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Although some photographers prefer to shoot everything by available light, an electronic flash unit is among the top five on my list of essential equipment. In indoor work, it can help produce brightly lit, colourful pictures without a tripod or the digital noise that's problematic at high ISO levels. Of course, flash is also ideal in outdoor photography. I use it often to brighten a backlit subject, to lighten shadows, to add a sparkle on dark days or to add a catch light in the eyes to make people pictures come to life.  

Regardless of the circumstances, flash photography can be particularly versatile when using a full-featured accessory unit. You might not take advantage of every single function initially, but it's great to have one that's fully equipped. As your willingness to experiment increases you won't be frustrated by an inability to achieve specific effects. If you're in the market for a new flash unit, consider the latest technology as well as the available features and how you might use them in serious image making.

High-Tech Flash Metering

The current technology is complicated in principle but simple to use in practice. Light reflected from the subject is transmitted through the lens (TTL) to a metering cell in the camera for analysis by a microcomputer. The "smart" flash (or Evaluative or Matrix) metering system considers a great deal of data: scene brightness, the color of the ambient light, lens focal length, the aperture in use and usually subject distance as well.

Most of the latest DSLRs also fire a very brief (imperceptible) burst of pre-flash light just before actually taking the photo. The camera's microcomputer employs this final bit of data for making calculations as to a suitable flash exposure. The system should then produce appropriate flash output for pleasing results even with a highly reflective subject such as a scene including a large mirror.

In bright outdoor conditions, the high-tech flash metering system will automatically reduce flash output. The ambient light remains the primary source of illumination with flash lightening (or filling-in) shadows, like those cast by the bill of a cap. All of the enthusiast-level DSLRs and flash units also allow you to control the output, using the Flash Exposure Compensation feature. If a flash photo seems too dark or too bright, you can easily modify the effect. Before taking the next shot, try setting -1 for a gentler flash effect or +1 for greater flash intensity.

Most Valuable Features

A built-in flash is certainly convenient, but it’s far from ideal in several respects. Its power output — and hence the effective range — is minimal. Because the flash is not far above the lens, the light can be blocked by a lens barrel, a problem particularly in tight close-ups. Red-eye frequently occurs for a similar reason: the flash is too close to the lens axis. More specifically, let's take a look at the benefits provided by many enthusiast-level flash units and the value of each function.

Note: A DSLR may provide some of the same features (e.g. compensation and bracketing) with its built-in flash, but an external flash unit offers many other benefits. Note too that some of the high-tech functions will work only with a digital SLR that includes support for those features. For full specifics about your own camera, check out the pertinent Magic Lantern Guide book or DVD.

High Power Output: The specs for every model include a Guide Number or GN, a measure of flash illuminating power such as "40 in meters (equivalent to 131 feet), at ISO 100 at the 85mm flash head setting, at f/4". (By comparison, a built-in flash may have a GN of 12.) That allows for shooting at a lower ISO level with a distant subject, for optimal image quality. Do note that the GN does not designate the actual "reach" of the light from flash, but it is useful for comparing the relative power of several models. (The flash Owner’s Manual and some manufacturers' Specs should include a chart providing data on the actual effective range in direct flash at various ISO levels and lens apertures.) 

High-Speed Flash Sync (HSS): Available with high-end flash units when used with many (but not all) DSLRs, HSS or Focal Plane or FP Sync modes employ interesting technology. When activated, it causes the flash unit fires numerous bursts of light during the entire exposure time. This allows for flash photography at any shutter speed, such 1/500 sec. or even faster. HSS is useful in very bright light and in close-up photography. It allows for shooting at much wider apertures (such as f/2.8) to minimize depth of field, to blur a cluttered background. Effective flash range is very limited so HSS is not intended for use with distant subjects.

Swivel/Tilt Head: Photo enthusiasts often appreciate the value of a flash head that can be tilted upward to bounce light from a ceiling or swiveled to the side for bouncing light from a wall. Either approach can provide softer light than direct on-camera flash.

Focus Assist Projector: Accessory flash units include a lamp that projects a bright red pattern onto a subject (up to 9 to 14 feet from the camera); that can be very useful to accelerate autofocus in low light. This large lamp is more effective than an in-camera focus-assist feature, providing faster, more reliable focus acquisition in dark locations.

Flash Exposure Bracketing: Some high-end flash units include this additional feature and it's supported by many (though not all) DSLRs. When it's selected, the camera will automatically produce three images, each with a different flash output: “normal” exposure, underexposure and overexposure. (A setting of +/- 0.67 often works well for Bracketing.) One of the photos should be close to ideal. Note too that you can set Flash Exposure Compensation (to a plus or minus level) before activating Compensation; that provides even greater versatility in serious flash photography.

Auto Zooming Head: Most recent flash units include a motorized head that adjusts automatically — with internal zooming of the flash tube — to match the angle of light to the lens focal length of the lens being used. (Manual zooming of the flash tube is usually available too.) Some models also include a built-in or accessory wide-angle diffuser that fits in front of the flash tube; it's very useful for spreading the light when flash is used with an ultra wide-angle lens.

LCD Data Panel: Common on full-featured flash units, a data panel provides information on the effective flash range at any aperture/ISO. The panel also makes it convenient to select the various functions, and it provides a quick reminder as to modes/options in current use.

