Smart Flash is Addictive
by Galen Rowell
Outdoor Photographer, October 1996

It's been eight years since I got hooked on smart flash. I especially remember an afternoon party where every slide came out with a perfect balance of fill and natural light. My Nikon 8008 was set on program mode with an SB-24 flash set at the -1.7 compensation ratio that I use to get natural- looking fill in landscape photographs.

When I later tried such casual shooting with either flash-equipped point-and-shoot cameras or more sophisticated SLRs with pop-up flashes, I could get acceptable one-hour prints for family use, but not well-exposed slides for publication. What caught my fancy, however, was that an occasional frame would come out just right.

In the last year, I've experimented with point-and-shoots and light SLRs for my lightweight adventures. What started me in this direction was Nikon's introduction of the N70 with the first programmable built-in smart flash. I tried "top-down" tests with the "smart" N70 and then began "bottom-up" tests with an old favorite Olympus XA and a super-light Canon Rebel-X SLR.

If you want to set the camera on autopilot and not worry about computing for the flash, there's no substitute for smart flash. If you prefer to have finer control over the image, however, the odds of balanced fill-flash with an unsmart camera can be greatly increased.

Without a smart flash, the desired -1.7 compensation ratio can be attained in a number of ways. A dedicated flash unit that ties the amount of light to the film speed setting on the camera can be fooled into doing the right thing for the wrong reason. First, set shutter speed and aperture for a manual exposure of the naturally lit background. Second, set the exposure compensation for -1.7 stops (or 1.5 if no 1.7 setting). For cameras without this adjustment, set the film speed 1.7 stops higher than normal; for example, ISO 320 instead of 100. Now you'll have a correct background exposure combined with flash that won't blow out natural shadows with overly bright fill. Remember to set your film speed back to the original ISO or you might end up with whole rolls of seriously underexposed pictures.

Things get more complicated when built-in flash units are relatively weak or not dedicated. Beginning from square one, you need to determine at what distance and apertures your unit will give you perfect looking flash fill. Don't trust the listed guide number for your flash unit. Logically, it should be too low; you need 1.7 stops less light for fill than for full-power flash in totally dark situations. Practically, guide numbers tend to be exaggerated, unless, that is, you're shooting in a narrow white hallway in a Hobbit house with five foot ceilings, where lots of light is reflected. In the open outdoors, effective guide numbers are cut in about half.

The concept of a guide number is far more simple than its relationship to light falling off in inverse proportion to the square of the distance makes it sound. Because the f-stops on your lens are also based on squares relating to the size of the lens opening, a flash guide number for a particular film speed remains constant as simply aperture times distance. A guide number of 64 means that at f8 a subject can be properly lit up to eight feet; at f16, four feet.

When I tested a Nikon N70 for outdoor fill with ISO 100 film, I began by setting the lens at f8 using the pop-up flash at distances varying from 2 feet to 8 feet. Without flash exposure compensation, only one of these distances would give a correct exposure, but the N70 with its built-in smart flash gave perfect exposures up to five feet, where they began to go dark. I concluded that 4.5 feet was the maximum distance, and eight times 4.5, or 36, was my guide number. To confirm this, I later tested fill up to 18 feet at f2 and down to about 2 feet at f16. I get a majority of properly exposed fill-flash images with the N70 simply by checking to make sure that I'm always within the limits of my experimental guide number.

With the Canon Rebel-X, exposures aren't so simple. The extremely light camera is very appealing for recreational use, but it lacks a flash exposure compensation setting and has a fill guide number that tests out at 30. Images made closer than the distance computed with the guide number are consistently overexposed for natural-looking fill. I go the extra step and use the guide number to select my distance and f-stop for accurate fill.

If successful fill-flash in the outdoors was merely a matter of choosing guide numbers, lots of people would be doing it with simple cameras. There are other complications. The two photographs printed here were made with and without fill-flash using a Canon Rebel-X on a climb of El Capitan in Yosemite. Without flash, the image is too dark. With flash at five feet at f5.6, the fill is excellent except for a strange round shadow on the rock at the left. The 20-35mm zoom lens interferes with the built-in flash's coverage area at close range with wide-angle settings.

There are three easy corrections for this problem. One is never to shoot wider than 28mm. The camera's manual recommends this limit as the coverage of the flash, but in practice the fill looks okay on this 20mm shot, even with a bit of fall-off at top and bottom. The second solution to the lens-shadow problem is to turn the camera 180 degrees so that the shadowed flash is against the natural light of the sky. The alternative of using the 28mm setting and backing farther away from my subject was not an option on a tiny ledge a thousand feet up a sheer rock face.

A major problem with overexposed backgrounds happens when a camera with a low flash synch speed and a weak flash is used in an auto-exposure mode. An aperture of f8 that works perfectly for fill at four feet will give a hideous two-stop or more overexposed background in bright daylight if the camera automatically limits shutter speeds to 1/90 sec. when the flash is popped up. The Nikon N70 is better at 1/125 sec. More sophisticated top-of-the-line Nikons and Canon SLRs have a far more versatile 1/250th synch speed, but lack a built-in flash.

In summary, any camera with a flash can give you perfect daylight fill if you know how to choose the situation. Depending on whether your flash unit is basic, dedicated, or smart, you will need to follow some or all of these procedures:

1. Compute your own experimental guide number and stay within its limit of shutter speed times aperture.

2. Don't let your camera choose a slow flash synch speed that will overexpose the natural light. (Many cameras have warning signals for this.)

3. Test wide-angle or large-diameter lenses for light fall-off and shadowing on close subjects.

4. When shooting verticals, turn the camera so that the flash is on the side of the lens that maximizes light on the subject and minimizes shadowing of wide or large lenses.

When all is said and done, it takes a focused mind as well as camera to make consistently good fill-flash exposures, unless you're willing to carry around that pro SLR with a 1/250 sec. synch speed and separate flash that allows you to shoot away at f8 or f11 in bright daylight. I'm still waiting for someone to come out with a perfect camera for my outdoor adventures. It should weigh less than a pound, zoom to 20mm with full flash coverage and no lens shadow, synch to 1/500sec. and have programmable flash exposure compensation. Until then, I've learned how to make do.

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