6.1. Flash Basics

How Flash Works

A flash is a very short burst of bright light that helps expose your photo when the ambient light is not enough. In digital photography, this light must be precisely timed to the camera shutter.

Modern camera flashes use a xenon tube. When you take a photo, a high voltage charge is released through the tube, which produces an intense pulse of white light. The duration of this pulse is extremely short, often between 1/1000 s and 1/20,000 s, even if your shutter speed is slower. This is why flash is so useful for freezing motion in low light.

The camera and flash talk to each other through electrical contacts on the hot shoe or through a cable or wireless trigger. The camera opens the shutter, the flash fires while the shutter is fully open, and then the shutter closes again. As long as your shutter speed is at or below the camera’s sync speed, the whole frame is illuminated evenly. Sync speed is usually around 1/160 s to 1/250 s and is covered in detail in the Flash Techniques chapter.

The light from the flash travels outwards in all directions, reflects off your subject and the surroundings, then returns to the lens where the sensor records it. The brightness of the flash in your final image depends on four main factors: flash power, distance from flash to subject, aperture, and ISO.

Shutter speed affects how much ambient light you see, but within normal sync limits it does not significantly change how much flash light reaches the sensor. This separation between flash light and ambient light is one of the most powerful aspects of flash photography.

Built-In vs External Flash

Most cameras and many smartphones include some kind of built-in flash. Larger cameras can also use external flashes, often called speedlights or flashguns, that slide into the camera hot shoe or work off-camera.

A built-in flash is integrated into the camera body. It is small, usually fixed in position, and fires straight ahead. It is convenient and automatic, which is useful for beginners, but the light is close to the lens and often harsh. This can create flat, unflattering light, strong shadows behind the subject, and red-eye in portraits, because the light is almost on the same axis as the lens and reflects directly back from the eyes.

An external flash is a separate unit with its own battery and controls. It is more powerful, recycles faster, and usually offers a tilting and rotating head so that you can bounce light off ceilings or walls. This makes the light softer and more directional and generally more flattering. External flashes also give you more control. You can adjust power in small steps, use different modes such as TTL and manual, add light modifiers, and often use the flash off-camera with wireless control.

Use a built-in flash when you need a quick snapshot, you have no other light source, and you do not want to carry extra gear. Switch to an external flash when you want better quality light, more power, faster shooting, and creative control over direction and softness of the light. For any serious portrait, event, or creative flash work, an external speedlight or studio strobe is strongly preferred.

Flash Power

Flash power describes how much light the flash emits in a single burst. On small flashes the power is adjustable, typically in fractional steps: full power (1/1), then 1/2, 1/4, 1/8, and so on down to 1/128 or even 1/256. Each step represents a one stop change in light. Reducing the power by one stop halves the amount of light.

At higher power settings, the flash tube discharges more energy and the light pulse lasts longer. This allows you to illuminate subjects that are farther away, or to use a smaller aperture or lower ISO while keeping proper exposure. The trade-offs are longer recycle times between shots and more heat in the flash.

At lower power settings, the flash pulse is very short. Recycle times are fast and battery life is better. Very low power settings are useful for close subjects, fill light in bright conditions, or when you want to balance flash subtly with ambient light.

When you change flash power, you directly change the flash exposure on your subject without affecting the ambient exposure. For example, if you lower flash power by one stop, you could open the aperture by one stop or increase ISO by one stop to keep the overall brightness the same, or you can keep camera settings and accept a darker flash-lit subject.

In real shooting, you balance flash power with aperture and ISO. If the flash is too bright, you can lower power, close the aperture, or lower ISO. If it is too dim, you can increase power, open the aperture, or raise ISO. Shutter speed mainly controls background brightness from ambient light, as long as you stay at or below sync speed.

Guide Numbers

The guide number (GN) of a flash is a simple way to describe its maximum power. It tells you how far the flash can effectively illuminate a subject at a given aperture and ISO. Manufacturers typically give a GN at ISO 100 and at a specific zoom setting of the flash head, for example GN 60 (meters, ISO 100, 200 mm).

The basic relationship is:

Distance is measured from the flash to the subject, usually in meters if the GN is given in meters. Suppose your flash has GN 40 at ISO 100. If your subject is 5 m away and you want a correct flash exposure at ISO 100, you can estimate:

$$f\text{-number} = \frac{40}{5} = 8$$

So you would start with f/8. If you open to f/4 at the same distance and ISO, the flash will overexpose at full power, so you would need to reduce flash power.

