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DCR Workshop: Flash Demystified, Part I
by David Rasnake -  11/26/2008

So you finally pulled the trigger and bought your new dream camera, your first DSLR. While you were at it, you got a great walk-around lens, maybe some key accessories like a battery grip and a bag. And of course, you threw an external flash in there as well, just for good measure. After all, being able to use an external flashgun (or better still, more than one of them) is one of the great advantages of a DSLR, right?

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But now what?

Many aspiring photographers who move up to DSLRs (or other hot-shoe equipped cameras) and external strobes quickly run up against a wall: they know that an external flash is supposed to give better results than their camera's built-in flash allows, but they're not always sure where to go beyond this recognition. Mount the flash, fire off a few shots, and you'll probably notice first off that the images you're getting look about the same as what your camera's built-in flash provides. So what gives?

External flashguns open up a whole realm of possibilities – in terms of how you control your light, how you direct it, and how you meter it – that your camera's built-in flash doesn't give you. As a rule, they're much more powerful than built-in flashes, recycle faster, and can do things that built-in flashes can't, like bounce light off the ceiling or put your light somewhere other than right atop your camera. But using external flash can also be one of the most intimidating aspects of photography to learn.

In this special November/December "double issue" of DCR Workshop, we're offering up a quick tutorial for new flash users aimed at answering the most basic questions we field about flash shooting: how does this thing work, and how do I make my flash pictures look less like flash pictures. In this week's article, we'll delve into the mysteries of metering and flash modes, and we'll be back next week with a follow-up piece on using flash bounce and other techniques to achieve different looks.


THE METERING MYSTERY
Power on that brand new flashgun for the first time and, assuming it's a newer variant with all of the modern amenities, you may be in for a surprise: this thing has nearly as many modes and controls as your DSLR itself!

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While different flash manufacturers (and even different models from the same maker) do things differently and sometimes use different nomenclature, the modes on most modern flash units are usually nothing more than variants on three basic methods for metering the output of the flash unit.

As with ambient-light exposures, when we talk about "metering" in the context of flashguns, we're talking about how the camera, the flash, or some combination of the two measure and control the amount of light the flash is putting out to provide a correct exposure. As noted above, in the world of flash photography, there are three basic ways to meter your exposure: manually (by using a flash burst of known power and adjust camera settings accordingly for correct exposure), automatically (by using a sensor on the flash to "switch off" the strobe when enough light has been distributed), and automatically with through-the-lens measurement (which uses a sensor inside the camera itself, rather than on the flash unit, to control burst power and duration).

All of this sounds confusing, but while the concepts can be hard to understand at first, putting it into practice (at a basic level) is relatively easy. That's because we're going to focus, after a brief explanation, on the auto options – and in particular, the TTL (or "through-the-lens") auto option – that modern flash units provide. But first, a quick history lesson in the progression of flash technology, as seen in the various settings a modern flashgun provides.

Manual Flash
In the beginning, there was only manual flash. And, in the opinion of many casual photographers at least, it was bad. Manual flash is a stone simple way of taking a flash picture, in that the external flash, on a trigger signal from the camera, simply fires. Thus, it's up to the photographer to take care of everything else – adjusting the camera's shutter speed to sync up with the flash burst, setting the aperture according to the film speed (or, in the case of digital cameras, the ISO setting), distance to subject, and other exposure requirements. The problem is, this requires an ability to estimate your distance to subject, as well as a knowledge of how to calculate exposure settings from guide numbers (a basic, common method for measuring a flash unit's power) if you were unlucky enough to have a flash that didn't give a printed table on the back for this purpose. All of these calculations could be a real headache, and without some experience, the chance of getting the settings right the first (or second, or third) time is pretty slim.

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Even though manual flash control has largely gone by the wayside for casual flash photography, most contemporary flashguns (like the Olympus model seen in the shot above) still have a manual control mode. These days, you'll even find dynamic manual modes on some higher-end flash units, which provide an adjusted guide number readout based on the ISO setting you enter. Most manual flash modes also allow you to shoot the flash at reduced power, which can be crucial for multi-lighting setups or exposure fine-tuning without having to make aperture changes.

In short, manual flash power control is a great option to have on your flashguns, and many serious flash shooters use nothing but manual control for the same reasons that many photographers prefer manual exposure control on their cameras: if you know how to approach, manually setting your exposure puts you completely in the drivers seat. But in my opinion, flash shooting is one area where it's easier to understand the theory behind what's going on (and in turn, how to most effectively use manual exposure control) only after you have some flash shooting experience under your belt. If you want to read up on manually controlling your new flashgun, there are several great resources on the web – we've even listed some in the conclusion of this article – but for the purposes of this tutorial we'll leave the discussion of your flash's manual mode at that.

Auto Flash
Perhaps surprisingly, basic auto flash metering has also been with us for awhile, and still shows up on most modern flashguns. Like manual flash, your flashgun's plain auto mode (not its TTL auto mode) doesn't actually communicate with the camera for its settings. Instead, the auto flash mode will specify aperture and ISO values to set on your camera (again, as with manual mode, shutter speed doesn't really matter – it only has to be within the camera's sync range, which tops out on most models somewhere beyond 1/125).

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The Olympus flash above is currently configured to work at ISO 100 with an aperture of f/5.6. Hence, I'd set my camera in manual mode with a 1/60 shutter speed (a good middle-of-the-road choice for sync), an aperture of f/5.6, and a sensitivity setting of ISO 100 and be ready to fire away. Most modern flashguns, and even many older ones with manual dials, will let you configure your combination of auto flash settings. For instance, if I wanted to shoot at a wider aperture for shallow depth of field, I can dial that in on my flash.

