Auto-sensor flash units offer an economical choice. The sensor on the front determines flash output. This strobe is different in that it features a secondary flash head, which serves as a fill light when the flash head is aimed at the ceiling for bounce flash. Another feature on this flash is its manually zoomed flash head. This flash also houses the auto/manual selector switch on the front, color-coded to match the two usable f/stops for any given film speed.

For Starters
A good, economic starting point is an auto-sensor, or, simply, automatic, flash. And it's fairly easy to use. If you were using a strictly manual flash, or using this flash in manual mode, you would have to change the f/stop on the camera lens for each change in flash-to-subject distance--if you or the subject moved relative to each other (it has to do with that pesky inverse square law, light falloff, and distance). With automatic flash, one f/stop covers a range of distances, so you have considerable leeway. However, if you step outside this range, you'll have to change the auto setting on the flash, with a corresponding lens aperture setting, to accommodate the increased distance. But you'll still be covered over a fairly wide range. The on-board flash calculator (dial, sliders, or table), coupled with distance readings from the lens barrel, is your guide.

Automatic flash units gained popularity for another reason. They incorporate thyristor circuitry that more efficiently uses the energy stored in the flash unit's capacitor, in essence recycling unused energy and channeling it back into the system, resulting in shorter flash durations and faster recycling times. This means that the flash would be ready sooner--all based on the auto-sensor reading, plus batteries effectively go further. As an analogy, think of a refrigerator closing automatically (and saving energy) the moment you made a decision, instead of waiting to be manually closed, which means you'll be able to open it again sooner (less time required for the vacuum seal to re-pressurize).

A typical use of electronic flash is freezing movement. The use of an auto-sensor, shoe mount strobe ensured the very short flash duration required to stop motion with this fast-spinning pottery wheel.

Select flash units even let you use power ratios, using less energy to start with. This option may exist for manual or automatic mode--it varies with the flash. Often it's used to support a winder mode, but works equally well if you know a nearby subject needs less light: Start with less, and save.

For auto-sensor flash to work properly, the sensor must remain aimed at the subject. If you tilt or rotate the entire flash unit that orients the sensor incorrectly, which will result in an erroneous flash exposure. Fortunately, most flash heads can be tilted or swiveled independently, with the sensor in place, in the original orientation, thereby preventing exposure error. Exposure settings with automatic, as well as manual flash need to be adjusted when most filters and lens extension are used to prevent underexposure.

TTL auto-flash, or simply TTL (Through The Lens) flash, bypasses the need to interpret a flash calculator because the flash receives all this data from the camera. With a few key settings in place on flash and camera, you're ready to go--with all data digitally displayed (where LCD panels are employed). Because light readings continue to be TTL, the use of filters and lens attachments is accommodated when the flash is directed to deliver its burst of light. TTL flash units often boast added functionality, with prices to match, easily costing up to several hundred dollars with all the bells and whistles; much less for modest TTL units.

This young couple withstood the freezing cold to provide an opportunity to illustrate balanced fill flash against a dusky backdrop. TTL flash simplifies the use of flash fill, by taking ambient light levels into consideration.

Making Attachments: The First Level
With a built-in flash, all the triggering circuitry--the stuff that tells the flash when to fire--is internal. An external flash needs a little extra, aside from its own batteries. Most flash units at the very least have a "hot shoe" contact at the base, or "foot," of the unit, in the form of a pin, which matches a similar contact point in the camera's hot shoe (routinely situated) atop the penta-prism housing--although some pro-level SLRs may not be equipped with a hot shoe. Releasing the shutter sets in motion a triggering process that results in the flash firing, with information relayed through this contact point. This normally follows the opening of the first shutter curtain. The camera shutter speed that ensures synchronized operation with conventional flash units is called "X sync."

As cameras and flash units became more sophisticated, the simple individual contact point became an array of matching multiple contacts (two or more--on flash and camera). We moved from simply providing the means to trigger the flash to adding some level of microprocessor-controlled electronic communication between flash and camera. Flash units were now "dedicated." Whereas earlier, any hot shoe flash could be used with any hot shoe capable camera, now the flash had to be matched specifically to the camera model (or system camera). This dedicated connection tells the camera--and you--when the flash is ready to fire (fully charged) and sets the correct flash sync speed on your focal plane shutter SLR automatically (assuming basic settings are in place).