Modeling Flash: Most recent high-end flash units provide this feature, firing a burst of light before you take an actual photo. This allows for previewing the flash effect. It’s particularly valuable when using two or more off-camera flash units. When activated, a burst of light is fired with the same intensity to be used when actually taking a photo. The brief lighting preview should enable an experienced photographer to determine whether the flash placement is suitable; this is most useful in multiple off-camera flash setups.

Rear Curtain Sync: Also called Second Curtain Sync (and often selected with the camera), this feature causes flash to fire at the end of the shutter opening time instead of the beginning. It’s intended for use in low light with a moving subject when using a long sync speed such as 1/8 sec. Unlike conventional “front curtain sync,” this alternative produces images with light streaks that follow the subject instead of preceding it for a more convincing impression of forward motion. Rear Curtain Sync is most useful with a high-powered flash that can pump out enough light to actually reach a moving subject in low light.

Wireless Off-Camera TTL Flash: Most brands of DSLRs support remote flash placement without a connecting cable from the camera’s hot-shoe to the flash unit. The built-in flash, or an accessory on-camera flash, will automatically trigger one or more remote flash units. Of course, this feature is available only when it's supported by the camera and the flash unit. Holding the flash unit off-camera — slightly above and to the side of the subject — can produce a very natural-looking lighting effect.

Note: There are significant differences re: wireless off-camera TTL flash procedures and compatibility with the various brands of DSLR cameras. In a subsequent part of this series about accessory flash, I'll provide specifics re:  every brand of DSLR. Of course, you can also find information on this topic in the pertinent Magic Lantern Guide book for you own specific camera http://www.larkbooks.com/catalog?section_key=8-4&limit=10.

Flash Compatibility

Because DSLRs employ high-tech flash metering systems, only specific "digitally dedicated" flash units are fully compatible with any camera. An old flash unit like the Vivitar 285 will not work and may actually damage the camera due to very high trigger voltage. Even relatively recent flash models, designed for 35mm film SLRs, are rarely suitable for use with a digital SLR. That's because they were designed to meter the light reflected off the film plane, a technology that does not apply in digital photography.

When researching flash units, check the specs to determine whether a model is specifically dedicated to your brand of digital SLR camera. You can find many such products, ranging from small, inexpensive models with modest power output and few features to large, very powerful pro units with numerous user-selectable functions. Suitable models are available from the camera manufacturers and from others. For example, the best Sigma, Sunpak, Quantum and Metz models provide most of the valuable features discussed below.  

Useful Flash Accessories

A versatile flash unit already includes many features, but better results are possible when using one of the few available accessories.

TTL Connecting Cables: If a camera or flash unit does not support Wireless Off-Camera TTL flash, a cable is required for connection. Every camera manufacturer markets at least a basic TTL cable for a single-flash unit. One end plugs into the camera's hot shoe and the other end connects to the remote flash unit. Some manufacturers also offer additional accessories for adding several extra flash units or for extending the length of the cable while maintaining full TTL automation.

More sophisticated devices — such as the Pocket Wizard radio-controlled accessories http://www.pocketwizard.com/ — are also available for triggering off-camera flash units, but they're quite expensive, intended for professional photographers.

Flash Brackets. In off-camera flash, holding the unit above and to the side of the subject often produces the most natural effect. It also helps to assure that shadows fall below the subject and not on the wall directly behind it, so they will not be obvious in the pictures. While you can hold a flash unit in your hand, most serious photographers eventually want a flash bracket. This type of accessory is available in brands such as Stroboframe http://www.tiffen.com/products.html?tablename=stroboframe and Custom Brackets http://www.custombrackets.com/Scripts/default.asp?storemode=y in various sizes and configurations.

Light Modifiers:  Adding a diffuser or a bounce/reflector accessory to the front of a flash unit can make for softer illumination, for a more flattering effect indoors and for softer shadows outdoors. These devices are available in brands such as LumiQuest, MilaGrid, Honl and Gary Fong, stocked by most major photo retailers. They differ significantly in style, but they all modify the light by softening or spreading it. They’re most effective when the subject is no more than six feet from the camera so they’re most appropriate for close-up photos of people and nature subjects.  

Filters: Especially when using flash only as gentle fill-in lighting under artificial illumination, a filter over the flash tube can be very useful. For example, under warm (orange) tungsten lighting, you might add an amber filter to balance the color of the two light sources: the lamps and the flash. This produces a more natural looking effect than combining pure white light with amber light. Filters of this type are available in brands such as Honl http://www.honlphoto.com/servlet/the-26/HonlPhoto-Filter-Kit-cln--Color/Detail, Lumiquest/Roscoe http://justlumiquest.com/fxtra_4.html and Lee http://www.leefilters.com/lighting/products/packs/category:Lighting%20Packs/

Master the Techniques

Flash has come a long way from the old days when photographers were required to make complex calculations to get a good exposure. With a DSLR camera, the entire process can be quite uncomplicated. After taking a basic approach in P mode, begin to use more advanced techniques, taking advantage of the additional features discussed earlier. Articles and books about flash photography can be very useful for jump-starting your education. For example, consider Chris George's Mastering Digital Flash Photography: The Complete Reference Guide (Lark Books, 2008).

With a digital camera, you have a definite benefit over photographers who use film: the ability to review photos immediately. If the effect is not quite right, try slightly different settings or a modified approach. Regardless of the equipment you’re using, read the instruction manuals closely for information on the pertinent features and for any tips or flash-range data. Once you become proficient with an accessory flash unit and the suitable techniques, you’ll wonder how you ever got along without them.