How Electronic Flash Works

Trick Photography And Special Effects

Trick Photography and Special Effects

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The bursts of light we call electronic flash are produced by a flash of photons generated by an electrical charge that is accumulated in a component called a capacitor and then directed through a glass tube containing xenon gas, which absorbs the energy and emits the brief flash. For the pop-up flash built into the EOS 40D, the full burst of light lasts about 1/1,000th of a second and provides enough illumination to shoot a subject 10 feet away at f/4 using the ISO 100 setting. In a more typical situation, you'd use ISO 200, f/5.6 to f/8 and photograph something 8 to 10 feet away. As you can see, the built-in flash is somewhat limited in range; you'll see why external flash units are often a good idea later in this chapter.

An electronic flash (whether built in or connected to the EOS 40D through a cable or the hot shoe) is triggered at the instant of exposure, during a period when the sensor is fully exposed by the shutter. As I mentioned earlier in this book, the 40D has a vertically traveling shutter that consists of two curtains. The first curtain opens and moves to the opposite side of the frame, at which point the shutter is completely open. The flash can be triggered at this point (so-called first-curtain sync), making the flash exposure. Then, after a delay that can vary from 30 seconds to 1/250th second (with the EOS 40D; other cameras may sync at a faster or slower speed), a second curtain begins moving across the sensor plane, covering up the sensor again. If the flash is triggered just before the second curtain starts to close, then second-curtain sync is used. In both cases, though, a shutter speed of 1/250th second is the maximum that can be used to take a photo.

If you happen to set the 40D's shutter to a faster speed in Tv or M mode, the camera will automatically adjust the shutter speed down to 1/250th second. In Av, P, or any of the Basic Zone modes, the 40D will never select a shutter speed higher than 1/250th second when using flash. Actually, in P mode, shutter speed is automatically set between 1/60th to 1/250th second when using flash.

Ghost Images

The difference might not seem like much, but whether you use first-curtain sync (the default setting) or second-curtain sync (an optional setting) can make a significant difference to your photograph if the ambient light in your scene also contributes to the image. At faster shutter speeds, particularly 1/250th second, there isn't much time for the ambient light to register, unless it is very bright. It's likely that the electronic flash will provide almost all the illumination, so first-curtain sync or second-curtain sync isn't very important.

However, at slower shutter speeds, or with very bright ambient light levels, there is a significant difference, particularly if your subject is moving, or the camera isn't steady. In any of those situations, the ambient light will register as a second image accompanying the flash exposure, and if there is movement (camera or subject), that additional image will not be in the same place as the flash exposure. It will show as a ghost image and, if the movement is significant enough, as a blurred ghost image trailing in front of or behind your subject in the direction of the movement.

When you're using first-curtain sync, the flash goes off the instant the shutter opens, producing an image of the subject on the sensor. Then, the shutter remains open for an additional period (30 seconds to 1/250th second, as I noted). If your subject is moving, say, towards the right side of the frame, the ghost image produced by the ambient light will produce a blur on the right side of the original subject image, making it look as if your sharp (flash-produced) image is chasing the ghost. For those of us who grew up with lightning-fast superheroes who always left a ghost trail behind them, that looks unnatural (see Figure 7.5).

Trilhas Eletronicas
Figure 7.5 First-curtain sync produces an image that trails in front of the flash exposure (top), while second-curtain sync creates a more "natural looking" trail behind the flash image.

So, Canon provides second-curtain sync to remedy the situation. In that mode, the shutter opens, as before. The shutter remains open for its designated duration, and the ghost image forms. If your subject moves from the left side of the frame to the right side, the ghost will move from left to right, too. Then, about 1.5 milliseconds before the second shutter curtain closes, the flash is triggered, producing a nice, sharp flash image ahead of the ghost image. Voilà! We have monsieur le Flash outrunning his own trailing image.