Automatic flash units have a flash calculator that indicates the usable f/stops for a given range of distances, for a given ISO/ASA. This one is slide-rule style; others are dial-shaped. Either way, use the f/stop that will cover the greatest distance you and the camera/flash expect to be from the subject. Then set the elector switch to the matching color-coded setting. Use the lash-test button as confirmation (an LED lights to indicate adequate auto coverage). At the top of this flash, you can also see the bounce flash angles--click stops used to tilt the flash head. Other flash heads may also swivel, for added versatility.

Why does sync speed matter? With focal plane shutter cameras (the typical 35mm SLR), if the shutter speed set on the camera is too fast, the shutter curtain movement and flash burst are out of sync and only part of the frame is exposed to flash. On the other hand, when too slow a speed is used, the ambient light may be enough to record a secondary, ghost image--which many photographers actually use to advantage, to add depth or a dynamic element to the picture, when done well.

Making A Truly Dedicated Connection
The next step up in dedicated technology is TTL auto-flash. Thanks to an enhanced pin/contact base array (which may be permanently attached or modular), all information is electronically transmitted not only from the camera to the flash, but also back from flash to camera--and to levels heretofore unknown, to provide the highest degree of accuracy in a flash exposure with the greatest versatility. This may involve a pre-flash that measures subject distance. Different camera technologies have their own way of measuring the flash, involving one or more sensors, but in each case, the light is measured through the lens--hence TTL--and off the film plane, and possibly tied into autofocusing.

Owing to its sophisticated electronics, TTL flash may be made to work with the existing light, automating and simplifying the process of fill flash. Depending on the camera and flash combination, the flash may actually reduce output so as to provide a more natural level of fill. However, you may still have to correct for subject brightness.

Zoom, Bounce, Twist
The simplest flash unit features a stationary head. More capable units may appear possessed, Exorcist fashion--practically spinning about on an axis, nodding their head, and magically zooming. How much of this do you need?

The automatic flash unit on the left features multi-dedicated flash contacts, which means it can be used as a dedicated strobe with a variety of SLRs. The flash on the right can only be used with specific camera models with TTL flash capability, from one manufacturer. The topmost pin is used to help secure the flash in the hot shoe, which has a similar dedicated pin array. Note: normally, automatic strobes are dedicated to one camera/series.

A zooming head largely does away with the need for accessory wide angle and telephoto panels to match the lens focal length and more efficiently use the light. Zooming is automatic (often with manual overrides) on TTL flash, and largely transparent to the end user, except for some LED or LCD indication on the flash. Other flash units may be equipped with a mechanically operated zoom head, with concomitant changes in the unit's appearance. Where an override option exists, use the tele-zoom setting to spotlight a person, or limit the reach of light at the wider settings. (The flash may also feature a built-in, manually positioned super-wide panel, to accommodate even shorter focal lengths.)

Bounce flash requires the head to tilt upward, at the very least, although a nearby wall may prove a more suitable reflecting surface--hence the swivel movement. Because overhead bounce flash tends to produce harsh shadows under the eyes, select strobes feature a built-in secondary flash tube, that, when activated, shunts some power away from the main flash head in order to provide fill.

TTL flash units employ various technologies to assist in outputting the proper amount of light. Zooming on this flash is internal, and designed to automatically match the lens focal length, within reason. Manual zoom overrides are available here, but not on all auto-zooming strobes.

Some strobes also feature a downward tilt, for close-ups. With the flash seated in the hot shoe, this downward, or negative, tilt may result in throwing the subject (partially) in shadow, because the lens or lens shade is blocking the light. A more effective solution would be to use the flash off-camera, with a sync cord, while diffusing the light, or to employ a ring flash or other flash specially designed for macro work.

News Flash
Even with ultrahigh-tech flash units, the concept of a Guide Number remains in vogue. It provides a measure of the unit's output, telling us how potentially powerful the light is. The higher the GN--Guide Number, the greater the output, assuming a level playing field--measured at ISO 100, at full power, at a standard focal length (if zooming/without panels), in feet. Don't expect much reach from a unit rated as GN 30, whereas one with a GN of 130 will go pretty far. Zooming the head to a tele position effectively increases the GN, with the reverse having a negative effect.

