error to get what they want, but if you understand a few basics, there's a better way.
First off, I'm not a flash expert. Most of my images are made in the outdoors using available light, but there are times where flash can make all the difference. And it is precisely times like these where I'm thankful that I finally took the time out to understand fill flash. It's really that difficult.
There are two main ways to use a flash for exposure: full flash and fill flash. With full flash, the flash is so bright that it completely overpowers the available light to the point where every other light source becomes irrelevant. You walk into a dark room and take a flash picture. You turn on the room lights and take another picture. Nothing about the exposure changes because the flash is bright enough to do the job completely on its own. With fill flash, the light from the flash is dialed back enough that it only augments the available light rather than overpowering it. You are adding additional light to fill in shadows or balance the overall lighting, but the basic exposure is still made by the available light, not the flash.
Making this basic distinction is the first thing to understand if you want to use your flash for more than just trial and error. These two types of flash are quite different in how you need to approach using your flash. When thinking of one, forget everything you know about the other. They really have almost nothing in common with each other.
Even in fill flash, there is some light coming from the flash. If there weren't, you could turn the flash off and put it back in your camera bag. But your flash only emits light during the actual exposure. Sure you can manually fire it without tripping the shutter, but it goes off so quickly there's really no way you could set your exposure before the flash shuts off again. Given this conundrum, you can't possibly use full manual exposure where both shutter speed and aperture are set by you in advance. You can't really use shutter priority either since your camera would have a hard time adjusting the aperture to compensate for subject reflectivity and other factors while the shutter is open. It's simply not possible to calculate in advance how much light from the flash will reach your subject and reflect back to you without trying it. There's a better way.
Set your camera to aperture priority and chose an aperture that will give you the depth of field you want. In this mode, your camera will control the shutter speed to make an exposure that works. When you fire the shutter and the flash goes off, the camera will automatically close the shutter when it determines that a "correct" exposure has been made. This is basically the same thing that happens with aperture priority when your flash isn't even present. When you turn your flash on and fire the shutter, your camera will see a bit more light during the exposure, but everything else remains the same.
Set your fill flash output to around -1.5 to -2 stops. Depending on how much trial and error you've already done, you may need to consult that thick instruction manual that came with your flash to find out how to do this. This will ensure that your flash doesn't compete with the basic ambient exposure lighting. Remember, with fill flash, your aim is simply to fill in the shadows a bit or to add a catch light in the eyes of your subject if it's an animal or person. Until you're more familiar with what to expect, it's better to use too little fill flash than too much. Once you set your flash this way, you should be able to shoot away as if the flash weren't even there.
As with basic aperture priority use, your camera will attempt to expose whatever you point it at as "medium toned." If your subject is brighter or darker than medium, use the Exposure Compensation control to adjust exposure, not the flash compensation setting.
Full Program Exposure mode can also be used, but this robs you of being able to control depth of field via the aperture setting so I can't recommend it.
Keep in mind that fill flash isn't intended to be your only light source. It only adds to the available light. Given this, the shutter speed will do what it needs to in order to get your exposure. It will generally stay open longer than your flash duration. If it doesn't, that's a sign you're using too much flash and its output is starting to compete with the ambient lighting. This also means that any subject or camera movement won't be frozen by the flash blast. The normal rules of tripod use and camera shake still apply. If your exposure won't freeze subject motion sufficiently without flash, don't rely on fill flash to take care of that for you. If your subject is moving too much, it may not be a good candidate for fill flash at all.
Also realize that the inverse square law I talked about last will work in your favor with fill flash so long as your subject is the closest thing to the camera. With everything else further away, the flash will have its greatest impact on your subject where you want it.
For outdoor shooting at dawn or dusk, color temperature can be a concern. At such times, the ambient lighting will be quite warm in color while the color temperature of your flash remains as daylight as ever. An inexpensive warming gel filter can be placed over the flash head to better match its light to the natural lighting. Roscoe and Lee are well known manufacturers of gel filters and you can find these at any good camera supply store. A single 20 x 24" sheet will last you a long time. Just cut out a piece big enough to cover your flash head and tape it on. Even though they make these things in a plethora of different shades, you really only need one unless you do a lot of this sort of work. Opt for one that is only modestly orange in color. All you really need to do is make sure your foreground fill light doesn't appear blue.