The Digital Photo Guy

Flash Photography Primer

by on Nov.03, 2010, under gear, Monday Morning Tips, Schedule, Webcast, Workshops

Flash in a Flash

In advance of the Flash Photography Workshop at Deer Park Winery and Auto Museum on November 13, here’s a quick MMT covering the basics of flash. I can’t cover all the details that normally take 4 hours but this will get you started.


Canon currently has two primary dedicated flashes, the 430EX II (US$280, above)and the 580EX II (US$445, below). The main differences between the two besides price are power (430 = GN43 vs 580 = GN58) and Master Mode. In multi-flash situations, the 430EX can only be used as a Slave while the 580EX II can be a Master or Slave. Is the 580EX II worth an extra US$165? Probably not unless you’re into flash photography. Having said that, my recommendation is to always buy the biggest, baddest flash you can afford as your first flash.


The biggest advantage of the 430EX II over its predecessor, the 420EX, is the inclusion of Manual Mode and the attendant LCD panel. Notice how the back looks amazingly like the 580EX II. Just like the difference between Canon’s entry level dSLRs and the high end models, the high-end 580EX II has a dial while the mid-range 430EX II has left/right buttons. The control buttons and LCD are needed for Manual Mode.

Before we get too far, let’s back up a moment and review GN (Guide Number). Back in the “good ol’ days”, GN was a reasonably accurate measure of flash power. That’s because GN was used to set the proper aperture. Inflated GN would have caused severe agida for photographers and a backlash against manufacturers. When Nikon i-TTL and Canon E-TTL become the norm, marketers got busy and the GN wars were on. Today, GN is only useful for comparing flashes from the same company. GN43 (Canon 430EX II) is less powerful than GN58 (Canon 580EX II). It’s near impossible to compare Canon 580EX II GN58 to Nikon SB900 GN48 because the Nikon is measured differently from the Canon. For more on the math behind GN, follow this link.

OK, back to our regularly scheduled program. A flash can be used in auto or manual modes, just like your dSLR. Canon auto flash mode is called E-TTL (Evaluative Through The Lens). E-TTL is about as close to magic as it gets without a witch doctor. At half-press ambient light is metered and just before full- press, a preflash is fired and flash exposure is metered. If all goes well, the camera knows how much ambient light is present and how much flash is needed to properly light the subject.

Unfortunately, just like a camera’s auto exposure meter, this balancing act between AE and Flash AE can get confused. As neat-o-whiz-bang as E-TTL is, it’s still a dumb computer. For true flash control, you need Manual Mode. The mechanics are pretty simple. Press the Mode button repeatedly until you get to M(anual).

With the flash in M, determine the distance to your subject and the desired aperture. For this example, let’s assume the subject is 10 feet away and you want f/5.6. When you aim at your subject and half-press, the distance scale along the bottom of the LCD will have a black stripe over it, indicating the near and far effective range. If the effective range encompasses 10 ft, you’re set. If not, press Sel/Set and turn the dial to a lower flash output (1:2, 1:4, 1:8, 1:16, 1:32, 1:64, 1:128). Repeat the process and recheck the effective distance. Of course, you can also increase or decrease ISO as well. If all this seems overly complicated, not to worry because the way most pros do it is much simpler.

In today’s digital world, most flash photographers have reverted to a time-honored method known as “by guess and by golly”. Just make a guess, take a photo and dial in more or less light based on your histogram. As your skills improve, you’ll be able to be dial it in after 1 or 2 test shots.

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