POASSB: Flash
Flash
Even for those who take photos primarily outside, flash is still useful. Existing light sources such as flames and street lights can create a dramatic picture, but such scenes are the minority. There are lots of good pictures to be taken that don't involve flames or posing under a light, and there are uses for flash in daylight. And don't forget museums, models, and the like, all of which are usually inside.
Flash units come in all sizes. The famed rail photographer O. Winston Link used setups with dozens of flashbulbs linked by hundreds of feet of wire. Although his setup, and similar ones that came later, are portable in the sense that they can be moved, they are not easily moved about. They require hours of set up time. There are also studio units, which run on AC power. At this point, I'll not deal with these (while reserving the right to add them later).
For those who want the ability to move about quickly, the answer is a small, battery-powered unit. In this genre, there are units that mount on the "hot shoe" on top of the camera, and larger ones with a handle that mounts to a bracket secured to the camera. Generally, the larger and more expensive a flash is, the more powerful, which means more light. However, some "handle mount" or "potato masher" units are made more for looks, and don't have any more power than some shoe-mounts. Some units are "dedicated" -- designed to work with a specific make or model. Dedicated units offer some nifty features. However, they are often more expensive or don't have the power of others. Some warnings: these units generally can't be used with a unit they aren't designed for (and may cause damage if that is attempted). A few independent manufacturers make dedicated units. They generally work well, but if you have trouble, you can get caught in a situation that must have been an inspiration to the computer industry: the flash maker will blame the camera maker, and the camera maker will blame the flash maker.
The output of small flash units is usually quoted in terms of a "guide number." The guide number, when divided by the distance, tells you the suggested aperture to use in manual mode. When comparing guide numbers, make sure you are looking at the same ISO speed for film or sensors (100 is the typical specification, but some will quote 400, as it makes a higher number), and the same measuring unit (feet or meters). For outdoor use, you typically want a guide number of 90 or better at ISO 100, measured in feet.
With its short duration, light from an unmodified electronic flash is very flat and directional. Because of this, you can only illluminate one plane (think of high school geometry). If something is 20 feet away, and something else is 10 feet away, your flash will expose one properly, and either under or over expose the other. Note how the fence posts in the foreground of the photo on the right have been washed out. A good part of learning to use flash well is learning to visualize the effects of this light, just as in any other type of photography. Of course, there is often little you can do about this--we're dealing with objects that you can't move, and often you have limited ability to move about. Thus, sometimes you have to choose the "least worst" possibility.
Sometimes there will be enough light in one part of the scene, but not in another. Film and digital chips do not have the ability to capture details in both bright (highlight) and dark (shadow) areas at the same time. You can shoot for one and ignore the other, or you can use your flash to fill in the darker areas. This works both day and night. Number boards, headlights, and flames are very bright, and if you don't "fill," you'll have a nicely-exposed light with a lot of dark shapes. You may want the silhouette effect, but it can be overdone, and is not very useful for modelers or investigators. Dedicated units allow for easy fill, as they will automatically balance the flash with existing light (although you may have to override the "automatic" fill setting that's designed for portraits if you're trying to fill in the side of a building). The general rule for an automatic unit (this means you have to try it yourself and make adjustments) is to set the camera to its flash sync speed (or, with care, a slower speed), then match the flash output range to the resulting exposure.
Most flash units these days provide automatic exposure control. This feature works in one of two ways: a sensor that cuts off the light when it signals a proper exposure (most common with non-dedicated units), or by a through-the-lens metering system (common with most dedicated units). In either case, if you read the instructions, they will typically refer to exposure in average rooms, where the walls and roof reflect some light. Outdoors at night is not an average room -- so your flash may not give you the right exposure (and if you're still into guide numbers or manual adjustments, the same applies). Give it a test at several distances to find out. It will probably work fine up to a point, and then rapidly fall off (this is because of the physics of light, which decreases exponentially). You may find that you can adjust the aperture and still get good exposures.
The shutter speed you use has little direct effect when using flash. The light is so fast and intense that it blasts everything else. However, you do need to coordinate a few matters. There is a shutter speed on the camera that is designated for flash use. This is because the shutter is completely open at that speed when the flash fires. If you shoot at a higher speed, you'll get some of the shutter in your picture. If you shoot at a lower speed, you'll start getting the existing light in your picture, which may or may not be good. Balancing flash and existing light is the idea behind fill flash, but it's also easy to get a blur if you go too far. Dedicated units typically tell the camera to use the "synch" (synchronized) speed, which is great if you're a little forgetful.
Smoke is composed of large amounts of small particles of oxidized material, borne about by hot gases or the wind. Fog is similar: it is a low-lying cloud, composed of water droplets. These particles and droplets will either reflect or absorb light. If all the light is either reflected or absorbed, the result is either a washed-out blob or a dark hole. So if 2 + 2 = 4 in the physics of light, you can't shoot, especially with a flash, in the middle of smoke or fog. If you're lucky, you get a diffuser that leaves you with an unclear photos. But in general, with or without a flash, they generally become opaque. The best advice here is to steer clear. Use a time exposure without flash; and be aware that smoke can be dangerous to your health.
Light from a flash mounted to a shoe or camera bracket is harsh and flat. The human eye relies on shadows to give a sense of depth. Thus, as on the left, a direct shot from flash can obscure details. Turning the flash to a 45 to 90 degree angle from the lens gives shadow detail and depth (right). This is fairly easy with a handle-mount unit: focus the camera (or confirm autofocus), and take the flash off the bracket. With a shoe mount you'll need a cord (and possibly an adapter if your camera doesn't have a cord terminal). Dedicated units require a special cord. This may be a real cord that locks in place to a pin terminal, or it may be a finicky shoe-to-shoe cord that requires more precise placement of the contacts than often seems to exist in manufacturing. Make sure such a cord works before you take it home.
Topics to come: reflections from Scotchlite and similar materials; diffusing light from electronic flash, use of studio lights. Suggestions and contributions are welcome, use the "e-mail me" banner on the home page.