So that's the principle behind using auto flash, but how does the flash actually work in this mode? Basically, the flash unit has a built in sensor (a thyristor, to be technical) that measures the amount of light being reflected off of whatever's in front of the flash. When the exposure reaches the amount of light needed for the values you've dialed in (or the flash's preset exposure values for auto mode, if it's not adjustable), the sensor within the flash sends a signal to turn off the flash.

To use an analogy, the flash's auto-mode sensor is like a bucket that you're "filling up" with light when you take a flash shot. When the bucket is full – that is, when enough light has been added to make a proper exposure – the sensor kills power to the flash, effectively stopping the "flow" of additional light.

The disadvantages of auto flash may be obvious by now. First, since the flash isn't really "talking" to the camera body, you have to manually configure your camera's settings to align with your flash's settings: if a dial gets knocked accidentally, you can end up with some missed exposures. Second, the thyristors used for measuring light with auto flash are primitive compared to the complex exposure control systems that modern cameras use. For this reason, auto flash exposures aren't always highly accurate depending on the subject. Tweaking exposure up or down isn't particularly hard with auto flash (the easiest way is to use the aperture, going with a narrower aperture if the first shot is overexposed, or a wider one if the shot is underexposed – the same kind of adjustments you'd make when shooting with ambient light), but the system can require multiple exposures to get everything dial in just right depending on your subject.

At a basic level, while thyristor auto flash automates the process of taking flash exposures considerably when compared to manual flash, compared to the kind of flash shooting experience you may be used to with a point-and-shoot camera (where the camera makes most of the calculations for proper flash exposure for you), it's still fairly primitive. As with manual flash exposure, you may find that you prefer auto flash to more advanced TTL auto options in some situations (the ability to use a newer flash on older camera bodies being one key advantage). But again, if you're just starting out with your flash, thyristor auto mode may seem a little clunky.

TTL Auto Flash
With the mysteries surrounding your new flash's manual and thyristor auto settings hopefully cleared up, we've come around to the mode where you'll probably be spending most of your time, at least initially: TTL auto. If you've been searching for that setting that automates the whole process, this is it.

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TTL flash metering, which uses active communication between the flash unit and the camera body, is like the "easy auto" mode of flash control. The camera is measuring the total quantity of light – what's coming from the flash, but also what's coming from ambient sources – as it hits the sensor. The bucket analogy from above works here as well, only now we've positioned the bucket that we're filling in the same place where the image is being recorded. As a result, the outcome tends to be more consistent than what you'll get from traditional auto flash: after all, the camera's processor – the same thing that sets exposure for your images using ambient light – is doing the calculations based on the amount of light actually gathered behind the lens.

All of that said, TTL flash metering has quirks of its own. Furthermore, many newer variants on the TTL (Canon's E-TTL system, for instance) idea actually use a pre-flash that fires imperceptibly just before the shutter opens to get an initial measurement, and these systems all have unique proclivities of their own depending on the specific system design. Some newer TTL systems also incorporate focal length data from the lens, which can further complicate matters in some situations. Ambient light and other conditions permitting, though, TTL metering does a good job in most situations of giving uniformly balanced, middle-of-the-histogram exposures on your average subject.

You'll find nearly as many opinions as there are advanced photographers on the use of TTL flash metering: some shooters who learned flash photography by calculating their own exposures detest some TTL system quirks, and many others who are looking for more dramatic lighting effects from one or more external flashguns find this kind of highly automated metering approach simply too limiting. After all, unless you tweak the system manually to perform otherwise, what you'll get from TTL metering by default is almost always a conservative, fairly "vanilla" exposure. But if you're just learning the ropes of flash photography, TTL metering provides a straightforward, largely automated in-road into external flash use – letting you experiment with bounce, light direction, even multiple flash units, without having to worry (too much, anyway) about controlling the flash's power and metering options.

Of course, if you need to you can fine-tune TTL exposures as well. Most flash units, camera bodies, or sometimes both, have a flash exposure compensation control. As mentioned, even TTL auto can be fooled at times, but by compensating your flash exposure up or down as needed, you should be able to get perfectly exposed results relatively easily in TTL auto mode. As you learn your camera and flash setup's proclivities, you'll also be able to more easily anticipate when the system is likely to miss and act accordingly.


CONCLUSIONS
Whew! If you're just dipping your toe into the world of flash photography using an external flash, that probably seems like a lot of nitty-gritty info to take in – and we haven't even gotten around to actually shooting with the flash yet. Thankfully, the basics from here forward get a little more practically focused: in next week's conclusion to our two-part series, we'll talk about some simple techniques for getting away from that flat, blasted look that gives flash photos a bad name, and tie it all back in with the info on metering modes presented above.

Recommended Reading
As I've alluded to throughout, this overview really doesn't even begin to scratch the surface of flash shooting. Lighting is a complex topic – broad enough to merit the attention of entire books on the subject. If you're itching to go beyond the basic ideas outlined in this series, however, there are some excellent online resources for doing just that. Moose Peterson's TTL Flash System page is often recommended as a starting point if you're wanting to dig a little deeper into what makes flashguns tick and how this affects their use in the field. And for in-depth tutorials, how-to articles, gear reviews, and lots of other excellent advice on flash shooting (especially manual shooting, and shooting with multi-flash setups), there's no better online resource in my opinion than veteran photographer David Hobby's Strobist. If you're looking for information and inspiration, check these guys out.

NEXT: Flash Demystified, Part II