Guide numbers scale with ISO. Because doubling ISO increases sensitivity by one stop, the effective GN scales by the square root of the ISO ratio. A good approximation is:

$$GN_{\text{new}} = GN_{100} \times \sqrt{\frac{ISO_{\text{new}}}{100}}$$

For example, if GN is 40 at ISO 100, then at ISO 400:

$$GN_{400} \approx 40 \times \sqrt{\frac{400}{100}} = 40 \times 2 = 80$$

In practice, modern cameras with TTL flash metering make manual GN calculations less necessary in everyday shooting. However, understanding guide numbers helps you judge how powerful a flash is, how far it can reach, and what apertures are realistic at certain distances and ISOs, especially when working manually or with simple flashes.

TTL Flash

TTL stands for Through The Lens. TTL flash metering is an automatic mode where the camera measures the light coming through the lens and adjusts flash output for you. This is extremely helpful in changing conditions, such as events or street scenes, where your distance to the subject and the background change constantly.

Most modern systems use a pre-flash. When you press the shutter, the flash fires a tiny, quick burst that is usually not noticeable. The camera’s metering system measures the reflected light from this pre-flash, then calculates how much power the main flash should output to reach the desired exposure. All of this happens just before the shutter fully opens.

TTL tries to produce a midtone brightness on your subject, similar to the way camera metering for ambient light works. It reads the scene, makes assumptions about what is important, and sets the flash power automatically. This is convenient, but it can be fooled by very dark, very light, or highly reflective scenes, or by strong backlighting. In those situations you often use flash exposure compensation, which is described later in this chapter.

TTL is ideal when you are moving around, your subject distance changes, or the background is unpredictable. You can focus on composition and timing, and let the camera handle flash power. However, TTL exposure may vary from shot to shot if the scene changes, so if you need perfect consistency across many frames, manual flash can be a better choice.

Even when you use TTL as your main mode, it is useful to think of it as a starting point rather than a final answer. You can nudge it brighter or darker with flash exposure compensation, or switch to manual flash when the lighting situation is stable and you want repeatable results.

Manual Flash

In manual flash mode, you choose the flash power yourself and it stays fixed until you change it. Power is usually set in fractions such as 1/1, 1/2, 1/4, down to 1/128 or lower. There is no pre-flash metering. The camera does not try to adjust flash output. This gives you full control and complete consistency once you have dialed in a good exposure.

Manual flash is especially useful in situations where the distance from flash to subject is constant, such as studio portraits, product photography, or a speaker standing at a podium. You can take a test shot, review the result, and then adjust power, aperture, or ISO until the exposure looks right. After that, every frame will look similar as long as nothing important changes.

The exposure control process is straightforward. If your test shot is too bright, you can reduce flash power by one stop, close the aperture by one stop, or lower ISO by one stop. If it is too dark, you do the opposite. Because flash and ambient light are separate, you can also adjust shutter speed within sync limits to change the background brightness without affecting the flash exposure significantly.

Manual flash is also important when working off-camera with multiple flashes. Each light can be set to a precise power level so that you can build lighting ratios between key, fill, and rim lights. TTL with multiple flashes is possible but can be less predictable and harder to troubleshoot for beginners.

Although manual flash may feel slower at first, it trains you to understand how distance, power, aperture, and ISO interact. Over time, you will learn to estimate a starting point for power and settings by eye, which makes your lighting process much more efficient and intentional.

Flash Exposure Compensation

Flash exposure compensation (FEC) is a control that tells the camera to adjust the strength of the flash relative to what TTL metering thinks is correct. It works only when the flash is in an automatic mode such as TTL. If you are using manual flash, FEC has no effect, because you have already chosen the power directly.

FEC is usually expressed in stops, often in one third stop increments. For example, +1.0 FEC tells the camera to make the flash one stop brighter than the metered value, which means doubling the flash output. A setting of −1.0 FEC makes the flash one stop darker, halving the output. These adjustments only affect the flash component, not the ambient exposure.

You use FEC whenever the camera’s automatic judgment about flash brightness does not match your creative intention. Common cases include backlit subjects, where the camera might underexpose the person in front of a bright window, or subjects wearing very dark or very light clothing that trick the meter. Adding positive FEC brightens the flash-lit subject. Negative FEC is helpful when you want subtle fill light, such as outdoors in sunlight where you only need a gentle lift in the shadows rather than a full flash exposure.

A useful approach is to keep your ambient exposure where you want it using shutter speed, aperture, and ISO, then use TTL flash with FEC to control how strong the flash appears relative to the scene. If a face looks a bit flat or too bright, dial the FEC down. If the face is too dark against a bright background, dial it up.

In practice, flash exposure compensation is the main fine tuning control you will use when working hand held with TTL flash. It lets you keep the speed and convenience of automatic flash metering, while still guiding the camera toward the look you want in your photos.

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