Exception to the Sync Rule

Triggering the electronic flash only when the shutter is completely open makes a lot of sense if you think about what's going on. To obtain shutter speeds faster than 1/250th second, the 40D exposes only part of the sensor at one time, by starting the second curtain on its journey before the first curtain has completely opened. That effectively provides a briefer exposure as a slit of passes over the surface of the sensor. If the flash were to fire during the time when the first and second curtains partially obscured the sensor, only the slit that was actually open would be exposed.

However, the 40D and certain Canon flashes provide a partial solution, called high-speed sync or FP sync (focal plane sync). Those flash units can fire a series of flashes consecutively in rapid succession, producing the illusion of a longer continuous flash, although at reduced intensity. These multiple flashes have a duration long enough to allow exposing the area of the sensor revealed by the traveling slit as it makes its full pass. However, the reduced intensity means that your flash's range is greatly reduced.

There is no setting to make on the EOS 40D to use this high-speed sync (other than to dial in a shutter speed higher than 1/250 second). Instead, you need to select this mode on the flash itself. With the 580EX II, the button to press is the High Speed Sync/Shutter Synchronization button, located just below the LCD between the Mode and Zoom buttons. The button is used to select second-curtain sync on the flash unit.

Determining Exposure

Calculating the proper exposure for an electronic flash photograph is a bit more complicated than determining the settings by continuous light. The right exposure isn't simply a function of how far away your subject is (which the 40D can figure out based on the autofocus distance that's locked in just prior to taking the picture). Various objects reflect more or less light at the same distance so, obviously, the camera needs to measure the amount of light reflected back and through the lens. Yet, as the flash itself isn't available for measuring until it's triggered, the 40D has nothing to measure.

The solution is to fire the flash twice. The initial shot is a preflash that can be analyzed, then followed by a main flash that's given exactly the calculated intensity needed to provide a correct exposure. As a result, the primary flash may be longer for distant objects and shorter for closer subjects, depending on the required intensity for exposure. This through-the-lens evaluative flash exposure system is called E-TTL II, and it operates whenever the pop-up internal flash is used, or you have attached a Canon dedicated flash unit to the 40D.

Guide Numbers

Guide numbers, usually abbreviated GN, are a way of specifying the power of an electronic flash in a way that can be used to determine the right f/stop to use at a particular shooting distance and ISO setting. In fact, before automatic flash units became prevalent, the GN was actually used to do just that. A GN is usually given as a pair of numbers for both feet and meters that represent the range at ISO 100. For example, the EOS 40D's built-in flash has a GN of 13/43 (meters/feet) at ISO 100. To calculate the right exposure at that ISO setting, you'd divide the guide number by the distance to arrive at the appropriate f/stop.

Using the 40D's built-in flash as an example, at ISO 100 with its GN of 43, if you wanted to shoot a subject at a distance of 10 feet, you'd use f/4.3 (43 divided by 10; round to f/4 for simplicity's sake). At 8 feet, an f/stop of f/5.4 (round up to f/5.6) would be used. Some quick mental calculations with the GN will give you any particular electronic flash's range. You can easily see that the built-in flash would begin to peter out at about 15 feet, where you'd need an aperture of f/2.8 at ISO 100. Of course, in the real world you'd probably bump the sensitivity up to a setting of ISO 400 so you could use a more practical f/5.6 at that distance.

Today, guide numbers are most useful for comparing the power of various flash units. You don't need to be a math genius to see that an electronic flash with a GN of, say, 165 would be a lot more powerful than your built-in flash (at ISO 100, you could use f/11 instead of f/2.8 at 15 feet).

Using the Built-In Flash

The Canon EOS 40D's built-in flash is a handy accessory because it is available as required, without the need to carry an external flash around with you constantly. This section explains how to use the flip-up flash in the various Basic Zone and Creative Zone modes.

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  • Tracie
    How can i use the flash light of canon 40d?
    1 year ago

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