Note: If you typically find the GN of your flash to be overrated (leading to underexposure) or underrated (overexposure), open up or close down the f/stop, respectively. In other words, if you detect about a full stop of underexposure--owing to an inflated GN, instead of using f/5.6 that the flash unit recommends, use f/4. Bracket exposures in half stops to be safe. You, of course, have more leeway with print film than with slide emulsions. Keep in mind that the GN rating is based on a flash used in a normal sized room, with fairly reflective walls. Consequently, when shooting in wide, open spaces or outdoors at night with auto-sensor, as well as manual, flash, increase exposure by one-half to one full stop. (Want to perform your own GN tests? Use this equation: GN (in feet) = f/stop x distance (flash to subject, in feet), at a distance of 10 ft, to simplify matters. You can then interpolate the flash calculator based on your findings.)

Many TTL flash units feature an LCD panel for a digital readout of settings. This flash also has a switch permitting it to work in wireless TTL mode, and to act as the master/triggering unit or as the slave/receiving flash. But considering the high cost, maybe you can do without such esoteric functionality.

With flash, it is not the camera's shutter speed that counts as part of the exposure. Instead, it's the flash duration. So, in our classic E(xposure) = I(ntensity) x T(ime) equation, "T" is now flash duration. "I" remains the lens aperture, or f/stop.

Moreover, at any given film speed, the f/stop determines the reach of the flash (without zooming the flash head or using accessory panels): the larger the f/stop, the greater the reach; the smaller the f/stop, the more restricted the distance covered by the flash. (In other words, to keep a distracting backdrop in relative or possibly total darkness, use a small f/stop designed to reach only the subject. This may also render intrusive background shadows invisible.)

Getting Wired
While a flash on-camera is convenient, the lighting that results is often flat and harsh, producing hard shadows that may make it difficult to discern the subject from the shadow directly behind. So, with conventional flash units, we turn to a sync, or "PC," cord. This is our wired connection from a terminal on the flash to another terminal on the camera, traditionally the "X-sync" terminal. (Not all flash units support a sync-cord connection.)

Dedicated auto-flash and TTL flash cameras take this one step further. The cable fits into the camera's hot shoe and extends from there to the flash. Or, the dedicated connection may instead attach to an electronic terminal on the camera, with a locking mechanism. A few dedicated automatic flash units allow the sensor to be removed from the flash and mounted atop the hot shoe, seated in an adapter with a cable attached for off-camera flash. Any way you look at it, this design ensures a more secure connection, as standard PC cords have a tendency to pull free or wear with use. Some TTL flash capable SLRs may still feature X sync, for use with conventional flash units, letting you directly attach a standard PC cord to the X terminal on the camera or indirectly to the hot shoe, via a hot shoe adapter.

Unless you plan to hand hold the off-camera flash, you'll need something to support it. One option is to mount it atop a flash bracket. Flash brackets come in all sizes and configurations. Some flash units today come with their own, convenient miniature stand. Numerous TTL flash units also support wireless TTL flash, when used together with the appropriate cameras.

Power Up
Normally, AA-size alkalines are used in external shoe mount strobes, providing the most number of pops, bested only by AA lithiums. Alkalines provide fairly consistent performance and are easy to buy. Lithium cells are not recommended for all flash units--only those specifically outlining their use, as these batteries have a higher initial power surge that may damage the unit. Most of the time, rechargeable NiCd (Nickel Cadmium) or Ni-MH (Nickel Metal Hydride, nickel hydride, or NiHy for short) can be used. But again, the manufacturer may advise against their use: there is a size variance and the nickel cells may not make contact at both ends. Fortunately, most flash units readily accept nickel-type rechargeables, even though they are of slightly lower voltage than alkalines. Nickel rechargeables recycle faster than alkaline and lithium. Caution: never mix battery types, and always use batteries with equal energy reserves. Fresh (or freshly charged) batteries are always best.

Replace the batteries when recycling times become excessive. Interestingly enough, if you use a digital camera that also uses AA cells, you may find that after the digital camera has bowed out, that there is sufficient life left in the batteries to use them in your flash.